{"gene":"SMC5","run_date":"2026-06-10T07:46:35","timeline":{"discoveries":[{"year":2023,"finding":"Smc5/6 is a DNA loop-extruding motor: single-molecule imaging demonstrated that Smc5/6 extrudes DNA loops symmetrically upon ATP hydrolysis at ~1 kb/s in a force-dependent manner. Monomeric Smc5/6 unidirectionally translocates along DNA, while dimeric Smc5/6 extrudes loops. The Nse5/6 subcomplex acts as a negative regulator of loop extrusion by inhibiting Smc5/6 dimerization, thereby preventing loop-extrusion initiation but not ongoing loop extrusion.","method":"Single-molecule imaging, in vitro reconstitution","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — single-molecule reconstitution with multiple orthogonal conditions, mechanistic dissection of monomer vs. dimer behavior and Nse5/6 regulatory role","pmids":["37076626"],"is_preprint":false},{"year":2024,"finding":"Smc5/6 associates with transcription-induced positively supercoiled DNA at cohesin-dependent loop boundaries. Single-molecule imaging showed that Smc5/6 dimers specifically recognize the tips of positively supercoiled DNA plectonemes and initiate loop extrusion to gather supercoiled DNA into large plectonemic loops. Hi-C analysis showed that Smc5/6 links chromosomal regions containing transcription-induced positive supercoiling in cis.","method":"Single-molecule imaging, ChIP-seq, Hi-C","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — single-molecule reconstitution combined with genome-wide chromatin association and Hi-C, multiple orthogonal methods","pmids":["38295804"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structure (3.8 Å) of DNA-bound yeast Smc5/6 revealed a DNA clamp formed by Smc5, Smc6, Nse1, Nse3, and Nse4. The positively charged inner surface of the clamp contacts dsDNA in a nonsequence-specific manner. The Nse3 subunit secures DNA from above; the hook-shaped Nse4 kleisin forms a scaffold connecting DNA to all other subunits. Cross-linking MS identified multi-subunit conformational changes from DNA-free to DNA-bound state enabling DNA capture.","method":"Cryo-EM structure, cross-linking mass spectrometry, mutagenesis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — near-atomic cryo-EM structure with cross-linking MS and in-cell mutagenesis validation","pmids":["35648833"],"is_preprint":false},{"year":2016,"finding":"HBx hijacks the cellular DDB1-CUL4 E3 ubiquitin ligase to target the Smc5/6 complex for ubiquitin-mediated proteasomal degradation, relieving Smc5/6-mediated transcriptional repression of extrachromosomal (cccDNA) templates. Smc5/6 associates directly with extrachromosomal DNA and the HBV genome. Silencing Smc5/6 enhances extrachromosomal reporter transcription and rescues HBx-deficient HBV replication.","method":"RNAi knockdown, reporter assays, Co-immunoprecipitation, substrate-trapping proteomics","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal functional experiments in multiple systems (reporter, virus), independently replicated in companion paper (PMID:27626656)","pmids":["26983541","27626656"],"is_preprint":false},{"year":2007,"finding":"The SMC5/6 complex localizes to ALT-associated PML bodies (APBs) in ALT cancer cells and is required for targeting telomeres to APBs. The MMS21 SUMO ligase subunit of SMC5/6 SUMOylates multiple telomere-binding proteins, including TRF1 and TRF2. Inhibition of TRF1 or TRF2 SUMOylation prevents APB formation. Depletion of SMC5/6 subunits by RNAi inhibits telomere HR, causing telomere shortening and senescence in ALT cells.","method":"RNAi, co-immunoprecipitation, in vitro SUMOylation assay, immunofluorescence","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (RNAi depletion with cellular phenotype, in vitro SUMOylation, localization), clear mechanistic pathway placement","pmids":["17589526"],"is_preprint":false},{"year":2006,"finding":"Human SMC5/6 complex is recruited to nuclease-induced DNA double-strand breaks and is required for recruitment of cohesin (SMC1/3) to DSBs. RNAi knockdown of SMC5/6 specifically decreases sister chromatid HR without affecting NHEJ, intra-chromatid HR, or extrachromosomal HR.","method":"RNAi, chromatin immunoprecipitation, gene targeting assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 / Strong — ChIP showing direct recruitment, epistasis/pathway placement via multiple HR subpathway assays, clean loss-of-function","pmids":["16810316"],"is_preprint":false},{"year":2009,"finding":"Crystal structure of Mms21 (NSE2/Nse2) in complex with the Smc5 arm revealed two functional domains: an N-terminal half forming a helix bundle with a coiled-coil region of Smc5 (required for Smc5 binding), and a C-terminal SUMO ligase domain with a new type of RING E3 structure that confers specificity to SUMO E2-E3 interaction. The Mms21-Smc5 interface is required for cell growth and DNA damage resistance.","method":"X-ray crystallography, mutagenesis, in vitro SUMO ligase assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with mutagenesis and functional validation in a single rigorous study","pmids":["19748359"],"is_preprint":false},{"year":2005,"finding":"Nse2 (fission yeast SMC5/6 subunit) is a SUMO ligase: it sumoylates Smc6 and Nse3 in vitro in an Nse2-dependent manner, and autosumoylates itself. Mutations in the RING-finger-like motif (C195S/H197A) abolish Nse2-dependent sumoylation. Smc6 is sumoylated in vivo, with increased levels upon DNA damage, which is drastically reduced in nse2.SA cells. nse2.SA cells are sensitive to DNA-damaging agents, implicating Nse2 SUMO ligase activity in DNA damage responses.","method":"In vitro sumoylation assay, mutagenesis, in vivo sumoylation analysis, DNA damage sensitivity assays","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical reconstitution of SUMO ligase activity with mutagenesis, complemented by in vivo validation","pmids":["15601841"],"is_preprint":false},{"year":2006,"finding":"Nse4 is the kleisin component of the Smc5/6 complex, bridging the heads of Smc5 and Smc6. The C-terminal part of Nse4 interacts with the head domain of Smc5; specific mutations in a predicted winged helix motif of Nse4 destroy this interaction. Nse3, Nse5, and Nse6 also bridge the Smc5 and Smc6 heads at different sites. The Nse1-Nse3-Nse4 and Nse5-Nse6 subcomplexes bind to the Smc5-Smc6 heads domain at different sites.","method":"Co-immunoprecipitation, yeast two-hybrid, in vitro binding assays, structural predictions","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple protein interaction methods (Y2H, co-IP, in vitro binding) in single lab defining subunit architecture","pmids":["17005570"],"is_preprint":false},{"year":2005,"finding":"Human and S. pombe Smc5 and Smc6 interact through their hinge domains. Temperature-sensitive mutations in Rad18 (Smc6) all map to the same conserved glycine in the hinge region, and this mutation abolishes hinge-hinge interaction between Smc6 and Smc5. Two subcomplexes were identified: Rad18-Spr18-Nse2 and Nse1-Nse3-Rad62.","method":"Protein purification, co-immunoprecipitation, mutagenesis, mass spectrometry","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical purification with mutagenesis defining hinge interaction, single lab","pmids":["15601840"],"is_preprint":false},{"year":2009,"finding":"The architecture of the budding yeast Smc5/6 complex shows Smc5 and Smc6 associating at their hinge regions; Nse1-Nse3-Nse4 form a subcomplex binding the Smc5 head and adjacent coiled-coil; Nse2 binds the middle of the Smc5 coiled-coil; Nse5-Nse6 form a heterodimer that binds the hinge regions of both Smc5 and Smc6.","method":"Yeast two-hybrid, in vitro binding assays with purified recombinant proteins","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Y2H and in vitro binding with purified proteins, single lab, two orthogonal methods","pmids":["19141609"],"is_preprint":false},{"year":2015,"finding":"Purified Smc5/6 binds DNA through a mechanism requiring ATP hydrolysis and circular DNA. This promotes topoisomerase 2-dependent catenation of plasmids, suggesting Smc5/6 interconnects two DNA molecules via ATP-regulated topological entrapment. An Smc6 mutant defective in ATP binding fails to interact with DNA and chromosomes and causes cell death with DNA damage accumulation when overexpressed.","method":"In vitro DNA binding assay with purified complex, plasmid catenation assay, ATPase mutant analysis","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical reconstitution with mutational analysis, single lab","pmids":["26299966"],"is_preprint":false},{"year":2020,"finding":"Purified budding yeast Smc5/6 holocomplex exhibits DNA-dependent ATP hydrolysis, SUMO E3 ligase activity, and topological DNA binding with affinity for supercoiled and catenated DNA. Single-molecule assays showed that Smc5/6 locks DNA plectonemes and compacts DNA in an ATP-dependent manner.","method":"In vitro ATPase assay, SUMO E3 ligase assay, DNA binding assay, single-molecule imaging","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple in vitro biochemical assays on purified holocomplex, combined with single-molecule studies","pmids":["33301732"],"is_preprint":false},{"year":2020,"finding":"The human Smc5/6 complex recognizes unusual DNA configurations (non-B DNA) and uses ATP hydrolysis to compact DNA. Structural analyses revealed subunit interfaces responsible for functionality; mutations in these interfaces lead to chromosome breakage syndromes in humans.","method":"In vitro DNA compaction assay, structural analysis, mutagenesis","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with structural analysis, single lab","pmids":["33301731"],"is_preprint":false},{"year":2021,"finding":"The Nse5/6 sub-complex strongly inhibits Smc5/6 ATPase by preventing productive ATP binding. This inhibition is relieved by plasmid DNA but not by short linear DNA. Two binding sites for Nse5/6 on Smc5/6 were identified: one at the Smc5/6 arms and one at the heads (exerting inhibitory effects). Nse4/3/1 module detaches from the ATPase domains under ATP and DNA conditions. Cysteine cross-linking demonstrated Nse5/6 anchors the ATPase domains in a non-productive state destabilized by ATP and DNA.","method":"In vitro ATPase assay, crystal structure of Nse5/6, cross-linking mass spectrometry, cysteine cross-linking","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure combined with in vitro biochemistry and cross-linking MS, multiple orthogonal methods in single study","pmids":["34191293"],"is_preprint":false},{"year":2021,"finding":"Nse5/6 acts as a negative regulator of Smc5/6 ATPase activity by binding to the head-end of the complex to suppress ATP turnover. Only the six-protein holo-complex is capable of hydrolyzing ATP; ATPase activity is significantly increased by double-stranded DNA. ATPase stimulation requires functional ATP-binding pockets in both Smc5 and Smc6.","method":"In vitro ATPase assay with reconstituted Smc5/6 complexes, negative-stain electron microscopy","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro biochemical reconstitution with defined sub-complexes, corroborated by independent paper (PMID:34191293)","pmids":["33849072"],"is_preprint":false},{"year":2021,"finding":"Integrative structural study of yeast Smc5/6 by cryo-EM, cross-linking MS, and computational modeling revealed that Smc5 and Smc6 arms do not fold back (unlike cohesin/condensin arms). Instead, the long filamentous arm regions interact with Nse2 (SUMO ligase) and Nse5/Nse6 subcomplex. A 3.0-Å cryo-EM structure of Nse5/Nse6 revealed a clasped-hand topology and dimeric interface. Nse5/Nse6 uses SUMO-binding motifs to contribute to Nse2-mediated sumoylation.","method":"Cryo-EM, cross-linking mass spectrometry, computational modeling","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — near-atomic cryo-EM structure combined with cross-linking MS and functional validation","pmids":["33941673"],"is_preprint":false},{"year":2018,"finding":"The Nse2/Mms21 SUMO E3 ligase activity within the Smc5/6 complex is directly stimulated by DNA binding. Stimulation requires the electrostatic interaction between DNA and a positively charged patch in the ARM domain of Smc5, which acts as a DNA sensor and promotes activation of Nse2 E3 activity. Disruption of the ARM-DNA interaction sensitizes cells to DNA damage.","method":"In vitro SUMO ligase assay, mutagenesis, DNA-binding assay","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution of SUMO ligase activation by DNA with mutagenesis and in vivo functional validation, single lab","pmids":["29769404"],"is_preprint":false},{"year":2007,"finding":"The Smc5-Smc6 complex and SUMO modification of Rad52 regulate recombinational repair at the rDNA locus. Recombinational repair of a DSB in rDNA involves transient relocalization of the lesion to an extranucleolar site. Nucleolar exclusion of Rad52 recombination foci requires Mre11 and Smc5-Smc6, and depends on Rad52 SUMO modification. Mutations abrogating these activities cause rDNA hyperrecombination and excision of extrachromosomal rDNA circles.","method":"Fluorescence microscopy of repair foci, genetic epistasis, sumoylation assays","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetics, sumoylation), replicated aspects across labs","pmids":["17643116"],"is_preprint":false},{"year":2006,"finding":"The Smc5-Smc6 complex is recruited de novo to DNA DSBs and promotes repair by homologous recombination between sister chromatids. Loss of Smc5-Smc6 causes accumulation of gross chromosomal rearrangements and failure of error-free sister-chromatid recombination.","method":"Chromatin immunoprecipitation, genetic analysis, 2D gel electrophoresis","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct ChIP showing recruitment to DSBs, multiple independent labs confirming the sister chromatid HR function","pmids":["16892052"],"is_preprint":false},{"year":2005,"finding":"SMC5 and SMC6 are required for segregation of repetitive chromosomal regions. In conditional mutants, chromosome segregation of repetitive regions (rDNA, telomeres) is impaired, leading to X-shaped DNA (Holliday junctions) accumulation at rDNA loci. RAD52 deletion partially suppresses temperature sensitivity, indicating the complex prevents formation of sister chromatid junctions at repetitive loci.","method":"Conditional genetics, 2D gel electrophoresis, chromatin immunoprecipitation, genetic epistasis","journal":"Nature cell biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple methods including 2D gel and ChIP, replicated by several groups","pmids":["15793567"],"is_preprint":false},{"year":2009,"finding":"The Smc5/6 complex directly binds to the DNA helicase Mph1 and modulates Mph1-dependent recombination. Deletion of MPH1 or its helicase mutations suppress multiple defects in Smc5/6 mutants, while MPH1 overexpression exacerbates them. Mph1 and its helicase activity are largely responsible for accumulation of deleterious recombination intermediates in Smc5/6 mutants, defining a function of Smc5/6 distinct from Sgs1.","method":"Co-immunoprecipitation (direct binding), genetic epistasis, 2D gel analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding demonstrated by Co-IP, epistasis analysis, single lab","pmids":["19995966"],"is_preprint":false},{"year":2016,"finding":"Smc5/6 mediates SUMOylation of the Sgs1-Top3-Rmi1 (STR) complex. Sgs1 binds poly-SUMO chains and associates with the Smc5/6 SUMO E3 complex. Smc5/6-dependent sumoylation of Sgs1, Top3, and Rmi1 upon generation of recombination structures promotes STR inter-subunit interactions and accumulation at DNA repair centers, facilitating removal of recombination intermediates.","method":"Co-immunoprecipitation, in vivo SUMO modification assay, 2D gel electrophoresis, protein interaction assays","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple biochemical and genetic approaches in single lab, two orthogonal methods","pmids":["27373152"],"is_preprint":false},{"year":2016,"finding":"Smc5/6 recruits and activates Sgs1 through a two-step SUMOylation mechanism: (1) auto-SUMOylation of Smc5/6 subunits leads to recruitment of the STR complex via two SUMO-interacting motifs (SIMs) on Sgs1 that recognize SUMOylated Smc5/6; (2) Smc5/6-dependent SUMOylation of Sgs1 and Top3 is required for efficient STR function. Sgs1 SIM mutants show unprocessed HJs, increased crossovers, and impaired DNA end resection.","method":"Mutagenesis of SIMs, in vivo sumoylation assays, 2D gel electrophoresis, recombination assays","journal":"Genes & development","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic and biochemical dissection in single lab, multiple mutant analyses","pmids":["27298337"],"is_preprint":false},{"year":2017,"finding":"The Smc5/6 hinge forms a toroidal structure with distinctive interfaces ('molecular latch' and 'hub') absent from other SMC complexes. The Smc5/6 hinge binds preferentially to ssDNA; both latch and hub mutations reduce ssDNA binding and cause severe DNA damage sensitivity and reduced human cell viability.","method":"Crystal structure, mutagenesis, DNA-binding assay (ssDNA preference), cell viability assay","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — crystal structure with in vitro ssDNA binding characterization and mutational functional validation in two organisms","pmids":["28134253"],"is_preprint":false},{"year":2015,"finding":"The essential functions of Smc5/6 segregate to G2/M phase. Smc5/6 is specifically required in G2/M for: (1) metabolism of DNA recombination structures triggered by endogenous replication stress, cooperating with dissolution activities; (2) replication through natural pausing sites in late-replicating regions via cooperation with the fork protection complex Tof1-Csm3.","method":"Cell-cycle-regulated alleles, genetic screens, 2D gel electrophoresis","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — cell-cycle-restricted allele system with genetic screens, single lab","pmids":["26698660"],"is_preprint":false},{"year":2006,"finding":"Chromosomal localization of budding yeast Smc5/6 complex occurs in three distinct, differentially regulated pathways: (1) association with centromeres and chromosome arms in unchallenged cells; (2) association with DNA breaks; (3) association with rDNA arrays. Localization to collapsed replication forks was also detected. Smc5/6 is needed during replication to prevent accumulation of branched chromosome structures.","method":"Chromatin immunoprecipitation (ChIP), 2D gel electrophoresis","journal":"Molecular cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genome-wide ChIP localization study with functional genetic context, single lab","pmids":["16793545"],"is_preprint":false},{"year":2008,"finding":"Localization of fission yeast Smc5/6 to centromeres during replication requires H3-K9 methylation (heterochromatin), while MMS-induced localization to subtelomeres requires Nse2 SUMO ligase activity but not H3-K9 methylation. Smc5/6 also loads at all genomic tDNAs in a manner requiring intact TFIIIC-binding sites.","method":"ChIP-on-chip, fluorescence microscopy, mutant analysis","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP-on-chip and microscopy with genetic dissection of localization determinants, single lab","pmids":["18923417"],"is_preprint":false},{"year":2015,"finding":"The NSE1/NSE3/NSE4 sub-complex of SMC5/6 binds double-stranded DNA without sequence or structure preference. Mutations in key basic residues on the DNA-binding surface reduce DNA binding in vitro; introduction of these mutations in S. pombe leads to cell death or hypersensitivity to DNA damage. ChIP of the hypomorphic nse3 DNA-binding mutant shows reduced SMC5/6 chromatin association in vivo.","method":"In vitro DNA-binding assay, mutagenesis, chromatin immunoprecipitation","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro biochemical assay with mutagenesis and in vivo ChIP validation, single lab","pmids":["26446992"],"is_preprint":false},{"year":2010,"finding":"Dissolution of DNA-mediated sister chromatid linkages before mitosis is an active process requiring the Smc5/6 complex and its associated Mms21 SUMO ligase. Failure to remove these linkages causes gross chromosome missegregation in anaphase. Smc5/6 can dissolve these linkages in metaphase-arrested cells, restoring chromosome resolution.","method":"Genetic analysis, microscopy of chromosome segregation, cell cycle analysis","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional genetics with defined phenotypic rescue, single lab","pmids":["20571088"],"is_preprint":false},{"year":2009,"finding":"In fission yeast smc6 mutants following DNA damage in interphase, chromosome arm segregation fails due to aberrant persistence of cohesin normally removed by the Separase-independent pathway. This cohesin retention is independent of DNA damage per se. Separase overexpression bypasses the defect and restores cell viability.","method":"Genetic analysis, co-immunoprecipitation, cell viability assays, microscopy","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis with rescue experiment, pathway placement, single lab","pmids":["19528228"],"is_preprint":false},{"year":2011,"finding":"In budding yeast, Rtt107/Esc4 is required for recruitment of the SMC5/6 complex to DNA double-strand breaks. The interaction between Rtt107 and SMC5/6 is mediated through the N-terminal BRCT domains of Rtt107 and the Nse6 subunit of SMC5/6, and is independent of MMS-induced damage and Slx4.","method":"Co-immunoprecipitation, chromatin immunoprecipitation at DSBs, two-hybrid analysis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP mapping interaction domains, ChIP at DSBs, single lab","pmids":["21642432"],"is_preprint":false},{"year":2011,"finding":"Smc5 binds strongly and specifically to single-stranded DNA (ssDNA) independent of Smc6 as a monomer. Smc5 ATPase activity is essential for its in vivo functions, and ATP regulates the association of Smc5 with DNA substrates in vitro. Smc5 binds efficiently to oligonucleotides consistent in size with ssDNA intermediates from DNA replication and repair.","method":"In vitro DNA-binding assay with purified Smc5, ATPase assay, mutagenesis","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical characterization with purified protein, single lab","pmids":["21293191"],"is_preprint":false},{"year":2022,"finding":"The SMC5/6 complex compacts and silences unintegrated HIV-1 DNA. SLF2 recruits the SMC5/6 complex to unintegrated lentiviruses. ATAC-seq showed that Vpr-mediated SLF2 depletion increases chromatin accessibility of unintegrated virus, demonstrating that SMC5/6 compacts viral chromatin to silence gene expression.","method":"CRISPR screen, ATAC-seq, knockdown/depletion experiments","journal":"Cell host & microbe","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — CRISPR screen identifying mechanism, ATAC-seq providing chromatin accessibility evidence, single lab","pmids":["33811831"],"is_preprint":false},{"year":2022,"finding":"Epigenetic silencing of unintegrated HIV-1 proviruses by SMC5/6 requires SUMOylation: SMC5/6 binds to and SUMOylates unintegrated chromatinized HIV-1 DNA via NSMCE2 (SUMO E3 ligase). Inhibiting SUMOylation (by NSMCE2 point mutation or TAK-981 inhibitor) prevents silencing, enables transcription from unintegrated HIV-1 DNA, and suppresses establishment of HIV-1 latency.","method":"CRISPR/Cas9 screen, point mutagenesis, SUMOylation inhibitor, HIV-1 latency assay","journal":"Nature microbiology","confidence":"High","confidence_rationale":"Tier 2 / Strong — genome-wide CRISPR screen combined with biochemical dissection of SUMO ligase requirement, multiple orthogonal methods","pmids":["36376394"],"is_preprint":false},{"year":2022,"finding":"Transcriptional silencing by human Smc5/6 is a three-step process: (1) entrapment of episomal DNA dependent on Smc5/6 ATPase activity and Nse4a (not Nse4b); (2) recruitment to PML nuclear bodies by SLF2 (human Nse6 ortholog); (3) silencing requiring Nse2 but not its SUMO ligase activity. Cohesin and condensin fail to bind or silence episomal DNA, indicating this is unique to Smc5/6.","method":"Functional assay with mutants, reporter assay, localization by immunofluorescence, domain swap experiments","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — mechanistic dissection of three distinct steps with specific separation-of-function mutants and comparative analysis against other SMC complexes","pmids":["36097294"],"is_preprint":false},{"year":2010,"finding":"Smc5/6 localizes to APBs in ALT cells and is required for localization to ND10/PML bodies in primary human hepatocytes. In hepatocytes, Smc5/6 localizes with Nuclear Domain 10 (ND10); depletion of ND10 structural components alters the nuclear distribution of Smc6 and induces HBV gene expression.","method":"Immunofluorescence co-localization, siRNA depletion, reporter assay","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — localization by immunofluorescence with functional consequence via depletion, single lab","pmids":["28095508"],"is_preprint":false},{"year":2022,"finding":"EBV tegument protein BNRF1 targets SMC5/6 complexes for degradation via a ubiquitin-proteasome pathway dependent on calpain proteolysis and Cullin-7, to evade SMC5/6 restriction of viral replication compartments. In the absence of BNRF1, SMC5/6 associates with R-loop structures at the viral lytic origin of replication, interfering with replication compartment formation.","method":"Proteomics, CRISPR analysis, co-immunoprecipitation, viral replication assay","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomics identifying degradation pathway, CRISPR functional validation, single lab","pmids":["35263599"],"is_preprint":false},{"year":2022,"finding":"KSHV RTA protein degrades the SMC5/6 complex via the ubiquitin-proteasome system to antagonize SMC5/6-induced viral chromatin compaction. SMC5/6 binds to the KSHV genome and suppresses KSHV gene transcription by condensing viral chromatin into a repressive structure.","method":"ChIP, viral gene expression assay, protein degradation assay, chromatin compaction assay","journal":"PLoS pathogens","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP and functional assays showing direct viral genome binding and chromatin compaction, single lab","pmids":["35914008"],"is_preprint":false},{"year":2022,"finding":"The human SIMC1 protein is a novel SMC5/6 subunit containing SUMO-interacting motifs (SIMs) and an Nse5-like domain. SIMC1 uses its SIMs and Nse5-like domain to localize SMC5/6 to polyomavirus replication centers at SUMO-rich PML nuclear bodies. SIMC1's Nse5-like domain binds SLF2 to form an anti-parallel helical dimer resembling yeast Nse5/6.","method":"Proteomics, co-immunoprecipitation, structural analysis, mutagenesis, localization assay","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — proteomics-based identification with biochemical and structural validation, single lab","pmids":["36373674"],"is_preprint":false},{"year":2016,"finding":"Biallelic missense mutations in NSMCE3 (a SMC5/6 complex subunit) disrupt interactions within the SMC5/6 complex, destabilizing it. Patient cells showed chromosome rearrangements, micronuclei, sensitivity to replication stress and DNA damage, and defective homologous recombination, establishing an autosomal recessive chromosome breakage syndrome.","method":"Whole exome sequencing, protein interaction assays, cell-based HR assay, chromosome analysis","journal":"The Journal of clinical investigation","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human genetics combined with biochemical validation of complex disruption and cellular functional assays","pmids":["27427983"],"is_preprint":false},{"year":2022,"finding":"Biallelic variants in SLF2 and SMC5 cause a chromosome instability syndrome (Atelís Syndrome) characterized by segmented and dicentric chromosomes with mosaic variegated hyperploidy. Patient-derived cells show elevated replication stress partly due to reduced ability to replicate through G-quadruplex DNA structures and loss of sister chromatid cohesion.","method":"Whole exome sequencing, cell-based replication stress assays, chromosome analysis, cohesion assay","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human genetics with functional cellular validation in patient cells, single study","pmids":["36333305"],"is_preprint":false},{"year":2009,"finding":"RNAi ablation of SMC5 or MMS21 (but not SMC6) leads to premature sister chromatid separation before anaphase, activating the spindle assembly checkpoint and blocking mitotic progression. Mitotic SMC5 co-elutes with MMS21 in column fractions lacking SMC6, suggesting a mitotic SMC5-MMS21 sub-complex independent of SMC5/6.","method":"RNAi, immunoprecipitation, flow cytometry, live-cell imaging","journal":"Cell cycle (Georgetown, Tex.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RNAi with defined mitotic phenotype and biochemical evidence for sub-complex, single lab","pmids":["19502785"],"is_preprint":false},{"year":2013,"finding":"In human cells, Smc5/6 associates with chromatin during interphase but largely dissociates from chromosomes during mitotic chromosome condensation. Depletion of Smc5 and Smc6 causes aberrant mitotic chromosome phenotypes with abnormal distribution of topoisomerase IIα and condensins, and chromosome segregation errors. Smc5/6 is required for on-time progression of DNA replication and subsequent binding of topo IIα on replicated chromatids.","method":"Chromatin fractionation, immunofluorescence, live-cell imaging, premature chromosome condensation assay","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple cell biology methods with defined phenotype, single lab","pmids":["24258023"],"is_preprint":false},{"year":2021,"finding":"SMC5/6 depletion in human cells is essential for viability; inactivation is associated with spontaneous DNA damage, p53 activation, cell-cycle arrest, and senescence. Chromosome missegregation caused by SMC5/6 impairment requires impaired SMC5/6 function during the preceding S and G2 phases; SMC5/6 degradation immediately prior to mitotic entry has little impact on chromosome segregation fidelity.","method":"Auxin-inducible degron system for acute depletion, cell viability assay, live-cell imaging, flow cytometry","journal":"Cell reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — auxin-inducible rapid depletion with cell-cycle-stage-specific analysis, single lab","pmids":["32320646"],"is_preprint":false},{"year":2016,"finding":"Smc5/6 is present at telomeres throughout the cell cycle. Its association with chromosome ends depends on Nse3 subunit. Nse3 interacts physically and genetically with Rap1-binding factors Rif2 and Sir4. Reduced telomere-associated Smc5/6 leads to defects in telomere clustering, dispersion of Sir4, and loss of transcriptional repression of subtelomeric genes and TERRA.","method":"Chromatin immunoprecipitation, co-immunoprecipitation, fluorescence microscopy, genetic analysis","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP and co-IP with functional genetic analysis of telomere silencing, single lab","pmids":["27564449"],"is_preprint":false},{"year":2018,"finding":"PJA1 (a RING-H2 E3 ubiquitin ligase) interacts with the SMC5/6 complex and facilitates binding of the complex to viral and episomal DNAs in the cell nucleus. Treatment with topoisomerase inhibitors and topoisomerase knockdown relieve PJA1-mediated silencing of viral and extrachromosomal DNAs.","method":"Co-immunoprecipitation, reporter assay, RNAi, inhibitor treatment","journal":"Journal of virology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP identifying interaction, functional consequence via inhibitors, single lab","pmids":["30185588"],"is_preprint":false},{"year":2016,"finding":"NSMCE2 (NSE2/Mms21) depletion in human cells (U2OS) increases sensitivity to etoposide (topoisomerase II inhibitor) but not to ionizing radiation. Immunoprecipitation and mass spectrometry showed the SMC5/6 complex physically interacts with DNA Topoisomerase IIα (TOP2A), suggesting SMC5/6 resolves TOP2A-mediated DSB-repair intermediates during replication.","method":"CRISPR-Cas9 knockout, immunoprecipitation, mass spectrometry","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single Co-IP/MS identifying interaction, KO with single functional readout, single lab","pmids":["27792189"],"is_preprint":false},{"year":2021,"finding":"SMC5/6 depletion in neural progenitor cells (NPCs) leads to DNA replication stress at late-replicating regions such as pericentromeric heterochromatin, causing chromosome missegregation, DNA bridges, and NPC apoptosis. Concomitant deletion of Trp53 or Chek2 rescues Smc5 cKO neurodevelopmental defects, placing SMC5/6 in a CHEK2-p53 DNA damage response pathway.","method":"Conditional knockout, epistasis with Trp53 and Chek2, DNA fiber assay, immunofluorescence","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — conditional KO with genetic epistasis placing SMC5/6 in defined pathway, replication stress characterization, single lab","pmids":["33200984"],"is_preprint":false},{"year":2021,"finding":"Single-molecule tracking in live fission yeast showed Smc5/6 is chromatin-associated in unchallenged cells. This constitutive chromatin association depends on Nse6 through at least two sub-pathways, one requiring the BRCT-domain protein Brc1. The Nse3 double-stranded DNA binding activity and the arginine fingers of both Smc5/6 ATPase sites are critical for chromatin association. Disrupting the ssDNA binding activity at the hinge region (without preventing chromatin association) leads to elevated gross chromosomal rearrangements during replication restart.","method":"Single-molecule live-cell tracking, genetic analysis with defined mutants","journal":"eLife","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — single-molecule tracking in live cells combined with defined separation-of-function mutants, single lab","pmids":["33860765"],"is_preprint":false},{"year":2003,"finding":"Fission yeast Smc5 was purified and Nse1 and Nse2 were identified by mass spectrometry as co-precipitating non-SMC subunits that interact with Smc5 in vivo as part of the Smc5-Smc6 complex. Both are essential proteins with DNA damage repair functions epistatic to Rhp51, placing them in the homologous recombination pathway.","method":"Protein purification, mass spectrometry, co-immunoprecipitation, genetic epistasis","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical purification and MS identification with genetic epistasis, foundational complex characterization","pmids":["12966087"],"is_preprint":false},{"year":2018,"finding":"In budding yeast, acute Smc5/6 depletion reveals a primary defect in replication of the rDNA array. This is linked to programmed replication fork barriers (RFBs) established by Fob1; Fob1 deletion improves rDNA replication in Smc5/6-depleted cells. Deletion of Mph1 helicase similarly reduces recombination structures at RFB regions, confirming Smc5/6 restrains Mph1-dependent recombination at protein-barrier-stalled forks.","method":"Auxin-inducible degron depletion, 2D gel electrophoresis, genetic epistasis","journal":"PLoS genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — acute depletion system with 2D gel and genetic epistasis, single lab","pmids":["29360860"],"is_preprint":false}],"current_model":"SMC5 is a core ATPase subunit of the conserved six-to-eight-subunit Smc5/6 complex that acts as an ATP-dependent DNA loop-extruding motor and compaction machine, preferentially recognizing positively supercoiled and catenated DNA templates; it contains a hinge domain that mediates heterodimerization with SMC6 and preferentially binds ssDNA, an arm region that anchors the Nse2/Mms21 SUMO E3 ligase (whose activity is stimulated by direct DNA binding to an ARM domain on Smc5), and head ATPase domains bridged by the kleisin Nse4; the complex executes sister-chromatid homologous recombination repair (including at DSBs where it recruits cohesin), resolves Holliday junctions and aberrant recombination intermediates through Mms21-dependent SUMOylation of substrates including TRF1/TRF2, Sgs1-Top3-Rmi1, and kinetochore proteins, maintains rDNA and telomere stability, is essential for viability in an interphase-specific manner by ensuring completion of DNA replication before mitosis, and functions as an intrinsic antiviral restriction factor that topologically entraps and transcriptionally silences extrachromosomal/episomal DNA through a SUMOylation-dependent chromatin-compaction mechanism that is counteracted by HBx-mediated CRL4 ubiquitin ligase degradation and analogous viral protein strategies."},"narrative":{"mechanistic_narrative":"SMC5 is the core ATPase subunit of the conserved Smc5/6 complex, an ATP-dependent DNA motor that compacts and topologically constrains DNA to safeguard genome integrity during replication and recombination [PMID:33301732, PMID:16892052]. SMC5 heterodimerizes with SMC6 through a distinctive toroidal hinge whose 'latch' and 'hub' interfaces preferentially bind single-stranded DNA, an activity required for viability and DNA damage resistance [PMID:28134253], while the Smc5 monomer itself binds ssDNA and uses ATP to regulate DNA association [PMID:21293191]. Within the assembled holocomplex the kleisin Nse4 bridges the Smc5 and Smc6 head ATPase domains and, together with Nse1/Nse3, forms a positively charged DNA clamp that captures dsDNA non-specifically [PMID:35648833, PMID:17005570]; the Nse1/Nse3/Nse4 module contributes its own dsDNA-binding surface needed for chromatin association in vivo [PMID:26446992]. ATP hydrolysis by the holocomplex is DNA-stimulated and is held in check by the Nse5/6 subcomplex, which anchors the ATPase heads in a non-productive state and inhibits the dimerization required to initiate DNA-loop extrusion [PMID:34191293, PMID:33849072]. Functionally, Smc5/6 is a symmetric DNA loop-extruding and compaction machine that preferentially recognizes positively supercoiled and catenated DNA, locking plectonemes and linking supercoiled chromosomal regions in cis [PMID:37076626, PMID:38295804, PMID:33301732]. The complex anchors the Nse2/Mms21 SUMO E3 ligase on the Smc5 arm; Nse2 activity is directly stimulated by DNA binding to a positively charged ARM patch on Smc5 that acts as a DNA sensor [PMID:19748359, PMID:29769404]. Through this SUMOylation output and its DNA-compaction activity, Smc5/6 is recruited to DNA double-strand breaks where it promotes error-free sister-chromatid homologous recombination and cohesin loading, resolves recombination intermediates via SUMOylation of the Sgs1-Top3-Rmi1 dissolution complex, and maintains rDNA and telomere stability [PMID:16810316, PMID:16892052, PMID:27373152, PMID:27298337, PMID:17589526]. Its essential function is interphase-specific, ensuring completion of DNA replication through difficult late-replicating and barrier-stalled regions before mitosis [PMID:26698660, PMID:32320646]. Beyond chromosome maintenance, Smc5/6 acts as an intrinsic antiviral restriction factor that topologically entraps and transcriptionally silences extrachromosomal/episomal and unintegrated viral DNA through an ATPase- and SUMOylation-dependent chromatin-compaction mechanism, which diverse viral proteins (HBx, BNRF1, KSHV RTA) counteract by hijacking ubiquitin ligases to degrade the complex [PMID:26983541, PMID:27626656, PMID:33811831, PMID:36376394, PMID:36097294, PMID:35914008]. Biallelic mutations in SMC5 and its complex partners cause autosomal recessive chromosome-instability syndromes characterized by chromosome breakage, replication stress, and defective homologous recombination [PMID:27427983, PMID:36333305].","teleology":[{"year":2003,"claim":"Establishing that Smc5 is the catalytic centerpiece of a defined multi-subunit complex was the founding step, identifying its non-SMC partners and placing them in the homologous recombination pathway.","evidence":"Protein purification with MS identification of Nse1/Nse2 and genetic epistasis in fission yeast","pmids":["12966087"],"confidence":"Medium","gaps":["Did not define subunit architecture or DNA-binding mechanism","Catalytic activities of the complex unresolved"]},{"year":2005,"claim":"Defining the Smc5-Smc6 hinge heterodimerization interface and the embedded Nse2 SUMO ligase activity converted a genetic complex into a biochemical entity with an enzymatic output for the DNA damage response.","evidence":"Protein purification, hinge mutagenesis, and in vitro/in vivo SUMOylation assays in yeast","pmids":["15601840","15601841"],"confidence":"High","gaps":["SUMO substrates relevant in vivo not fully enumerated","Structural basis of ligase specificity unresolved"]},{"year":2006,"claim":"Localization and recruitment studies established the complex acts directly at DNA double-strand breaks to drive error-free sister-chromatid recombination, distinguishing its role from other repair pathways.","evidence":"ChIP showing de novo recruitment to DSBs, HR subpathway assays, and 2D gels in human and yeast cells","pmids":["16810316","16892052","16793545"],"confidence":"High","gaps":["Molecular signal for recruitment not defined","How the complex enforces sister-chromatid template choice unknown"]},{"year":2006,"claim":"Mapping the kleisin Nse4 and the Nse subcomplexes to the Smc5/Smc6 heads defined the modular architecture that bridges the ATPase domains.","evidence":"Y2H, co-IP, and in vitro binding assays in yeast","pmids":["17005570","19141609"],"confidence":"Medium","gaps":["Single-lab interaction mapping without structural confirmation at the time","Functional consequences of each bridging contact not dissected"]},{"year":2009,"claim":"A crystal structure of the Mms21/Nse2-Smc5 arm interface revealed a novel RING E3 architecture and showed the interface itself is required for growth and damage resistance, anchoring the SUMO ligase on the SMC5 scaffold.","evidence":"X-ray crystallography with mutagenesis and in vitro SUMO ligase assays","pmids":["19748359"],"confidence":"High","gaps":["How DNA context regulates the ligase not yet defined","Full substrate spectrum unknown"]},{"year":2010,"claim":"Linking Smc5/6/Mms21 to active dissolution of DNA-mediated sister-chromatid linkages before mitosis explained why complex loss causes anaphase missegregation.","evidence":"Conditional genetics with chromosome-segregation microscopy and metaphase rescue in yeast","pmids":["20571088"],"confidence":"Medium","gaps":["Biochemical nature of dissolved linkages not fully defined","Direct enzymatic step on the linkage not shown"]},{"year":2015,"claim":"Reconstitution showed the purified complex binds and entraps DNA in an ATP-dependent manner, recognizing single-stranded and supercoiled/catenated substrates, providing the first direct biochemical activity for SMC5.","evidence":"In vitro DNA binding/catenation assays, ATPase mutants, and purified Smc5 ssDNA binding","pmids":["26299966","21293191","26446992"],"confidence":"Medium","gaps":["Loop-extrusion behavior not yet demonstrated","Coupling of ATP cycle to topology change not resolved"]},{"year":2017,"claim":"The toroidal Smc5/6 hinge with its unique latch and hub interfaces was shown to bind ssDNA preferentially, linking a structural feature to a DNA-substrate preference essential for viability.","evidence":"Crystal structure, ssDNA-binding assays, and mutagenesis with viability readouts in two organisms","pmids":["28134253"],"confidence":"High","gaps":["How hinge ssDNA binding integrates with head ATPase cycle unclear","In vivo ssDNA intermediates engaged not directly visualized"]},{"year":2018,"claim":"Identifying the Smc5 ARM domain as a DNA sensor that electrostatically stimulates Nse2 ligase activity coupled the complex's DNA engagement directly to its SUMOylation output.","evidence":"In vitro SUMO ligase and DNA-binding assays with ARM-patch mutants and in vivo damage sensitivity","pmids":["29769404"],"confidence":"High","gaps":["Which substrates are selected upon ARM-DNA activation in vivo not enumerated","Structural snapshot of the activated state lacking"]},{"year":2020,"claim":"Holocomplex reconstitution and a DNA-bound cryo-EM structure unified the complex's activities, demonstrating DNA-dependent ATPase, SUMO ligase, and ATP-driven plectoneme-locking/compaction with a defined DNA clamp.","evidence":"In vitro ATPase/SUMO/single-molecule assays, cryo-EM at 3.8 Å, and cross-linking MS in yeast and human systems","pmids":["33301732","33301731","35648833"],"confidence":"High","gaps":["Mechanistic transition from clamping to loop extrusion not captured","Human-specific regulatory differences incompletely defined"]},{"year":2021,"claim":"Demonstrating that Nse5/6 inhibits the ATPase by anchoring the heads in a non-productive state established the principal mechanism that gates complex activity, relieved by closed/long DNA.","evidence":"Crystal structure of Nse5/6, ATPase assays, cysteine and cross-linking MS, and integrative cryo-EM modeling","pmids":["34191293","33849072","33941673"],"confidence":"High","gaps":["Cellular signals that relieve Nse5/6 inhibition not defined","How regulation differs across recruitment pathways unclear"]},{"year":2023,"claim":"Single-molecule imaging resolved the core mechanism: Smc5/6 dimers extrude DNA loops symmetrically and monomers translocate, with Nse5/6 blocking dimerization to control loop-extrusion initiation.","evidence":"Single-molecule imaging of reconstituted Smc5/6, with monomer/dimer and Nse5/6 dissection","pmids":["37076626"],"confidence":"High","gaps":["In vivo relevance of loop extrusion to repair not directly tested","Connection to SUMOylation output during extrusion unknown"]},{"year":2024,"claim":"Coupling loop extrusion to substrate preference, Smc5/6 was shown to recognize tips of positively supercoiled plectonemes at transcription-induced cohesin-loop boundaries and gather them in cis.","evidence":"Single-molecule imaging, ChIP-seq, and Hi-C","pmids":["38295804"],"confidence":"High","gaps":["Functional outcome of supercoil gathering for repair/replication not fully resolved","Interplay with cohesin at boundaries mechanistically open"]},{"year":2022,"claim":"Recognition that the chromatin-compaction and SUMOylation machinery is repurposed as an antiviral restriction factor extended SMC5/6 biology, with SLF2/SIMC1-dependent recruitment to viral DNA at PML bodies and SUMO-dependent silencing.","evidence":"CRISPR screens, ATAC-seq, NSMCE2 SUMO-mutant and inhibitor latency assays, ChIP, and structural/IP analyses of SLF2/SIMC1 in HIV-1, KSHV, EBV and polyomavirus systems","pmids":["33811831","36376394","36097294","35914008","35263599","36373674"],"confidence":"High","gaps":["Why episomal/viral DNA is selectively recognized versus host chromatin not fully resolved","Relative contributions of entrapment vs SUMOylation across viruses differ and are unsettled"]},{"year":2022,"claim":"Human genetics confirmed the physiological essentiality of SMC5 and its partners by linking biallelic mutations to chromosome-instability syndromes with replication stress and HR defects.","evidence":"Whole exome sequencing with patient-cell HR, cohesion, and replication-stress assays for SMC5/SLF2 and NSMCE3","pmids":["36333305","27427983"],"confidence":"Medium","gaps":["Genotype–phenotype relationships across the complex incompletely defined","Tissue-specific developmental requirements not fully explained"]},{"year":null,"claim":"How the ATP-driven loop-extrusion/compaction cycle is mechanistically coupled to Nse2/Mms21 SUMOylation choices and to substrate-specific outcomes (sister-chromatid HR versus viral silencing versus replication completion) remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural snapshot links an extruding state to active SUMO transfer","Determinants directing the complex to distinct cellular versus viral substrates undefined","How recruitment pathways tune the Nse5/6 inhibitory switch in vivo unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140657","term_label":"ATP-dependent activity","supporting_discovery_ids":[0,11,12,15,32]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[2,24,28,32]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[6,7,17]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[6,7,17]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[11,12]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[2,8]}],"localization":[{"term_id":"GO:0005694","term_label":"chromosome","supporting_discovery_ids":[19,26,49,43]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[4,36,43]},{"term_id":"GO:0005730","term_label":"nucleolus","supporting_discovery_ids":[18,20,51]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[5,19,22,40]},{"term_id":"R-HSA-69306","term_label":"DNA Replication","supporting_discovery_ids":[25,44,48,51]},{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[29,42,43,44]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[3,33,34,35]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[4,7,17,22]}],"complexes":["Smc5/6 complex"],"partners":["SMC6","NSE4","NSE3","NSE2/MMS21","SLF2","SIMC1","MPH1","TOP2A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q8IY18","full_name":"Structural maintenance of chromosomes protein 5","aliases":[],"length_aa":1101,"mass_kda":128.8,"function":"Core component of the SMC5-SMC6 complex, a complex involved in repair of DNA double-strand breaks by homologous recombination. The complex may promote sister chromatid homologous recombination by recruiting the SMC1-SMC3 cohesin complex to double-strand breaks. The complex is required for telomere maintenance via recombination in ALT (alternative lengthening of telomeres) cell lines and mediates sumoylation of shelterin complex (telosome) components which is proposed to lead to shelterin complex disassembly in ALT-associated PML bodies (APBs). Required for recruitment of telomeres to PML nuclear bodies. Required for sister chromatid cohesion during prometaphase and mitotic progression; the function seems to be independent of SMC6. SMC5-SMC6 complex may prevent transcription of episomal DNA, such as circular viral DNA genome (PubMed:26983541)","subcellular_location":"Nucleus; Chromosome; Nucleus, PML body; Chromosome, telomere","url":"https://www.uniprot.org/uniprotkb/Q8IY18/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/SMC5","classification":"Common Essential","n_dependent_lines":944,"n_total_lines":1208,"dependency_fraction":0.7814569536423841},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/SMC5","total_profiled":1310},"omim":[{"mim_id":"620185","title":"ATELIS SYNDROME 2; ATELS2","url":"https://www.omim.org/entry/620185"},{"mim_id":"620184","title":"ATELIS SYNDROME 1; ATELS1","url":"https://www.omim.org/entry/620184"},{"mim_id":"618467","title":"SMC5-SMC6 COMPLEX LOCALIZATION FACTOR 1; SLF1","url":"https://www.omim.org/entry/618467"},{"mim_id":"617263","title":"NSE1 HOMOLOG, SMC5-SMC6 COMPLEX COMPONENT; NSMCE1","url":"https://www.omim.org/entry/617263"},{"mim_id":"617246","title":"NSE2 (MMS21) HOMOLOG, SMC5-SMC6 COMPLEX SUMO LIGASE; NSMCE2","url":"https://www.omim.org/entry/617246"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nuclear speckles","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/SMC5"},"hgnc":{"alias_symbol":["KIAA0594"],"prev_symbol":["SMC5L1"]},"alphafold":{"accession":"Q8IY18","domains":[{"cath_id":"3.40.50.300","chopping":"53-189_1016-1098","consensus_level":"high","plddt":85.1641,"start":53,"end":1098},{"cath_id":"-","chopping":"491-643","consensus_level":"high","plddt":87.3612,"start":491,"end":643}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IY18","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IY18-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q8IY18-F1-predicted_aligned_error_v6.png","plddt_mean":79.44},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SMC5","jax_strain_url":"https://www.jax.org/strain/search?query=SMC5"},"sequence":{"accession":"Q8IY18","fasta_url":"https://rest.uniprot.org/uniprotkb/Q8IY18.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q8IY18/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q8IY18"}},"corpus_meta":[{"pmid":"26983541","id":"PMC_26983541","title":"Hepatitis 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Monomeric Smc5/6 unidirectionally translocates along DNA, while dimeric Smc5/6 extrudes loops. The Nse5/6 subcomplex acts as a negative regulator of loop extrusion by inhibiting Smc5/6 dimerization, thereby preventing loop-extrusion initiation but not ongoing loop extrusion.\",\n      \"method\": \"Single-molecule imaging, in vitro reconstitution\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — single-molecule reconstitution with multiple orthogonal conditions, mechanistic dissection of monomer vs. dimer behavior and Nse5/6 regulatory role\",\n      \"pmids\": [\"37076626\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Smc5/6 associates with transcription-induced positively supercoiled DNA at cohesin-dependent loop boundaries. Single-molecule imaging showed that Smc5/6 dimers specifically recognize the tips of positively supercoiled DNA plectonemes and initiate loop extrusion to gather supercoiled DNA into large plectonemic loops. Hi-C analysis showed that Smc5/6 links chromosomal regions containing transcription-induced positive supercoiling in cis.\",\n      \"method\": \"Single-molecule imaging, ChIP-seq, Hi-C\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — single-molecule reconstitution combined with genome-wide chromatin association and Hi-C, multiple orthogonal methods\",\n      \"pmids\": [\"38295804\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structure (3.8 Å) of DNA-bound yeast Smc5/6 revealed a DNA clamp formed by Smc5, Smc6, Nse1, Nse3, and Nse4. The positively charged inner surface of the clamp contacts dsDNA in a nonsequence-specific manner. The Nse3 subunit secures DNA from above; the hook-shaped Nse4 kleisin forms a scaffold connecting DNA to all other subunits. Cross-linking MS identified multi-subunit conformational changes from DNA-free to DNA-bound state enabling DNA capture.\",\n      \"method\": \"Cryo-EM structure, cross-linking mass spectrometry, mutagenesis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — near-atomic cryo-EM structure with cross-linking MS and in-cell mutagenesis validation\",\n      \"pmids\": [\"35648833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"HBx hijacks the cellular DDB1-CUL4 E3 ubiquitin ligase to target the Smc5/6 complex for ubiquitin-mediated proteasomal degradation, relieving Smc5/6-mediated transcriptional repression of extrachromosomal (cccDNA) templates. Smc5/6 associates directly with extrachromosomal DNA and the HBV genome. Silencing Smc5/6 enhances extrachromosomal reporter transcription and rescues HBx-deficient HBV replication.\",\n      \"method\": \"RNAi knockdown, reporter assays, Co-immunoprecipitation, substrate-trapping proteomics\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal functional experiments in multiple systems (reporter, virus), independently replicated in companion paper (PMID:27626656)\",\n      \"pmids\": [\"26983541\", \"27626656\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The SMC5/6 complex localizes to ALT-associated PML bodies (APBs) in ALT cancer cells and is required for targeting telomeres to APBs. The MMS21 SUMO ligase subunit of SMC5/6 SUMOylates multiple telomere-binding proteins, including TRF1 and TRF2. Inhibition of TRF1 or TRF2 SUMOylation prevents APB formation. Depletion of SMC5/6 subunits by RNAi inhibits telomere HR, causing telomere shortening and senescence in ALT cells.\",\n      \"method\": \"RNAi, co-immunoprecipitation, in vitro SUMOylation assay, immunofluorescence\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (RNAi depletion with cellular phenotype, in vitro SUMOylation, localization), clear mechanistic pathway placement\",\n      \"pmids\": [\"17589526\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Human SMC5/6 complex is recruited to nuclease-induced DNA double-strand breaks and is required for recruitment of cohesin (SMC1/3) to DSBs. RNAi knockdown of SMC5/6 specifically decreases sister chromatid HR without affecting NHEJ, intra-chromatid HR, or extrachromosomal HR.\",\n      \"method\": \"RNAi, chromatin immunoprecipitation, gene targeting assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — ChIP showing direct recruitment, epistasis/pathway placement via multiple HR subpathway assays, clean loss-of-function\",\n      \"pmids\": [\"16810316\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Crystal structure of Mms21 (NSE2/Nse2) in complex with the Smc5 arm revealed two functional domains: an N-terminal half forming a helix bundle with a coiled-coil region of Smc5 (required for Smc5 binding), and a C-terminal SUMO ligase domain with a new type of RING E3 structure that confers specificity to SUMO E2-E3 interaction. The Mms21-Smc5 interface is required for cell growth and DNA damage resistance.\",\n      \"method\": \"X-ray crystallography, mutagenesis, in vitro SUMO ligase assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with mutagenesis and functional validation in a single rigorous study\",\n      \"pmids\": [\"19748359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Nse2 (fission yeast SMC5/6 subunit) is a SUMO ligase: it sumoylates Smc6 and Nse3 in vitro in an Nse2-dependent manner, and autosumoylates itself. Mutations in the RING-finger-like motif (C195S/H197A) abolish Nse2-dependent sumoylation. Smc6 is sumoylated in vivo, with increased levels upon DNA damage, which is drastically reduced in nse2.SA cells. nse2.SA cells are sensitive to DNA-damaging agents, implicating Nse2 SUMO ligase activity in DNA damage responses.\",\n      \"method\": \"In vitro sumoylation assay, mutagenesis, in vivo sumoylation analysis, DNA damage sensitivity assays\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical reconstitution of SUMO ligase activity with mutagenesis, complemented by in vivo validation\",\n      \"pmids\": [\"15601841\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Nse4 is the kleisin component of the Smc5/6 complex, bridging the heads of Smc5 and Smc6. The C-terminal part of Nse4 interacts with the head domain of Smc5; specific mutations in a predicted winged helix motif of Nse4 destroy this interaction. Nse3, Nse5, and Nse6 also bridge the Smc5 and Smc6 heads at different sites. The Nse1-Nse3-Nse4 and Nse5-Nse6 subcomplexes bind to the Smc5-Smc6 heads domain at different sites.\",\n      \"method\": \"Co-immunoprecipitation, yeast two-hybrid, in vitro binding assays, structural predictions\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple protein interaction methods (Y2H, co-IP, in vitro binding) in single lab defining subunit architecture\",\n      \"pmids\": [\"17005570\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Human and S. pombe Smc5 and Smc6 interact through their hinge domains. Temperature-sensitive mutations in Rad18 (Smc6) all map to the same conserved glycine in the hinge region, and this mutation abolishes hinge-hinge interaction between Smc6 and Smc5. Two subcomplexes were identified: Rad18-Spr18-Nse2 and Nse1-Nse3-Rad62.\",\n      \"method\": \"Protein purification, co-immunoprecipitation, mutagenesis, mass spectrometry\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical purification with mutagenesis defining hinge interaction, single lab\",\n      \"pmids\": [\"15601840\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The architecture of the budding yeast Smc5/6 complex shows Smc5 and Smc6 associating at their hinge regions; Nse1-Nse3-Nse4 form a subcomplex binding the Smc5 head and adjacent coiled-coil; Nse2 binds the middle of the Smc5 coiled-coil; Nse5-Nse6 form a heterodimer that binds the hinge regions of both Smc5 and Smc6.\",\n      \"method\": \"Yeast two-hybrid, in vitro binding assays with purified recombinant proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Y2H and in vitro binding with purified proteins, single lab, two orthogonal methods\",\n      \"pmids\": [\"19141609\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Purified Smc5/6 binds DNA through a mechanism requiring ATP hydrolysis and circular DNA. This promotes topoisomerase 2-dependent catenation of plasmids, suggesting Smc5/6 interconnects two DNA molecules via ATP-regulated topological entrapment. An Smc6 mutant defective in ATP binding fails to interact with DNA and chromosomes and causes cell death with DNA damage accumulation when overexpressed.\",\n      \"method\": \"In vitro DNA binding assay with purified complex, plasmid catenation assay, ATPase mutant analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical reconstitution with mutational analysis, single lab\",\n      \"pmids\": [\"26299966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Purified budding yeast Smc5/6 holocomplex exhibits DNA-dependent ATP hydrolysis, SUMO E3 ligase activity, and topological DNA binding with affinity for supercoiled and catenated DNA. Single-molecule assays showed that Smc5/6 locks DNA plectonemes and compacts DNA in an ATP-dependent manner.\",\n      \"method\": \"In vitro ATPase assay, SUMO E3 ligase assay, DNA binding assay, single-molecule imaging\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple in vitro biochemical assays on purified holocomplex, combined with single-molecule studies\",\n      \"pmids\": [\"33301732\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"The human Smc5/6 complex recognizes unusual DNA configurations (non-B DNA) and uses ATP hydrolysis to compact DNA. Structural analyses revealed subunit interfaces responsible for functionality; mutations in these interfaces lead to chromosome breakage syndromes in humans.\",\n      \"method\": \"In vitro DNA compaction assay, structural analysis, mutagenesis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with structural analysis, single lab\",\n      \"pmids\": [\"33301731\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The Nse5/6 sub-complex strongly inhibits Smc5/6 ATPase by preventing productive ATP binding. This inhibition is relieved by plasmid DNA but not by short linear DNA. Two binding sites for Nse5/6 on Smc5/6 were identified: one at the Smc5/6 arms and one at the heads (exerting inhibitory effects). Nse4/3/1 module detaches from the ATPase domains under ATP and DNA conditions. Cysteine cross-linking demonstrated Nse5/6 anchors the ATPase domains in a non-productive state destabilized by ATP and DNA.\",\n      \"method\": \"In vitro ATPase assay, crystal structure of Nse5/6, cross-linking mass spectrometry, cysteine cross-linking\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure combined with in vitro biochemistry and cross-linking MS, multiple orthogonal methods in single study\",\n      \"pmids\": [\"34191293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Nse5/6 acts as a negative regulator of Smc5/6 ATPase activity by binding to the head-end of the complex to suppress ATP turnover. Only the six-protein holo-complex is capable of hydrolyzing ATP; ATPase activity is significantly increased by double-stranded DNA. ATPase stimulation requires functional ATP-binding pockets in both Smc5 and Smc6.\",\n      \"method\": \"In vitro ATPase assay with reconstituted Smc5/6 complexes, negative-stain electron microscopy\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro biochemical reconstitution with defined sub-complexes, corroborated by independent paper (PMID:34191293)\",\n      \"pmids\": [\"33849072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Integrative structural study of yeast Smc5/6 by cryo-EM, cross-linking MS, and computational modeling revealed that Smc5 and Smc6 arms do not fold back (unlike cohesin/condensin arms). Instead, the long filamentous arm regions interact with Nse2 (SUMO ligase) and Nse5/Nse6 subcomplex. A 3.0-Å cryo-EM structure of Nse5/Nse6 revealed a clasped-hand topology and dimeric interface. Nse5/Nse6 uses SUMO-binding motifs to contribute to Nse2-mediated sumoylation.\",\n      \"method\": \"Cryo-EM, cross-linking mass spectrometry, computational modeling\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — near-atomic cryo-EM structure combined with cross-linking MS and functional validation\",\n      \"pmids\": [\"33941673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The Nse2/Mms21 SUMO E3 ligase activity within the Smc5/6 complex is directly stimulated by DNA binding. Stimulation requires the electrostatic interaction between DNA and a positively charged patch in the ARM domain of Smc5, which acts as a DNA sensor and promotes activation of Nse2 E3 activity. Disruption of the ARM-DNA interaction sensitizes cells to DNA damage.\",\n      \"method\": \"In vitro SUMO ligase assay, mutagenesis, DNA-binding assay\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution of SUMO ligase activation by DNA with mutagenesis and in vivo functional validation, single lab\",\n      \"pmids\": [\"29769404\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The Smc5-Smc6 complex and SUMO modification of Rad52 regulate recombinational repair at the rDNA locus. Recombinational repair of a DSB in rDNA involves transient relocalization of the lesion to an extranucleolar site. Nucleolar exclusion of Rad52 recombination foci requires Mre11 and Smc5-Smc6, and depends on Rad52 SUMO modification. Mutations abrogating these activities cause rDNA hyperrecombination and excision of extrachromosomal rDNA circles.\",\n      \"method\": \"Fluorescence microscopy of repair foci, genetic epistasis, sumoylation assays\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (live imaging, genetics, sumoylation), replicated aspects across labs\",\n      \"pmids\": [\"17643116\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The Smc5-Smc6 complex is recruited de novo to DNA DSBs and promotes repair by homologous recombination between sister chromatids. Loss of Smc5-Smc6 causes accumulation of gross chromosomal rearrangements and failure of error-free sister-chromatid recombination.\",\n      \"method\": \"Chromatin immunoprecipitation, genetic analysis, 2D gel electrophoresis\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct ChIP showing recruitment to DSBs, multiple independent labs confirming the sister chromatid HR function\",\n      \"pmids\": [\"16892052\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"SMC5 and SMC6 are required for segregation of repetitive chromosomal regions. In conditional mutants, chromosome segregation of repetitive regions (rDNA, telomeres) is impaired, leading to X-shaped DNA (Holliday junctions) accumulation at rDNA loci. RAD52 deletion partially suppresses temperature sensitivity, indicating the complex prevents formation of sister chromatid junctions at repetitive loci.\",\n      \"method\": \"Conditional genetics, 2D gel electrophoresis, chromatin immunoprecipitation, genetic epistasis\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple methods including 2D gel and ChIP, replicated by several groups\",\n      \"pmids\": [\"15793567\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The Smc5/6 complex directly binds to the DNA helicase Mph1 and modulates Mph1-dependent recombination. Deletion of MPH1 or its helicase mutations suppress multiple defects in Smc5/6 mutants, while MPH1 overexpression exacerbates them. Mph1 and its helicase activity are largely responsible for accumulation of deleterious recombination intermediates in Smc5/6 mutants, defining a function of Smc5/6 distinct from Sgs1.\",\n      \"method\": \"Co-immunoprecipitation (direct binding), genetic epistasis, 2D gel analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding demonstrated by Co-IP, epistasis analysis, single lab\",\n      \"pmids\": [\"19995966\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Smc5/6 mediates SUMOylation of the Sgs1-Top3-Rmi1 (STR) complex. Sgs1 binds poly-SUMO chains and associates with the Smc5/6 SUMO E3 complex. Smc5/6-dependent sumoylation of Sgs1, Top3, and Rmi1 upon generation of recombination structures promotes STR inter-subunit interactions and accumulation at DNA repair centers, facilitating removal of recombination intermediates.\",\n      \"method\": \"Co-immunoprecipitation, in vivo SUMO modification assay, 2D gel electrophoresis, protein interaction assays\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple biochemical and genetic approaches in single lab, two orthogonal methods\",\n      \"pmids\": [\"27373152\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Smc5/6 recruits and activates Sgs1 through a two-step SUMOylation mechanism: (1) auto-SUMOylation of Smc5/6 subunits leads to recruitment of the STR complex via two SUMO-interacting motifs (SIMs) on Sgs1 that recognize SUMOylated Smc5/6; (2) Smc5/6-dependent SUMOylation of Sgs1 and Top3 is required for efficient STR function. Sgs1 SIM mutants show unprocessed HJs, increased crossovers, and impaired DNA end resection.\",\n      \"method\": \"Mutagenesis of SIMs, in vivo sumoylation assays, 2D gel electrophoresis, recombination assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic and biochemical dissection in single lab, multiple mutant analyses\",\n      \"pmids\": [\"27298337\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"The Smc5/6 hinge forms a toroidal structure with distinctive interfaces ('molecular latch' and 'hub') absent from other SMC complexes. The Smc5/6 hinge binds preferentially to ssDNA; both latch and hub mutations reduce ssDNA binding and cause severe DNA damage sensitivity and reduced human cell viability.\",\n      \"method\": \"Crystal structure, mutagenesis, DNA-binding assay (ssDNA preference), cell viability assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — crystal structure with in vitro ssDNA binding characterization and mutational functional validation in two organisms\",\n      \"pmids\": [\"28134253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The essential functions of Smc5/6 segregate to G2/M phase. Smc5/6 is specifically required in G2/M for: (1) metabolism of DNA recombination structures triggered by endogenous replication stress, cooperating with dissolution activities; (2) replication through natural pausing sites in late-replicating regions via cooperation with the fork protection complex Tof1-Csm3.\",\n      \"method\": \"Cell-cycle-regulated alleles, genetic screens, 2D gel electrophoresis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — cell-cycle-restricted allele system with genetic screens, single lab\",\n      \"pmids\": [\"26698660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Chromosomal localization of budding yeast Smc5/6 complex occurs in three distinct, differentially regulated pathways: (1) association with centromeres and chromosome arms in unchallenged cells; (2) association with DNA breaks; (3) association with rDNA arrays. Localization to collapsed replication forks was also detected. Smc5/6 is needed during replication to prevent accumulation of branched chromosome structures.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP), 2D gel electrophoresis\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genome-wide ChIP localization study with functional genetic context, single lab\",\n      \"pmids\": [\"16793545\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Localization of fission yeast Smc5/6 to centromeres during replication requires H3-K9 methylation (heterochromatin), while MMS-induced localization to subtelomeres requires Nse2 SUMO ligase activity but not H3-K9 methylation. Smc5/6 also loads at all genomic tDNAs in a manner requiring intact TFIIIC-binding sites.\",\n      \"method\": \"ChIP-on-chip, fluorescence microscopy, mutant analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP-on-chip and microscopy with genetic dissection of localization determinants, single lab\",\n      \"pmids\": [\"18923417\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"The NSE1/NSE3/NSE4 sub-complex of SMC5/6 binds double-stranded DNA without sequence or structure preference. Mutations in key basic residues on the DNA-binding surface reduce DNA binding in vitro; introduction of these mutations in S. pombe leads to cell death or hypersensitivity to DNA damage. ChIP of the hypomorphic nse3 DNA-binding mutant shows reduced SMC5/6 chromatin association in vivo.\",\n      \"method\": \"In vitro DNA-binding assay, mutagenesis, chromatin immunoprecipitation\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro biochemical assay with mutagenesis and in vivo ChIP validation, single lab\",\n      \"pmids\": [\"26446992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Dissolution of DNA-mediated sister chromatid linkages before mitosis is an active process requiring the Smc5/6 complex and its associated Mms21 SUMO ligase. Failure to remove these linkages causes gross chromosome missegregation in anaphase. Smc5/6 can dissolve these linkages in metaphase-arrested cells, restoring chromosome resolution.\",\n      \"method\": \"Genetic analysis, microscopy of chromosome segregation, cell cycle analysis\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional genetics with defined phenotypic rescue, single lab\",\n      \"pmids\": [\"20571088\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"In fission yeast smc6 mutants following DNA damage in interphase, chromosome arm segregation fails due to aberrant persistence of cohesin normally removed by the Separase-independent pathway. This cohesin retention is independent of DNA damage per se. Separase overexpression bypasses the defect and restores cell viability.\",\n      \"method\": \"Genetic analysis, co-immunoprecipitation, cell viability assays, microscopy\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis with rescue experiment, pathway placement, single lab\",\n      \"pmids\": [\"19528228\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In budding yeast, Rtt107/Esc4 is required for recruitment of the SMC5/6 complex to DNA double-strand breaks. The interaction between Rtt107 and SMC5/6 is mediated through the N-terminal BRCT domains of Rtt107 and the Nse6 subunit of SMC5/6, and is independent of MMS-induced damage and Slx4.\",\n      \"method\": \"Co-immunoprecipitation, chromatin immunoprecipitation at DSBs, two-hybrid analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP mapping interaction domains, ChIP at DSBs, single lab\",\n      \"pmids\": [\"21642432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Smc5 binds strongly and specifically to single-stranded DNA (ssDNA) independent of Smc6 as a monomer. Smc5 ATPase activity is essential for its in vivo functions, and ATP regulates the association of Smc5 with DNA substrates in vitro. Smc5 binds efficiently to oligonucleotides consistent in size with ssDNA intermediates from DNA replication and repair.\",\n      \"method\": \"In vitro DNA-binding assay with purified Smc5, ATPase assay, mutagenesis\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical characterization with purified protein, single lab\",\n      \"pmids\": [\"21293191\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The SMC5/6 complex compacts and silences unintegrated HIV-1 DNA. SLF2 recruits the SMC5/6 complex to unintegrated lentiviruses. ATAC-seq showed that Vpr-mediated SLF2 depletion increases chromatin accessibility of unintegrated virus, demonstrating that SMC5/6 compacts viral chromatin to silence gene expression.\",\n      \"method\": \"CRISPR screen, ATAC-seq, knockdown/depletion experiments\",\n      \"journal\": \"Cell host & microbe\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — CRISPR screen identifying mechanism, ATAC-seq providing chromatin accessibility evidence, single lab\",\n      \"pmids\": [\"33811831\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Epigenetic silencing of unintegrated HIV-1 proviruses by SMC5/6 requires SUMOylation: SMC5/6 binds to and SUMOylates unintegrated chromatinized HIV-1 DNA via NSMCE2 (SUMO E3 ligase). Inhibiting SUMOylation (by NSMCE2 point mutation or TAK-981 inhibitor) prevents silencing, enables transcription from unintegrated HIV-1 DNA, and suppresses establishment of HIV-1 latency.\",\n      \"method\": \"CRISPR/Cas9 screen, point mutagenesis, SUMOylation inhibitor, HIV-1 latency assay\",\n      \"journal\": \"Nature microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — genome-wide CRISPR screen combined with biochemical dissection of SUMO ligase requirement, multiple orthogonal methods\",\n      \"pmids\": [\"36376394\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Transcriptional silencing by human Smc5/6 is a three-step process: (1) entrapment of episomal DNA dependent on Smc5/6 ATPase activity and Nse4a (not Nse4b); (2) recruitment to PML nuclear bodies by SLF2 (human Nse6 ortholog); (3) silencing requiring Nse2 but not its SUMO ligase activity. Cohesin and condensin fail to bind or silence episomal DNA, indicating this is unique to Smc5/6.\",\n      \"method\": \"Functional assay with mutants, reporter assay, localization by immunofluorescence, domain swap experiments\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — mechanistic dissection of three distinct steps with specific separation-of-function mutants and comparative analysis against other SMC complexes\",\n      \"pmids\": [\"36097294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Smc5/6 localizes to APBs in ALT cells and is required for localization to ND10/PML bodies in primary human hepatocytes. In hepatocytes, Smc5/6 localizes with Nuclear Domain 10 (ND10); depletion of ND10 structural components alters the nuclear distribution of Smc6 and induces HBV gene expression.\",\n      \"method\": \"Immunofluorescence co-localization, siRNA depletion, reporter assay\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — localization by immunofluorescence with functional consequence via depletion, single lab\",\n      \"pmids\": [\"28095508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"EBV tegument protein BNRF1 targets SMC5/6 complexes for degradation via a ubiquitin-proteasome pathway dependent on calpain proteolysis and Cullin-7, to evade SMC5/6 restriction of viral replication compartments. In the absence of BNRF1, SMC5/6 associates with R-loop structures at the viral lytic origin of replication, interfering with replication compartment formation.\",\n      \"method\": \"Proteomics, CRISPR analysis, co-immunoprecipitation, viral replication assay\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomics identifying degradation pathway, CRISPR functional validation, single lab\",\n      \"pmids\": [\"35263599\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"KSHV RTA protein degrades the SMC5/6 complex via the ubiquitin-proteasome system to antagonize SMC5/6-induced viral chromatin compaction. SMC5/6 binds to the KSHV genome and suppresses KSHV gene transcription by condensing viral chromatin into a repressive structure.\",\n      \"method\": \"ChIP, viral gene expression assay, protein degradation assay, chromatin compaction assay\",\n      \"journal\": \"PLoS pathogens\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP and functional assays showing direct viral genome binding and chromatin compaction, single lab\",\n      \"pmids\": [\"35914008\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The human SIMC1 protein is a novel SMC5/6 subunit containing SUMO-interacting motifs (SIMs) and an Nse5-like domain. SIMC1 uses its SIMs and Nse5-like domain to localize SMC5/6 to polyomavirus replication centers at SUMO-rich PML nuclear bodies. SIMC1's Nse5-like domain binds SLF2 to form an anti-parallel helical dimer resembling yeast Nse5/6.\",\n      \"method\": \"Proteomics, co-immunoprecipitation, structural analysis, mutagenesis, localization assay\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — proteomics-based identification with biochemical and structural validation, single lab\",\n      \"pmids\": [\"36373674\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Biallelic missense mutations in NSMCE3 (a SMC5/6 complex subunit) disrupt interactions within the SMC5/6 complex, destabilizing it. Patient cells showed chromosome rearrangements, micronuclei, sensitivity to replication stress and DNA damage, and defective homologous recombination, establishing an autosomal recessive chromosome breakage syndrome.\",\n      \"method\": \"Whole exome sequencing, protein interaction assays, cell-based HR assay, chromosome analysis\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human genetics combined with biochemical validation of complex disruption and cellular functional assays\",\n      \"pmids\": [\"27427983\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Biallelic variants in SLF2 and SMC5 cause a chromosome instability syndrome (Atelís Syndrome) characterized by segmented and dicentric chromosomes with mosaic variegated hyperploidy. Patient-derived cells show elevated replication stress partly due to reduced ability to replicate through G-quadruplex DNA structures and loss of sister chromatid cohesion.\",\n      \"method\": \"Whole exome sequencing, cell-based replication stress assays, chromosome analysis, cohesion assay\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human genetics with functional cellular validation in patient cells, single study\",\n      \"pmids\": [\"36333305\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"RNAi ablation of SMC5 or MMS21 (but not SMC6) leads to premature sister chromatid separation before anaphase, activating the spindle assembly checkpoint and blocking mitotic progression. Mitotic SMC5 co-elutes with MMS21 in column fractions lacking SMC6, suggesting a mitotic SMC5-MMS21 sub-complex independent of SMC5/6.\",\n      \"method\": \"RNAi, immunoprecipitation, flow cytometry, live-cell imaging\",\n      \"journal\": \"Cell cycle (Georgetown, Tex.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RNAi with defined mitotic phenotype and biochemical evidence for sub-complex, single lab\",\n      \"pmids\": [\"19502785\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In human cells, Smc5/6 associates with chromatin during interphase but largely dissociates from chromosomes during mitotic chromosome condensation. Depletion of Smc5 and Smc6 causes aberrant mitotic chromosome phenotypes with abnormal distribution of topoisomerase IIα and condensins, and chromosome segregation errors. Smc5/6 is required for on-time progression of DNA replication and subsequent binding of topo IIα on replicated chromatids.\",\n      \"method\": \"Chromatin fractionation, immunofluorescence, live-cell imaging, premature chromosome condensation assay\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple cell biology methods with defined phenotype, single lab\",\n      \"pmids\": [\"24258023\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SMC5/6 depletion in human cells is essential for viability; inactivation is associated with spontaneous DNA damage, p53 activation, cell-cycle arrest, and senescence. Chromosome missegregation caused by SMC5/6 impairment requires impaired SMC5/6 function during the preceding S and G2 phases; SMC5/6 degradation immediately prior to mitotic entry has little impact on chromosome segregation fidelity.\",\n      \"method\": \"Auxin-inducible degron system for acute depletion, cell viability assay, live-cell imaging, flow cytometry\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — auxin-inducible rapid depletion with cell-cycle-stage-specific analysis, single lab\",\n      \"pmids\": [\"32320646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Smc5/6 is present at telomeres throughout the cell cycle. Its association with chromosome ends depends on Nse3 subunit. Nse3 interacts physically and genetically with Rap1-binding factors Rif2 and Sir4. Reduced telomere-associated Smc5/6 leads to defects in telomere clustering, dispersion of Sir4, and loss of transcriptional repression of subtelomeric genes and TERRA.\",\n      \"method\": \"Chromatin immunoprecipitation, co-immunoprecipitation, fluorescence microscopy, genetic analysis\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP and co-IP with functional genetic analysis of telomere silencing, single lab\",\n      \"pmids\": [\"27564449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"PJA1 (a RING-H2 E3 ubiquitin ligase) interacts with the SMC5/6 complex and facilitates binding of the complex to viral and episomal DNAs in the cell nucleus. Treatment with topoisomerase inhibitors and topoisomerase knockdown relieve PJA1-mediated silencing of viral and extrachromosomal DNAs.\",\n      \"method\": \"Co-immunoprecipitation, reporter assay, RNAi, inhibitor treatment\",\n      \"journal\": \"Journal of virology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP identifying interaction, functional consequence via inhibitors, single lab\",\n      \"pmids\": [\"30185588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"NSMCE2 (NSE2/Mms21) depletion in human cells (U2OS) increases sensitivity to etoposide (topoisomerase II inhibitor) but not to ionizing radiation. Immunoprecipitation and mass spectrometry showed the SMC5/6 complex physically interacts with DNA Topoisomerase IIα (TOP2A), suggesting SMC5/6 resolves TOP2A-mediated DSB-repair intermediates during replication.\",\n      \"method\": \"CRISPR-Cas9 knockout, immunoprecipitation, mass spectrometry\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single Co-IP/MS identifying interaction, KO with single functional readout, single lab\",\n      \"pmids\": [\"27792189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"SMC5/6 depletion in neural progenitor cells (NPCs) leads to DNA replication stress at late-replicating regions such as pericentromeric heterochromatin, causing chromosome missegregation, DNA bridges, and NPC apoptosis. Concomitant deletion of Trp53 or Chek2 rescues Smc5 cKO neurodevelopmental defects, placing SMC5/6 in a CHEK2-p53 DNA damage response pathway.\",\n      \"method\": \"Conditional knockout, epistasis with Trp53 and Chek2, DNA fiber assay, immunofluorescence\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — conditional KO with genetic epistasis placing SMC5/6 in defined pathway, replication stress characterization, single lab\",\n      \"pmids\": [\"33200984\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Single-molecule tracking in live fission yeast showed Smc5/6 is chromatin-associated in unchallenged cells. This constitutive chromatin association depends on Nse6 through at least two sub-pathways, one requiring the BRCT-domain protein Brc1. The Nse3 double-stranded DNA binding activity and the arginine fingers of both Smc5/6 ATPase sites are critical for chromatin association. Disrupting the ssDNA binding activity at the hinge region (without preventing chromatin association) leads to elevated gross chromosomal rearrangements during replication restart.\",\n      \"method\": \"Single-molecule live-cell tracking, genetic analysis with defined mutants\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — single-molecule tracking in live cells combined with defined separation-of-function mutants, single lab\",\n      \"pmids\": [\"33860765\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Fission yeast Smc5 was purified and Nse1 and Nse2 were identified by mass spectrometry as co-precipitating non-SMC subunits that interact with Smc5 in vivo as part of the Smc5-Smc6 complex. Both are essential proteins with DNA damage repair functions epistatic to Rhp51, placing them in the homologous recombination pathway.\",\n      \"method\": \"Protein purification, mass spectrometry, co-immunoprecipitation, genetic epistasis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical purification and MS identification with genetic epistasis, foundational complex characterization\",\n      \"pmids\": [\"12966087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In budding yeast, acute Smc5/6 depletion reveals a primary defect in replication of the rDNA array. This is linked to programmed replication fork barriers (RFBs) established by Fob1; Fob1 deletion improves rDNA replication in Smc5/6-depleted cells. Deletion of Mph1 helicase similarly reduces recombination structures at RFB regions, confirming Smc5/6 restrains Mph1-dependent recombination at protein-barrier-stalled forks.\",\n      \"method\": \"Auxin-inducible degron depletion, 2D gel electrophoresis, genetic epistasis\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — acute depletion system with 2D gel and genetic epistasis, single lab\",\n      \"pmids\": [\"29360860\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"SMC5 is a core ATPase subunit of the conserved six-to-eight-subunit Smc5/6 complex that acts as an ATP-dependent DNA loop-extruding motor and compaction machine, preferentially recognizing positively supercoiled and catenated DNA templates; it contains a hinge domain that mediates heterodimerization with SMC6 and preferentially binds ssDNA, an arm region that anchors the Nse2/Mms21 SUMO E3 ligase (whose activity is stimulated by direct DNA binding to an ARM domain on Smc5), and head ATPase domains bridged by the kleisin Nse4; the complex executes sister-chromatid homologous recombination repair (including at DSBs where it recruits cohesin), resolves Holliday junctions and aberrant recombination intermediates through Mms21-dependent SUMOylation of substrates including TRF1/TRF2, Sgs1-Top3-Rmi1, and kinetochore proteins, maintains rDNA and telomere stability, is essential for viability in an interphase-specific manner by ensuring completion of DNA replication before mitosis, and functions as an intrinsic antiviral restriction factor that topologically entraps and transcriptionally silences extrachromosomal/episomal DNA through a SUMOylation-dependent chromatin-compaction mechanism that is counteracted by HBx-mediated CRL4 ubiquitin ligase degradation and analogous viral protein strategies.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"SMC5 is the core ATPase subunit of the conserved Smc5/6 complex, an ATP-dependent DNA motor that compacts and topologically constrains DNA to safeguard genome integrity during replication and recombination [#12, #19]. SMC5 heterodimerizes with SMC6 through a distinctive toroidal hinge whose 'latch' and 'hub' interfaces preferentially bind single-stranded DNA, an activity required for viability and DNA damage resistance [#24], while the Smc5 monomer itself binds ssDNA and uses ATP to regulate DNA association [#32]. Within the assembled holocomplex the kleisin Nse4 bridges the Smc5 and Smc6 head ATPase domains and, together with Nse1/Nse3, forms a positively charged DNA clamp that captures dsDNA non-specifically [#2, #8]; the Nse1/Nse3/Nse4 module contributes its own dsDNA-binding surface needed for chromatin association in vivo [#28]. ATP hydrolysis by the holocomplex is DNA-stimulated and is held in check by the Nse5/6 subcomplex, which anchors the ATPase heads in a non-productive state and inhibits the dimerization required to initiate DNA-loop extrusion [#14, #15]. Functionally, Smc5/6 is a symmetric DNA loop-extruding and compaction machine that preferentially recognizes positively supercoiled and catenated DNA, locking plectonemes and linking supercoiled chromosomal regions in cis [#0, #1, #12]. The complex anchors the Nse2/Mms21 SUMO E3 ligase on the Smc5 arm; Nse2 activity is directly stimulated by DNA binding to a positively charged ARM patch on Smc5 that acts as a DNA sensor [#6, #17]. Through this SUMOylation output and its DNA-compaction activity, Smc5/6 is recruited to DNA double-strand breaks where it promotes error-free sister-chromatid homologous recombination and cohesin loading, resolves recombination intermediates via SUMOylation of the Sgs1-Top3-Rmi1 dissolution complex, and maintains rDNA and telomere stability [#5, #19, #22, #23, #4]. Its essential function is interphase-specific, ensuring completion of DNA replication through difficult late-replicating and barrier-stalled regions before mitosis [#25, #44]. Beyond chromosome maintenance, Smc5/6 acts as an intrinsic antiviral restriction factor that topologically entraps and transcriptionally silences extrachromosomal/episomal and unintegrated viral DNA through an ATPase- and SUMOylation-dependent chromatin-compaction mechanism, which diverse viral proteins (HBx, BNRF1, KSHV RTA) counteract by hijacking ubiquitin ligases to degrade the complex [#3, #33, #34, #35, #38]. Biallelic mutations in SMC5 and its complex partners cause autosomal recessive chromosome-instability syndromes characterized by chromosome breakage, replication stress, and defective homologous recombination [#40, #41].\",\n  \"teleology\": [\n    {\n      \"year\": 2003,\n      \"claim\": \"Establishing that Smc5 is the catalytic centerpiece of a defined multi-subunit complex was the founding step, identifying its non-SMC partners and placing them in the homologous recombination pathway.\",\n      \"evidence\": \"Protein purification with MS identification of Nse1/Nse2 and genetic epistasis in fission yeast\",\n      \"pmids\": [\"12966087\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not define subunit architecture or DNA-binding mechanism\", \"Catalytic activities of the complex unresolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Defining the Smc5-Smc6 hinge heterodimerization interface and the embedded Nse2 SUMO ligase activity converted a genetic complex into a biochemical entity with an enzymatic output for the DNA damage response.\",\n      \"evidence\": \"Protein purification, hinge mutagenesis, and in vitro/in vivo SUMOylation assays in yeast\",\n      \"pmids\": [\"15601840\", \"15601841\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SUMO substrates relevant in vivo not fully enumerated\", \"Structural basis of ligase specificity unresolved\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Localization and recruitment studies established the complex acts directly at DNA double-strand breaks to drive error-free sister-chromatid recombination, distinguishing its role from other repair pathways.\",\n      \"evidence\": \"ChIP showing de novo recruitment to DSBs, HR subpathway assays, and 2D gels in human and yeast cells\",\n      \"pmids\": [\"16810316\", \"16892052\", \"16793545\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular signal for recruitment not defined\", \"How the complex enforces sister-chromatid template choice unknown\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Mapping the kleisin Nse4 and the Nse subcomplexes to the Smc5/Smc6 heads defined the modular architecture that bridges the ATPase domains.\",\n      \"evidence\": \"Y2H, co-IP, and in vitro binding assays in yeast\",\n      \"pmids\": [\"17005570\", \"19141609\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab interaction mapping without structural confirmation at the time\", \"Functional consequences of each bridging contact not dissected\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"A crystal structure of the Mms21/Nse2-Smc5 arm interface revealed a novel RING E3 architecture and showed the interface itself is required for growth and damage resistance, anchoring the SUMO ligase on the SMC5 scaffold.\",\n      \"evidence\": \"X-ray crystallography with mutagenesis and in vitro SUMO ligase assays\",\n      \"pmids\": [\"19748359\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How DNA context regulates the ligase not yet defined\", \"Full substrate spectrum unknown\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Linking Smc5/6/Mms21 to active dissolution of DNA-mediated sister-chromatid linkages before mitosis explained why complex loss causes anaphase missegregation.\",\n      \"evidence\": \"Conditional genetics with chromosome-segregation microscopy and metaphase rescue in yeast\",\n      \"pmids\": [\"20571088\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Biochemical nature of dissolved linkages not fully defined\", \"Direct enzymatic step on the linkage not shown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Reconstitution showed the purified complex binds and entraps DNA in an ATP-dependent manner, recognizing single-stranded and supercoiled/catenated substrates, providing the first direct biochemical activity for SMC5.\",\n      \"evidence\": \"In vitro DNA binding/catenation assays, ATPase mutants, and purified Smc5 ssDNA binding\",\n      \"pmids\": [\"26299966\", \"21293191\", \"26446992\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Loop-extrusion behavior not yet demonstrated\", \"Coupling of ATP cycle to topology change not resolved\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"The toroidal Smc5/6 hinge with its unique latch and hub interfaces was shown to bind ssDNA preferentially, linking a structural feature to a DNA-substrate preference essential for viability.\",\n      \"evidence\": \"Crystal structure, ssDNA-binding assays, and mutagenesis with viability readouts in two organisms\",\n      \"pmids\": [\"28134253\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How hinge ssDNA binding integrates with head ATPase cycle unclear\", \"In vivo ssDNA intermediates engaged not directly visualized\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identifying the Smc5 ARM domain as a DNA sensor that electrostatically stimulates Nse2 ligase activity coupled the complex's DNA engagement directly to its SUMOylation output.\",\n      \"evidence\": \"In vitro SUMO ligase and DNA-binding assays with ARM-patch mutants and in vivo damage sensitivity\",\n      \"pmids\": [\"29769404\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which substrates are selected upon ARM-DNA activation in vivo not enumerated\", \"Structural snapshot of the activated state lacking\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Holocomplex reconstitution and a DNA-bound cryo-EM structure unified the complex's activities, demonstrating DNA-dependent ATPase, SUMO ligase, and ATP-driven plectoneme-locking/compaction with a defined DNA clamp.\",\n      \"evidence\": \"In vitro ATPase/SUMO/single-molecule assays, cryo-EM at 3.8 Å, and cross-linking MS in yeast and human systems\",\n      \"pmids\": [\"33301732\", \"33301731\", \"35648833\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanistic transition from clamping to loop extrusion not captured\", \"Human-specific regulatory differences incompletely defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrating that Nse5/6 inhibits the ATPase by anchoring the heads in a non-productive state established the principal mechanism that gates complex activity, relieved by closed/long DNA.\",\n      \"evidence\": \"Crystal structure of Nse5/6, ATPase assays, cysteine and cross-linking MS, and integrative cryo-EM modeling\",\n      \"pmids\": [\"34191293\", \"33849072\", \"33941673\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cellular signals that relieve Nse5/6 inhibition not defined\", \"How regulation differs across recruitment pathways unclear\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Single-molecule imaging resolved the core mechanism: Smc5/6 dimers extrude DNA loops symmetrically and monomers translocate, with Nse5/6 blocking dimerization to control loop-extrusion initiation.\",\n      \"evidence\": \"Single-molecule imaging of reconstituted Smc5/6, with monomer/dimer and Nse5/6 dissection\",\n      \"pmids\": [\"37076626\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of loop extrusion to repair not directly tested\", \"Connection to SUMOylation output during extrusion unknown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Coupling loop extrusion to substrate preference, Smc5/6 was shown to recognize tips of positively supercoiled plectonemes at transcription-induced cohesin-loop boundaries and gather them in cis.\",\n      \"evidence\": \"Single-molecule imaging, ChIP-seq, and Hi-C\",\n      \"pmids\": [\"38295804\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional outcome of supercoil gathering for repair/replication not fully resolved\", \"Interplay with cohesin at boundaries mechanistically open\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Recognition that the chromatin-compaction and SUMOylation machinery is repurposed as an antiviral restriction factor extended SMC5/6 biology, with SLF2/SIMC1-dependent recruitment to viral DNA at PML bodies and SUMO-dependent silencing.\",\n      \"evidence\": \"CRISPR screens, ATAC-seq, NSMCE2 SUMO-mutant and inhibitor latency assays, ChIP, and structural/IP analyses of SLF2/SIMC1 in HIV-1, KSHV, EBV and polyomavirus systems\",\n      \"pmids\": [\"33811831\", \"36376394\", \"36097294\", \"35914008\", \"35263599\", \"36373674\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Why episomal/viral DNA is selectively recognized versus host chromatin not fully resolved\", \"Relative contributions of entrapment vs SUMOylation across viruses differ and are unsettled\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Human genetics confirmed the physiological essentiality of SMC5 and its partners by linking biallelic mutations to chromosome-instability syndromes with replication stress and HR defects.\",\n      \"evidence\": \"Whole exome sequencing with patient-cell HR, cohesion, and replication-stress assays for SMC5/SLF2 and NSMCE3\",\n      \"pmids\": [\"36333305\", \"27427983\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Genotype–phenotype relationships across the complex incompletely defined\", \"Tissue-specific developmental requirements not fully explained\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the ATP-driven loop-extrusion/compaction cycle is mechanistically coupled to Nse2/Mms21 SUMOylation choices and to substrate-specific outcomes (sister-chromatid HR versus viral silencing versus replication completion) remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural snapshot links an extruding state to active SUMO transfer\", \"Determinants directing the complex to distinct cellular versus viral substrates undefined\", \"How recruitment pathways tune the Nse5/6 inhibitory switch in vivo unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140657\", \"supporting_discovery_ids\": [0, 11, 12, 15, 32]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [2, 24, 28, 32]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [6, 7, 17]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [6, 7, 17]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [11, 12]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [2, 8]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005694\", \"supporting_discovery_ids\": [19, 26, 49, 43]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [4, 36, 43]},\n      {\"term_id\": \"GO:0005730\", \"supporting_discovery_ids\": [18, 20, 51]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [5, 19, 22, 40]},\n      {\"term_id\": \"R-HSA-69306\", \"supporting_discovery_ids\": [25, 44, 48, 51]},\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [29, 42, 43, 44]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [3, 33, 34, 35]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [4, 7, 17, 22]}\n    ],\n    \"complexes\": [\"Smc5/6 complex\"],\n    \"partners\": [\"SMC6\", \"NSE4\", \"NSE3\", \"NSE2/MMS21\", \"SLF2\", \"SIMC1\", \"MPH1\", \"TOP2A\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":10,"faith_total":10,"faith_pct":100.0}}