{"gene":"CDC34","run_date":"2026-04-28T17:28:52","timeline":{"discoveries":[{"year":1988,"finding":"CDC34 encodes a ubiquitin-conjugating enzyme (E2) required for the G1-to-S phase transition; the bacterially expressed product catalyzes covalent attachment of ubiquitin to histones H2A and H2B in vitro, demonstrating E2 activity.","method":"In vitro ubiquitination assay with bacterially expressed protein; sequence homology to RAD6","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution, foundational paper with 401 citations, replicated by subsequent work","pmids":["2842867"],"is_preprint":false},{"year":1991,"finding":"Cdc34 (and its rabbit homolog E2(32K)) preferentially catalyze processive multiubiquitination via Lys-48 of ubiquitin, distinguishing them from RAD6/E2(20K) which use non-Lys48 linkages; Cdc34 shows specificity for BSA rather than core histones.","method":"In vitro ubiquitin conjugation assay with native, reductively methylated, and K48R ubiquitin variants; kinetic analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro biochemical reconstitution with multiple ubiquitin variants, clear mechanistic distinction","pmids":["1848239"],"is_preprint":false},{"year":1992,"finding":"The 125-residue C-terminal tail of Cdc34 (specifically residues 171–244) is a portable determinant of cell cycle function; transplanting this tail onto the RAD6 catalytic domain creates a chimeric E2 that performs both CDC34 and RAD6 functions in vivo.","method":"Chimeric E2 constructs expressed in yeast; in vivo complementation of cdc34 and rad6 mutants; deletion analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — genetic complementation with chimeric constructs, replicated in companion paper (Silver et al.)","pmids":["1639076","1639075"],"is_preprint":false},{"year":1993,"finding":"Bacterially expressed Cdc34 catalyzes its own ubiquitination (autoubiquitination) via intramolecular transfer forming predominantly a single Lys48-linked multiubiquitin chain, with major linkage sites mapping to C-terminal lysines (K273, K277, K293, K294).","method":"In vitro autoubiquitination assay; hydroxylamine cleavage; site-directed mutagenesis of C-terminal lysines","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with mutagenesis, defines linkage specificity mechanistically","pmids":["8383676"],"is_preprint":false},{"year":1994,"finding":"The Cdc34 C-terminal tail (39 residues adjacent to the catalytic domain) mediates Cdc34 self-association (dimerization) in vitro and in vivo, and this self-association is required for cell cycle function.","method":"Chemical cross-linking; biophysical analysis; in vivo phenotypic analysis of CDC34 derivatives in cdc34 mutant strains","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — cross-linking combined with in vivo complementation, multiple methods","pmids":["7929378"],"is_preprint":false},{"year":1994,"finding":"Cdc34 (Ubc3) is itself a substrate for both ubiquitination and phosphorylation in vivo; immunochemical localization places it in the nucleus, suggesting nuclear substrates.","method":"In vivo labeling; immunochemical localization","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 3 — direct detection in vivo but single lab, no mechanistic follow-up on the modification writers","pmids":["8164658"],"is_preprint":false},{"year":1995,"finding":"Genetic and biochemical evidence demonstrates a noncovalent ubiquitin-binding site on Cdc34; overexpression of ubiquitin suppresses cdc34 temperature-sensitive alleles in an allele-specific manner, and chemical cross-linking confirms a specific noncovalent Ub-Cdc34 interaction.","method":"Genetic suppression screen; chemical cross-linking; in vivo ubiquitin overexpression","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — orthogonal genetic and biochemical methods, single lab","pmids":["7721857"],"is_preprint":false},{"year":1995,"finding":"Dual mutation of CDC34 active-site cysteine (C95S) and conserved Leu99 (L99S) generates a dominant-negative Cdc34 that blocks cell growth and inhibits in vitro ubiquitination of the Cdc34 substrate Cln2.","method":"Site-directed mutagenesis; in vitro ubiquitination assay; overexpression growth assay in yeast","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — active-site mutagenesis combined with in vitro reconstitution and in vivo phenotype","pmids":["7592826"],"is_preprint":false},{"year":1995,"finding":"Kinetochore protein Cbf2p (Ndc10p) is ubiquitinated in vivo by Cdc34; purified Cdc34 catalyzes Cbf2p-monoubiquitin conjugate formation in vitro; overexpression of CDC34 suppresses the ndc10-1 temperature-sensitive mutation.","method":"In vitro ubiquitination assay with purified proteins; in vivo anti-ubiquitin immunoprecipitation; genetic suppression","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro reconstitution plus in vivo evidence, single lab","pmids":["7651401"],"is_preprint":false},{"year":1997,"finding":"Multiubiquitination of Sic1 requires cyclin/Cdc28 protein kinase activity, the Cdc34 E2, and Cdc4; the N-terminal 160 residues of Sic1 are necessary and sufficient for Cdc34-dependent ubiquitination.","method":"In vitro reconstitution with DEAE-fractionated yeast extracts; cdc4ts mutant extracts; SIC1 deletion analysis","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1 — biochemical reconstitution with genetic mutant validation and substrate mapping","pmids":["9285816"],"is_preprint":false},{"year":1997,"finding":"Cdc34 is required for initiation of DNA replication in Xenopus egg extracts; Cdc34p in a large molecular size complex regulates initiation function of Cdk2-cyclin E, likely through degradation of the Xenopus CDK inhibitor Xic1.","method":"Xenopus egg extract depletion/add-back experiments; sizing column fractionation","journal":"Science","confidence":"Medium","confidence_rationale":"Tier 2 — functional rescue in a cell-free system, single lab","pmids":["9287222"],"is_preprint":false},{"year":1998,"finding":"Cdc53 functions as a scaffold protein within the SCF E3 complex, containing independent binding sites for Cdc34 and Skp1; Skp1 bridges Cdc53 to three F-box proteins (Cdc4, Met30, Grr1), conferring substrate specificity on a common Cdc34-Cdc53-Skp1 E2/E3 core.","method":"In vivo co-immunoprecipitation; two-hybrid; genetic epistasis","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP and genetic analysis, multiple F-box proteins tested, high citations","pmids":["9499404"],"is_preprint":false},{"year":1998,"finding":"Cdc34 and the F-box protein Met30 are required for degradation of the Cdk-inhibitory kinase Swe1; Met30 physically binds Swe1 in vivo and extracts from cdc34 or met30 mutants fail to polyubiquitinate Swe1.","method":"GST pull-down; in vitro ubiquitination assay with mutant extracts; genetic interaction screen","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods: pulldown, in vitro ubiquitination, genetics","pmids":["9716410"],"is_preprint":false},{"year":1998,"finding":"Human CDC34 associates in vivo with CUL-1 and the F-box protein p45(SKP2) as part of an SCF-type E3 ubiquitin ligase complex, establishing conservation of the SCF pathway in human cells.","method":"Co-immunoprecipitation in vivo from human cells","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 3 — co-IP in vivo, single lab, no in vitro reconstitution","pmids":["9430629"],"is_preprint":false},{"year":1998,"finding":"Cdc34-mediated degradation of Wee1 kinase in Xenopus egg extracts is required for timely entry into mitosis; this proteolysis is inhibited when DNA replication is blocked, linking the DNA replication checkpoint to Wee1 stability.","method":"Xenopus egg extract biochemical assay; Cdc34 immunodepletion; DNA replication block","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — reconstitution in cell-free system with immunodepletion, clear mechanistic link","pmids":["9836638"],"is_preprint":false},{"year":1999,"finding":"The RING-H2 protein Hrt1 (Rbx1/Roc1) is a subunit of SCF identified by mass spectrometry; it binds Cdc34 directly, stimulates SCF E3 activity, and enables reconstitution of Cln2 ubiquitination; SCF and Cdc53/Hrt1 activate Cdc34 autoubiquitination by a mechanism independent of reactive thiols.","method":"Mass spectrometry; recombinant protein reconstitution; in vitro ubiquitination assay; conditional genetic inactivation","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 1 — MS identification + reconstitution + mutagenesis, multiple orthogonal methods, >350 citations","pmids":["10385629"],"is_preprint":false},{"year":1999,"finding":"Human Cdc34 ubiquitinates the transcription factor repressors hICERIIγ and hATF5 in mammalian cells; both hCdc34- and hRad6B-dependent ubiquitin-mediated proteolysis abrogates their transcriptional repression of cAMP-induced genes.","method":"Transfection assay; dominant-negative and antisense Cdc34 constructs; ubiquitination assay in mammalian cells","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 — dominant-negative and antisense approaches with functional readout, single lab","pmids":["10373550"],"is_preprint":false},{"year":2000,"finding":"Cdc34/SCF(Met30) ubiquitinates the transcription factor Met4, causing transcriptional repression without proteolysis; ubiquitinated Met4 associates with target promoters but fails to form functional transcription complexes; deletion of MET4 suppresses lethality of met30 mutants.","method":"Genetic epistasis (MET4 deletion suppression); chromatin immunoprecipitation; metabolic labeling to assess Met4 stability","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1–2 — genetic suppression + biochemical analysis of protein stability + ChIP, multiple methods, >240 citations","pmids":["10975521"],"is_preprint":false},{"year":2000,"finding":"SCFβ-TRCP together with Cdc34 (Ubc3) catalyzes phosphorylation-dependent ubiquitination of IκBα; Ubc4 is also capable but is ~19-fold more efficient in THP.1 cells; Cdc34 associates with SCFβ-TRCP isolated from human cells.","method":"In vitro reconstitution with recombinant components; ubiquitination assay; pull-down of Cdc34 from human cell extracts","journal":"Oncogene","confidence":"High","confidence_rationale":"Tier 1 — biochemical reconstitution with recombinant components","pmids":["10918611"],"is_preprint":false},{"year":2001,"finding":"Herpes simplex virus 1 ICP0 binds Cdc34 via its RING finger (exon 2) and acts as an E3 ubiquitin ligase that promotes ubiquitination of Cdc34 in vitro; in infected cells, Cdc34 undergoes increased ICP0-dependent dynamic interaction with proteasomes.","method":"In vitro ubiquitination assay; RING-domain binding assay; co-immunoprecipitation in infected cells","journal":"PNAS","confidence":"Medium","confidence_rationale":"Tier 2 — in vitro ubiquitination plus in vivo co-IP, single lab","pmids":["11447293"],"is_preprint":false},{"year":2001,"finding":"Human CDC34 protein is localized to distinct nuclear and cytoplasmic speckles during interphase; nuclear localization depends on specific C-terminal CDC34 sequences; in anaphase CDC34 colocalizes with β-tubulin at the mitotic spindle.","method":"Immunofluorescence; subcellular fractionation; deletion mutant analysis","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 — direct localization by immunofluorescence and fractionation, with sequence determinant identification","pmids":["10769200"],"is_preprint":false},{"year":2001,"finding":"Elevated Cdc34 protein levels at prophase selectively block CENP-E kinesin from associating with kinetochores, causing chromosome congression failure and prometaphase arrest; this effect is not rescued by proteasome inhibitors, suggesting a non-proteolytic ubiquitination role.","method":"Microinjection of bacterially expressed Cdc34 into mammalian cells; immunofluorescence; electron microscopy of kinetochores","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct injection experiment with specific phenotypic and molecular readout, single lab","pmids":["11514588"],"is_preprint":false},{"year":2001,"finding":"Human CDC34 is phosphorylated in proliferating cells; the regulatory β-subunit of CK2 interacts with CDC34 in vivo; recombinant CK2 phosphorylates CDC34 at five C-terminal sites (S203, S222, S231, T233, S236); mutating these sites abolishes in vivo phosphorylation and shifts nuclear CDC34 to the cytoplasm.","method":"Yeast two-hybrid; co-immunoprecipitation in transfected cells; in vitro CK2 phosphorylation; site-directed mutagenesis; immunofluorescence","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods identifying writer kinase, phosphorylation sites, and functional consequence on localization","pmids":["11546811"],"is_preprint":false},{"year":2003,"finding":"Formation of the Cdc34-ubiquitin thioester increases the dissociation rate of Cdc34 from the SCF RING domain; release of ubiquitin-charged Cdc34 from the RING is essential for ubiquitination of the SCF(Cdc4)-bound substrate Sic1, as shown by F72V Cdc34 mutant with increased RING affinity that is unable to ubiquitinate Sic1.","method":"Kinetic binding measurements; mutagenesis (F72V Cdc34); in vitro ubiquitination assay","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with engineered affinity mutant and kinetic measurements, defines a mechanistic step","pmids":["13678584"],"is_preprint":false},{"year":2003,"finding":"Cdc34 self-association in vivo depends on integrity of the Cdc34-ubiquitin thioester and key catalytic domain residues (active-site C95, S73, S97, and the catalytic domain insertion); self-association is required for multi-ubiquitin chain assembly and cell cycle function.","method":"Co-immunoprecipitation; site-directed mutagenesis of catalytic residues; in vivo complementation","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP with systematic mutagenesis, mechanistically links thioester formation to self-association and chain assembly","pmids":["12861024"],"is_preprint":false},{"year":2005,"finding":"Sic1 ubiquitination by SCF(Cdc4)/Cdc34 occurs in two mechanistically distinct steps: slow rate-limiting attachment of the first ubiquitin, followed by rapid processive elongation of K48-linked chains; an acidic loop conserved in Cdc34 orthologs is required for processivity and K48-linkage specificity during chain elongation but not for first ubiquitin attachment.","method":"In vitro ubiquitination assay with purified recombinant proteins; acidic loop mutagenesis; kinetic analysis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — reconstitution + mutagenesis separating two mechanistic steps, >230 citations","pmids":["16360039"],"is_preprint":false},{"year":2005,"finding":"Proximity-induced dimerization of human Cdc34 (via GST fusion or FKBP-AP20187 chemical dimerization) constitutively activates K48-linked polyubiquitin chain synthesis independently of SCF, suggesting SCF may activate Cdc34 by converting it from an inactive monomer to an active dimer.","method":"GST-fusion dimerization; chemical inducer of dimerization (FKBP-AP20187); in vitro ubiquitin chain synthesis assay","journal":"PNAS","confidence":"Medium","confidence_rationale":"Tier 1 — in vitro reconstitution with two dimerization strategies, single lab","pmids":["16210246"],"is_preprint":false},{"year":2007,"finding":"Human Cdc34 employs distinct sites for mono- vs. polyubiquitination: the conserved charged stretch (residues 143–153) and acidic loop residues D102/D103 are required for Ub-Ub ligation (polychain assembly) but not for substrate monoubiquitination; N85 and S138 coordinate the attacking lysine for catalysis in both steps.","method":"Site-directed mutagenesis; in vitro ubiquitination assay with IκBα as substrate; GST-Cdc34 dimerization rescue experiments","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis with in vitro reconstitution, multiple distinct functional sites mapped","pmids":["17698585"],"is_preprint":false},{"year":2007,"finding":"CK2 phosphorylates yeast Cdc34 C-terminal tail at S207 and S216 (and human Cdc34 at S203, S222, S231) in vitro; this phosphorylation stimulates Cdc34 ubiquitination activity toward Sic1 in SCFCdc4-dependent reactions; phosphosite alanine mutants with altered activity show corresponding changes in Sic1 degradation and cell cycle progression speed.","method":"In vitro CK2 phosphorylation; in vitro ubiquitination assay; yeast cell cycle synchronization and progression assay with cdc34 phosphosite mutants","journal":"The Biochemical journal","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro kinase assay + in vitro ubiquitination + in vivo cell cycle phenotype, single lab but multiple methods","pmids":["17461777"],"is_preprint":false},{"year":2008,"finding":"CK2 phosphorylates the catalytic domain of Cdc34 at S130 and S167 in vitro and in vivo; these phosphorylation events strongly stimulate Cdc34 ubiquitin-charging activity; the S130A/S167A double mutant fails to complement a cdc34-2ts strain.","method":"Mass spectrometry; in vitro CK2 phosphorylation; ubiquitin charging assay; yeast complementation in cka1Δcka2-8ts background","journal":"Cell cycle","confidence":"High","confidence_rationale":"Tier 1–2 — MS-confirmed phosphosites, in vitro activity assay, in vivo complementation, single lab","pmids":["18418079"],"is_preprint":false},{"year":2009,"finding":"The Cdc34 acidic C-terminal tail contributes both to submicromolar binding affinity for SCF(Cdc4) and to catalysis; the functional requirement for the tail is its acidity; Cdc34 retains partial function when fused to the C-terminus of Cul1 even without the tail.","method":"Binding affinity measurements; in vitro ubiquitination assay; tail deletion and charge mutants; Cdc34-Cul1 fusion proteins","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with multiple mutants and fusion proteins separating binding vs. catalytic roles","pmids":["19875449"],"is_preprint":false},{"year":2010,"finding":"Polyubiquitination of IκBα by SCF(βTrCP2) begins with rapid monoubiquitination by UbcH5 at K21/K22, followed by Cdc34-mediated K48-linked polyubiquitin chain elongation using the substrate-linked ubiquitin as a receptor; the IκBα K21-linked ubiquitin makes direct contacts with Cdc34 and the SCF RING subcomplex.","method":"Biochemical reconstitution with UbcH5 and Cdc34; ubiquitin-fused substrate constructs; in vitro ubiquitination assay","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — reconstitution defining mechanistic handoff between two E2 enzymes","pmids":["20347421"],"is_preprint":false},{"year":2010,"finding":"The human Cdc34 C-terminus contains two non-covalent ubiquitin-binding sites (UBS1: residues 206–215; UBS2: 216–225) mapped by NMR; UBS1 aromatic residues (F206, Y207, Y210, Y211) interact near ubiquitin Lys48 and C-terminus; Y210 is specifically required for donor ubiquitin transfer in SCF-dependent reactions.","method":"NMR chemical shift perturbation; mutagenesis; in vitro IκBα ubiquitination reconstitution","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — NMR structural mapping plus in vitro functional reconstitution with mutagenesis","pmids":["20353940"],"is_preprint":false},{"year":2010,"finding":"Residues surrounding acceptor lysines on Sic1 and ubiquitin are critical for Cdc34-mediated ubiquitination independent of SCF; key residues composing the Cdc34 catalytic core alter lysine preference and determine whether Cdc34 monoubiquitinates or polyubiquitinates Sic1.","method":"In vitro ubiquitination assay; mutagenesis of Cdc34 catalytic core residues and substrate lysine-flanking residues","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 1 — systematic mutagenesis combined with in vitro reconstitution","pmids":["20194622"],"is_preprint":false},{"year":2011,"finding":"The I44A mutation in ubiquitin profoundly inhibits donor ubiquitin discharge from Cdc34; computationally predicted compensatory mutations in Cdc34 rescue this defect; the Cdc34-ubiquitin interaction at I44 promotes efficient deprotonation of the acceptor lysine for ubiquitin transfer.","method":"In vitro ubiquitination assay; site-directed mutagenesis (ubiquitin I44A and Cdc34 compensatory mutants); hydroxylamine acceptor experiments at varying pH","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 — reconstitution with rationally designed compensatory mutations and pH-dependence experiments","pmids":["21474069"],"is_preprint":false},{"year":2011,"finding":"A small molecule CC0651 selectively inhibits human Cdc34 by inserting into a cryptic allosteric pocket distant from the catalytic site, causing conformational changes in E2 secondary structure; it does not affect E1 charging or E3 binding but blocks ubiquitin discharge to acceptor lysines; cells show p27(Kip1) accumulation.","method":"Crystal structure of Cdc34-CC0651 complex; in vitro ubiquitination assay; cell-based p27 accumulation assay; thioester formation assay","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — crystal structure + in vitro activity assay + cell-based validation, defines allosteric inhibition mechanism","pmids":["21683433"],"is_preprint":false},{"year":2011,"finding":"The CDC34 C-terminus is intrinsically disordered but intramolecularly interacts with the catalytically bound ubiquitin in the Cdc34-Ub thioester complex; the C-terminus contacts a lysine-rich face of ubiquitin (K6, K11, K29, K33) in a two-state equilibrium, suggesting a role in directing K48-linked chain formation.","method":"NMR spectroscopy; CDC34-Ub disulfide mimetic; chemical shift perturbation analysis","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — NMR structural characterization of thioester mimetic with functional implications","pmids":["21296085"],"is_preprint":false},{"year":2013,"finding":"The Cdc34 acidic loop promotes SCF-Cdc34 interaction and suppresses the pKa of an ionizing species on ubiquitin or Cdc34; two glutamic acid residues on the distal side collaborate with a conserved histidine on the proximal side to promote catalysis.","method":"In vitro ubiquitination assay; mutagenesis; pKa measurements; quantification of E2-E3 binding","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis with in vitro reconstitution defining two distinct mechanistic roles for the loop","pmids":["24129577"],"is_preprint":false},{"year":2014,"finding":"The Cdc34-SCF interaction occurs in multiple conformations where several residues from the Cdc34 acidic C-terminal tail contact a broad region of the SCF basic canyon; similar contacts occur with Cul2, implicating a conserved mechanism across cullin-RING ligases.","method":"Protein cross-linking followed by mass spectrometry; binding interface mapping","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — cross-linking mass spectrometry, single lab","pmids":["25425648"],"is_preprint":false},{"year":2009,"finding":"let-7 microRNA directly down-regulates Cdc34 via its 3′-UTR, reducing Cdc34 protein levels, stabilizing the SCF substrate Wee1 kinase, and causing G2/M accumulation in primary human fibroblasts; siRNA knockdown of Wee1 reverses the G2/M phenotype.","method":"Luciferase reporter assay (3′-UTR); microarray; siRNA knockdown; Western blot; flow cytometry","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — direct reporter assay for miRNA targeting + epistasis with Wee1 siRNA, single lab","pmids":["19126550"],"is_preprint":false},{"year":2019,"finding":"Crystal structures of Cdc34 alone, in complex with E1 (Uba1), and as a Cdc34~Ub thioester mimetic reveal: unique E1-binding mode requiring conformational changes in both Uba1 and Cdc34 for transthiolation; the C-terminal extension contacts the catalytically bound ubiquitin, stabilizing a closed conformation critical for ubiquitin discharge.","method":"X-ray crystallography (Cdc34 alone, Cdc34-E1 complex, Cdc34~Ub mimetic); mutagenesis; cell-based assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 — multiple crystal structures with functional validation by mutagenesis and cell assays","pmids":["31341161"],"is_preprint":false},{"year":2020,"finding":"CDC34 competes with the E3 ligase c-Cbl to bind EGFR at Y1045, thereby inhibiting EGFR polyubiquitination and degradation and promoting lung carcinogenesis; knockdown of CDC34 inhibits NSCLC tumor formation in mouse models.","method":"siRNA library screen; co-immunoprecipitation; overexpression/knockdown in vitro and in vivo xenograft model","journal":"EBioMedicine","confidence":"Medium","confidence_rationale":"Tier 2 — co-IP defining competition mechanism plus in vivo mouse model, single lab","pmids":["32114396"],"is_preprint":false},{"year":2010,"finding":"COP9 signalosome (CSN) protects UBC3/Cdc34 from SCF(βTrCP)-mediated proteasomal degradation; downregulation of CSN4 or CSN5 induces ubiquitination and degradation of Cdc34; this requires the acidic C-terminal extension of Cdc34 which is sufficient to impart SCF(βTrCP)-sensitivity to UBCH5 enzymes.","method":"RNAi knockdown of CSN subunits; domain swap (acidic tail transplant onto UBCH5); proteasome inhibitor experiments","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — domain swap and RNAi defining specific interaction, single lab","pmids":["20378537"],"is_preprint":false}],"current_model":"CDC34/Ube2R1 is a Lys48-specific ubiquitin-conjugating enzyme (E2) that functions as part of SCF cullin-RING E3 complexes to drive proteasomal degradation of key cell cycle regulators (Sic1, Cln2, Wee1, p27, IκBα, Met4, and others) by catalyzing processive K48-linked polyubiquitin chain synthesis via a two-step mechanism (rate-limiting first-Ub attachment followed by rapid chain elongation), with its unique acidic C-terminal tail mediating electrostatic binding to the SCF basic canyon and promoting catalysis, its acidic loop controlling K48-linkage specificity and polyubiquitin processivity, CK2-mediated phosphorylation of both the catalytic domain and tail stimulating E2 charging and activity, and release of ubiquitin-charged Cdc34 from the RING subunit being an essential step that permits multisite substrate ubiquitination."},"narrative":{"teleology":[{"year":1988,"claim":"Established that CDC34 encodes a ubiquitin-conjugating enzyme required for the G1/S transition, answering the fundamental question of what biochemical activity underlies CDC34 cell-cycle function.","evidence":"Bacterially expressed Cdc34 catalyzed ubiquitin attachment to histones in vitro; sequence homology to RAD6 E2","pmids":["2842867"],"confidence":"High","gaps":["Physiological substrates unknown","No E3 ligase partner identified","Mechanism of cell cycle arrest not defined"]},{"year":1991,"claim":"Demonstrated that Cdc34 preferentially assembles K48-linked polyubiquitin chains processively, distinguishing its linkage specificity from other E2s like RAD6.","evidence":"In vitro conjugation assays with wild-type and K48R ubiquitin variants; kinetic analysis","pmids":["1848239"],"confidence":"High","gaps":["Structural basis for K48 specificity unknown","No physiological substrate tested"]},{"year":1992,"claim":"Revealed that the unique 125-residue C-terminal tail is a portable functional determinant sufficient to confer CDC34 cell-cycle activity when grafted onto RAD6, separating catalytic core from regulatory tail functions.","evidence":"Chimeric E2 constructs complementing cdc34 and rad6 mutants in yeast","pmids":["1639076","1639075"],"confidence":"High","gaps":["Biochemical role of the tail (binding vs. catalysis) not resolved","Tail binding partners unidentified"]},{"year":1994,"claim":"Identified Cdc34 self-association (dimerization) mediated by the C-terminal tail as a requirement for cell-cycle function, raising the question of whether oligomerization activates chain assembly.","evidence":"Chemical cross-linking and in vivo complementation of cdc34 mutants with tail deletions","pmids":["7929378"],"confidence":"High","gaps":["Whether dimerization is required for catalysis or SCF recruitment unclear","Stoichiometry at SCF not determined"]},{"year":1997,"claim":"Connected CDC34 to its first well-defined physiological substrate: Sic1 degradation requires Cdc34 and Cdc4, with phosphorylation by Cln/Cdc28 kinases triggering ubiquitination of the Sic1 N-terminus.","evidence":"In vitro reconstitution with DEAE-fractionated yeast extracts and cdc4-ts mutant extracts","pmids":["9285816"],"confidence":"High","gaps":["SCF complex composition not fully defined","Mechanism of phosphodegron recognition unclear"]},{"year":1998,"claim":"Defined the SCF architecture: Cdc53 scaffolds independent binding sites for Cdc34 and Skp1, and Skp1 bridges to F-box proteins (Cdc4, Met30, Grr1), establishing the modular E3 complex through which Cdc34 targets multiple substrates including Swe1.","evidence":"Reciprocal co-IP, two-hybrid, and genetic epistasis in yeast; Swe1 ubiquitination assays with mutant extracts","pmids":["9499404","9716410"],"confidence":"High","gaps":["RING subunit not yet identified","How Cdc34 is recruited to cullin not structurally resolved"]},{"year":1999,"claim":"Identified the RING protein Hrt1/Rbx1 as a critical SCF subunit that directly binds Cdc34 and stimulates E3 activity, completing the four-subunit SCF model and enabling full reconstitution of substrate ubiquitination.","evidence":"Mass spectrometry identification of Hrt1; recombinant reconstitution of Cln2 ubiquitination","pmids":["10385629"],"confidence":"High","gaps":["Structural basis of RING-Cdc34 interaction unknown","Whether RING activates Cdc34 allosterically or by proximity not resolved"]},{"year":2000,"claim":"Expanded the functional repertoire of Cdc34/SCF to include non-proteolytic ubiquitination: SCF(Met30)-mediated ubiquitination of Met4 represses transcription without triggering degradation, and Cdc34/SCF(βTrCP) catalyzes phosphorylation-dependent ubiquitination of IκBα.","evidence":"ChIP and metabolic labeling showing stable ubiquitinated Met4 at promoters; reconstitution of IκBα ubiquitination with recombinant components","pmids":["10975521","10918611"],"confidence":"High","gaps":["Mechanism distinguishing proteolytic from non-proteolytic ubiquitination unclear","Chain length determinants for proteasomal targeting unknown"]},{"year":2001,"claim":"Identified CK2 as the kinase phosphorylating CDC34 at five C-terminal sites, with phosphorylation controlling nuclear localization and linking proliferative signaling to E2 regulation.","evidence":"Yeast two-hybrid, co-IP with CK2β, in vitro kinase assay, phosphosite mutagenesis altering nuclear localization","pmids":["11546811"],"confidence":"High","gaps":["Whether phosphorylation affects catalytic activity not yet tested","Phosphorylation dynamics during cell cycle unknown"]},{"year":2003,"claim":"Demonstrated that ubiquitin charging triggers release of Cdc34 from the SCF RING domain, and that this dissociation is essential for substrate ubiquitination—explaining why Cdc34 must cycle on and off SCF during multisite ubiquitination.","evidence":"Kinetic binding measurements and F72V Cdc34 mutant with increased RING affinity that fails to ubiquitinate Sic1","pmids":["13678584"],"confidence":"High","gaps":["How released Cdc34~Ub reaches the substrate lysine not visualized","Whether multiple Cdc34 molecules act simultaneously unclear"]},{"year":2005,"claim":"Separated Cdc34 catalysis into two kinetically distinct steps—slow first-ubiquitin attachment and rapid processive chain elongation—and identified the acidic loop as selectively required for processivity and K48-linkage specificity during elongation.","evidence":"In vitro reconstitution with purified recombinant SCF(Cdc4)/Cdc34; acidic loop mutagenesis; kinetic analysis","pmids":["16360039"],"confidence":"High","gaps":["Structural basis for how the acidic loop orients acceptor ubiquitin not determined","Whether the two-step model applies to all SCF substrates unclear"]},{"year":2007,"claim":"Mapped distinct catalytic determinants for monoubiquitination versus polyubiquitination on Cdc34, and showed that CK2 phosphorylation of the C-terminal tail stimulates SCF-dependent ubiquitination activity in vitro and cell-cycle progression in vivo.","evidence":"Systematic mutagenesis of Cdc34 catalytic core and charged stretch with IκBα reconstitution; CK2 phosphorylation of tail sites and Sic1 ubiquitination assays; yeast cell-cycle synchronization","pmids":["17698585","17461777"],"confidence":"High","gaps":["Whether tail phosphorylation affects SCF binding affinity not quantified","Structural model of phosphorylated tail at SCF lacking"]},{"year":2008,"claim":"Extended CK2 regulation to the Cdc34 catalytic domain itself: phosphorylation at S130/S167 stimulates ubiquitin charging (E1→E2 transfer), and these sites are essential for cell viability.","evidence":"MS-confirmed phosphosites; ubiquitin-charging assay; complementation of cdc34-2ts yeast","pmids":["18418079"],"confidence":"High","gaps":["Structural mechanism by which catalytic-domain phosphorylation enhances charging unknown","Interplay between tail and core phosphorylation not dissected"]},{"year":2009,"claim":"Demonstrated that the acidic C-terminal tail contributes both sub-micromolar SCF binding affinity and catalytic function through its net negative charge, with the tail contacting the SCF basic canyon—and that let-7 miRNA post-transcriptionally regulates CDC34 levels to control Wee1 stability and G2/M progression.","evidence":"Binding affinity measurements and tail charge mutants; Cdc34-Cul1 fusion proteins; let-7 3′-UTR reporter assays and Wee1 epistasis by siRNA","pmids":["19875449","19126550"],"confidence":"High","gaps":["Atomic resolution structure of tail–canyon interaction not available","Physiological significance of let-7 regulation in vivo not confirmed in animal models"]},{"year":2010,"claim":"Revealed a two-E2 relay for IκBα ubiquitination—UbcH5 primes monoubiquitination, then Cdc34 elongates K48-linked chains—and mapped two non-covalent ubiquitin-binding sites in the Cdc34 C-terminus by NMR, with Y210 required for donor ubiquitin transfer.","evidence":"Reconstitution with UbcH5 and Cdc34 on IκBα; NMR chemical shift perturbation mapping of UBS1/UBS2; mutagenesis","pmids":["20347421","20353940"],"confidence":"High","gaps":["Whether two-E2 relay is general for all SCF substrates unknown","How UBS sites coordinate with acidic loop during chain elongation not structurally resolved"]},{"year":2011,"claim":"Provided structural and pharmacological insights: NMR showed the disordered C-terminus contacts thioester-bound ubiquitin in a closed conformation directing K48 specificity; ubiquitin I44 promotes acceptor lysine deprotonation; and the allosteric inhibitor CC0651 blocks ubiquitin discharge from a cryptic pocket, stabilizing p27 in cells.","evidence":"NMR of Cdc34~Ub disulfide mimetic; compensatory mutagenesis with pH-dependence; crystal structure of Cdc34-CC0651 complex; cell-based p27 accumulation","pmids":["21296085","21474069","21683433"],"confidence":"High","gaps":["CC0651 selectivity mechanism across E2 family not fully explained","Full closed-state structure of Cdc34~Ub at SCF not determined"]},{"year":2019,"claim":"Crystal structures of Cdc34 alone, with E1, and as a Ub thioester mimetic revealed a unique E1-binding mode requiring conformational changes in both enzymes for transthiolation, and confirmed the C-terminal extension stabilizes a closed conformation critical for ubiquitin discharge.","evidence":"X-ray crystallography of three Cdc34 states; mutagenesis; cell-based assays","pmids":["31341161"],"confidence":"High","gaps":["No structure of Cdc34 bound to an intact SCF-substrate complex","Dynamics of conformational switching during catalytic cycle not captured"]},{"year":null,"claim":"A complete structural view of Cdc34 engaged with an SCF-substrate complex during catalysis, the mechanistic basis for how CK2 phosphorylation at both core and tail sites coordinately regulate activity, and the generality of the two-E2 priming/elongation relay across SCF substrates remain open questions.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No cryo-EM or crystal structure of Cdc34~Ub bound to SCF with substrate","Relative contributions of core vs. tail CK2 phosphorylation not quantitatively dissected in vivo","Whether two-E2 relay (UbcH5 priming → Cdc34 elongation) applies to substrates beyond IκBα not established"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,3,25,27,31]},{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,25,34]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[5,20,22]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[20]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[0,9,14,39]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[1,25,27,31,35]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[18,41]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[18]}],"complexes":["SCF (Skp1-Cul1-F-box-Rbx1)"],"partners":["CUL1","SKP1","RBX1","CDC4","MET30","CSNK2B","BTRC"],"other_free_text":[]},"mechanistic_narrative":"CDC34 (Ube2R1) is a ubiquitin-conjugating enzyme (E2) that functions as the dedicated E2 partner of SCF (Skp1–Cullin–F-box) cullin-RING E3 ligase complexes to drive Lys48-linked polyubiquitination and proteasomal degradation of cell-cycle regulators and signaling proteins, thereby controlling the G1/S transition, S-phase entry, and mitotic timing [PMID:2842867, PMID:9285816, PMID:9836638]. CDC34 catalyzes ubiquitin transfer in two mechanistically distinct steps—rate-limiting attachment of the first ubiquitin to substrate, followed by rapid processive K48-linked chain elongation—with the conserved acidic loop conferring linkage specificity and processivity, and an intrinsically disordered acidic C-terminal tail mediating electrostatic docking to the SCF basic canyon and intramolecular contact with the thioester-linked ubiquitin to orient chain assembly [PMID:16360039, PMID:19875449, PMID:21296085]. CK2 phosphorylation of both the catalytic domain (S130, S167) and the C-terminal tail stimulates ubiquitin charging and SCF-dependent ubiquitination activity, coupling CDC34 function to proliferative signaling [PMID:11546811, PMID:18418079]. Through partnership with distinct F-box substrate receptors, CDC34/SCF targets Sic1, p27, Wee1, Swe1, IκBα, Met4, and other substrates for ubiquitin-dependent regulation, and a selective allosteric inhibitor (CC0651) blocks ubiquitin discharge and stabilizes p27 in cells [PMID:10385629, PMID:21683433]."},"prefetch_data":{"uniprot":{"accession":"P49427","full_name":"Ubiquitin-conjugating enzyme E2 R1","aliases":["(E3-independent) E2 ubiquitin-conjugating enzyme R1","E2 ubiquitin-conjugating enzyme R1","Ubiquitin-conjugating enzyme E2-32 kDa complementing","Ubiquitin-conjugating enzyme E2-CDC34","Ubiquitin-protein ligase R1"],"length_aa":236,"mass_kda":26.7,"function":"E2 ubiquitin-conjugating enzyme that accepts ubiquitin from an E1 ubiquitin-activating protein, and catalyzes its covalent attachment to other proteins by an E3 ubiquitin-protein ligase complex (PubMed:10329681, PubMed:17588522, PubMed:20061386, PubMed:38326650). In vitro catalyzes 'Lys-48'-linked polyubiquitination (PubMed:22496338). Cooperates with the E2 UBCH5C and the SCF(FBXW11) E3 ligase complex for the polyubiquitination of NFKBIA leading to its subsequent proteasomal degradation (PubMed:10329681, PubMed:10918611, PubMed:17698585). Performs ubiquitin chain elongation building ubiquitin chains from the UBE2D3-primed NFKBIA-linked ubiquitin. UBE2D3 acts as an initiator E2, priming the phosphorylated NFKBIA target at positions 'Lys-21' and/or 'Lys-22' with a monoubiquitin. Cooperates with the SCF(SKP2) E3 ligase complex to regulate cell proliferation through ubiquitination and degradation of MYBL2 and KIP1 (PubMed:10871850, PubMed:15652359, PubMed:19112177). Involved in ubiquitin conjugation and degradation of CREM isoform ICERIIgamma and ATF15 resulting in abrogation of ICERIIgamma- and ATF5-mediated repression of cAMP-induced transcription during both meiotic and mitotic cell cycles. Involved in the regulation of the cell cycle G2/M phase through its targeting of the WEE1 kinase for ubiquitination and degradation (PubMed:19126550). Also involved in the degradation of beta-catenin (PubMed:12037680). Is target of human herpes virus 1 protein ICP0, leading to ICP0-dependent dynamic interaction with proteasomes (PubMed:11805320, PubMed:12060736)","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/P49427/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/CDC34","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/CDC34","total_profiled":1310},"omim":[{"mim_id":"613336","title":"MEMBRANE-ASSOCIATED RING-CH FINGER PROTEIN 9; MARCHF9","url":"https://www.omim.org/entry/613336"},{"mim_id":"613335","title":"MEMBRANE-ASSOCIATED RING-CH FINGER PROTEIN 8; MARCHF8","url":"https://www.omim.org/entry/613335"},{"mim_id":"613332","title":"MEMBRANE-ASSOCIATED RING-CH FINGER PROTEIN 2; MARCHF2","url":"https://www.omim.org/entry/613332"},{"mim_id":"613331","title":"MEMBRANE-ASSOCIATED RING-CH FINGER PROTEIN 1; MARCHF1","url":"https://www.omim.org/entry/613331"},{"mim_id":"612598","title":"RING FINGER PROTEIN 11; RNF11","url":"https://www.omim.org/entry/612598"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear speckles","reliability":"Supported"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/CDC34"},"hgnc":{"alias_symbol":["E2-CDC34","UBE2R1","UBC3"],"prev_symbol":[]},"alphafold":{"accession":"P49427","domains":[{"cath_id":"3.10.110.10","chopping":"7-189","consensus_level":"high","plddt":94.9444,"start":7,"end":189}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P49427","model_url":"https://alphafold.ebi.ac.uk/files/AF-P49427-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P49427-F1-predicted_aligned_error_v6.png","plddt_mean":85.06},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=CDC34","jax_strain_url":"https://www.jax.org/strain/search?query=CDC34"},"sequence":{"accession":"P49427","fasta_url":"https://rest.uniprot.org/uniprotkb/P49427.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P49427/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P49427"}},"corpus_meta":[{"pmid":"2842867","id":"PMC_2842867","title":"The yeast cell cycle 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vitro, demonstrating E2 activity.\",\n      \"method\": \"In vitro ubiquitination assay with bacterially expressed protein; sequence homology to RAD6\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution, foundational paper with 401 citations, replicated by subsequent work\",\n      \"pmids\": [\"2842867\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"Cdc34 (and its rabbit homolog E2(32K)) preferentially catalyze processive multiubiquitination via Lys-48 of ubiquitin, distinguishing them from RAD6/E2(20K) which use non-Lys48 linkages; Cdc34 shows specificity for BSA rather than core histones.\",\n      \"method\": \"In vitro ubiquitin conjugation assay with native, reductively methylated, and K48R ubiquitin variants; kinetic analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical reconstitution with multiple ubiquitin variants, clear mechanistic distinction\",\n      \"pmids\": [\"1848239\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1992,\n      \"finding\": \"The 125-residue C-terminal tail of Cdc34 (specifically residues 171–244) is a portable determinant of cell cycle function; transplanting this tail onto the RAD6 catalytic domain creates a chimeric E2 that performs both CDC34 and RAD6 functions in vivo.\",\n      \"method\": \"Chimeric E2 constructs expressed in yeast; in vivo complementation of cdc34 and rad6 mutants; deletion analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic complementation with chimeric constructs, replicated in companion paper (Silver et al.)\",\n      \"pmids\": [\"1639076\", \"1639075\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"Bacterially expressed Cdc34 catalyzes its own ubiquitination (autoubiquitination) via intramolecular transfer forming predominantly a single Lys48-linked multiubiquitin chain, with major linkage sites mapping to C-terminal lysines (K273, K277, K293, K294).\",\n      \"method\": \"In vitro autoubiquitination assay; hydroxylamine cleavage; site-directed mutagenesis of C-terminal lysines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with mutagenesis, defines linkage specificity mechanistically\",\n      \"pmids\": [\"8383676\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The Cdc34 C-terminal tail (39 residues adjacent to the catalytic domain) mediates Cdc34 self-association (dimerization) in vitro and in vivo, and this self-association is required for cell cycle function.\",\n      \"method\": \"Chemical cross-linking; biophysical analysis; in vivo phenotypic analysis of CDC34 derivatives in cdc34 mutant strains\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — cross-linking combined with in vivo complementation, multiple methods\",\n      \"pmids\": [\"7929378\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"Cdc34 (Ubc3) is itself a substrate for both ubiquitination and phosphorylation in vivo; immunochemical localization places it in the nucleus, suggesting nuclear substrates.\",\n      \"method\": \"In vivo labeling; immunochemical localization\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — direct detection in vivo but single lab, no mechanistic follow-up on the modification writers\",\n      \"pmids\": [\"8164658\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Genetic and biochemical evidence demonstrates a noncovalent ubiquitin-binding site on Cdc34; overexpression of ubiquitin suppresses cdc34 temperature-sensitive alleles in an allele-specific manner, and chemical cross-linking confirms a specific noncovalent Ub-Cdc34 interaction.\",\n      \"method\": \"Genetic suppression screen; chemical cross-linking; in vivo ubiquitin overexpression\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal genetic and biochemical methods, single lab\",\n      \"pmids\": [\"7721857\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Dual mutation of CDC34 active-site cysteine (C95S) and conserved Leu99 (L99S) generates a dominant-negative Cdc34 that blocks cell growth and inhibits in vitro ubiquitination of the Cdc34 substrate Cln2.\",\n      \"method\": \"Site-directed mutagenesis; in vitro ubiquitination assay; overexpression growth assay in yeast\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — active-site mutagenesis combined with in vitro reconstitution and in vivo phenotype\",\n      \"pmids\": [\"7592826\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1995,\n      \"finding\": \"Kinetochore protein Cbf2p (Ndc10p) is ubiquitinated in vivo by Cdc34; purified Cdc34 catalyzes Cbf2p-monoubiquitin conjugate formation in vitro; overexpression of CDC34 suppresses the ndc10-1 temperature-sensitive mutation.\",\n      \"method\": \"In vitro ubiquitination assay with purified proteins; in vivo anti-ubiquitin immunoprecipitation; genetic suppression\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro reconstitution plus in vivo evidence, single lab\",\n      \"pmids\": [\"7651401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Multiubiquitination of Sic1 requires cyclin/Cdc28 protein kinase activity, the Cdc34 E2, and Cdc4; the N-terminal 160 residues of Sic1 are necessary and sufficient for Cdc34-dependent ubiquitination.\",\n      \"method\": \"In vitro reconstitution with DEAE-fractionated yeast extracts; cdc4ts mutant extracts; SIC1 deletion analysis\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical reconstitution with genetic mutant validation and substrate mapping\",\n      \"pmids\": [\"9285816\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Cdc34 is required for initiation of DNA replication in Xenopus egg extracts; Cdc34p in a large molecular size complex regulates initiation function of Cdk2-cyclin E, likely through degradation of the Xenopus CDK inhibitor Xic1.\",\n      \"method\": \"Xenopus egg extract depletion/add-back experiments; sizing column fractionation\",\n      \"journal\": \"Science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional rescue in a cell-free system, single lab\",\n      \"pmids\": [\"9287222\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Cdc53 functions as a scaffold protein within the SCF E3 complex, containing independent binding sites for Cdc34 and Skp1; Skp1 bridges Cdc53 to three F-box proteins (Cdc4, Met30, Grr1), conferring substrate specificity on a common Cdc34-Cdc53-Skp1 E2/E3 core.\",\n      \"method\": \"In vivo co-immunoprecipitation; two-hybrid; genetic epistasis\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP and genetic analysis, multiple F-box proteins tested, high citations\",\n      \"pmids\": [\"9499404\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Cdc34 and the F-box protein Met30 are required for degradation of the Cdk-inhibitory kinase Swe1; Met30 physically binds Swe1 in vivo and extracts from cdc34 or met30 mutants fail to polyubiquitinate Swe1.\",\n      \"method\": \"GST pull-down; in vitro ubiquitination assay with mutant extracts; genetic interaction screen\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods: pulldown, in vitro ubiquitination, genetics\",\n      \"pmids\": [\"9716410\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Human CDC34 associates in vivo with CUL-1 and the F-box protein p45(SKP2) as part of an SCF-type E3 ubiquitin ligase complex, establishing conservation of the SCF pathway in human cells.\",\n      \"method\": \"Co-immunoprecipitation in vivo from human cells\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — co-IP in vivo, single lab, no in vitro reconstitution\",\n      \"pmids\": [\"9430629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Cdc34-mediated degradation of Wee1 kinase in Xenopus egg extracts is required for timely entry into mitosis; this proteolysis is inhibited when DNA replication is blocked, linking the DNA replication checkpoint to Wee1 stability.\",\n      \"method\": \"Xenopus egg extract biochemical assay; Cdc34 immunodepletion; DNA replication block\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution in cell-free system with immunodepletion, clear mechanistic link\",\n      \"pmids\": [\"9836638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"The RING-H2 protein Hrt1 (Rbx1/Roc1) is a subunit of SCF identified by mass spectrometry; it binds Cdc34 directly, stimulates SCF E3 activity, and enables reconstitution of Cln2 ubiquitination; SCF and Cdc53/Hrt1 activate Cdc34 autoubiquitination by a mechanism independent of reactive thiols.\",\n      \"method\": \"Mass spectrometry; recombinant protein reconstitution; in vitro ubiquitination assay; conditional genetic inactivation\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — MS identification + reconstitution + mutagenesis, multiple orthogonal methods, >350 citations\",\n      \"pmids\": [\"10385629\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Human Cdc34 ubiquitinates the transcription factor repressors hICERIIγ and hATF5 in mammalian cells; both hCdc34- and hRad6B-dependent ubiquitin-mediated proteolysis abrogates their transcriptional repression of cAMP-induced genes.\",\n      \"method\": \"Transfection assay; dominant-negative and antisense Cdc34 constructs; ubiquitination assay in mammalian cells\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — dominant-negative and antisense approaches with functional readout, single lab\",\n      \"pmids\": [\"10373550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Cdc34/SCF(Met30) ubiquitinates the transcription factor Met4, causing transcriptional repression without proteolysis; ubiquitinated Met4 associates with target promoters but fails to form functional transcription complexes; deletion of MET4 suppresses lethality of met30 mutants.\",\n      \"method\": \"Genetic epistasis (MET4 deletion suppression); chromatin immunoprecipitation; metabolic labeling to assess Met4 stability\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic suppression + biochemical analysis of protein stability + ChIP, multiple methods, >240 citations\",\n      \"pmids\": [\"10975521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"SCFβ-TRCP together with Cdc34 (Ubc3) catalyzes phosphorylation-dependent ubiquitination of IκBα; Ubc4 is also capable but is ~19-fold more efficient in THP.1 cells; Cdc34 associates with SCFβ-TRCP isolated from human cells.\",\n      \"method\": \"In vitro reconstitution with recombinant components; ubiquitination assay; pull-down of Cdc34 from human cell extracts\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical reconstitution with recombinant components\",\n      \"pmids\": [\"10918611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Herpes simplex virus 1 ICP0 binds Cdc34 via its RING finger (exon 2) and acts as an E3 ubiquitin ligase that promotes ubiquitination of Cdc34 in vitro; in infected cells, Cdc34 undergoes increased ICP0-dependent dynamic interaction with proteasomes.\",\n      \"method\": \"In vitro ubiquitination assay; RING-domain binding assay; co-immunoprecipitation in infected cells\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — in vitro ubiquitination plus in vivo co-IP, single lab\",\n      \"pmids\": [\"11447293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human CDC34 protein is localized to distinct nuclear and cytoplasmic speckles during interphase; nuclear localization depends on specific C-terminal CDC34 sequences; in anaphase CDC34 colocalizes with β-tubulin at the mitotic spindle.\",\n      \"method\": \"Immunofluorescence; subcellular fractionation; deletion mutant analysis\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct localization by immunofluorescence and fractionation, with sequence determinant identification\",\n      \"pmids\": [\"10769200\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Elevated Cdc34 protein levels at prophase selectively block CENP-E kinesin from associating with kinetochores, causing chromosome congression failure and prometaphase arrest; this effect is not rescued by proteasome inhibitors, suggesting a non-proteolytic ubiquitination role.\",\n      \"method\": \"Microinjection of bacterially expressed Cdc34 into mammalian cells; immunofluorescence; electron microscopy of kinetochores\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct injection experiment with specific phenotypic and molecular readout, single lab\",\n      \"pmids\": [\"11514588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"Human CDC34 is phosphorylated in proliferating cells; the regulatory β-subunit of CK2 interacts with CDC34 in vivo; recombinant CK2 phosphorylates CDC34 at five C-terminal sites (S203, S222, S231, T233, S236); mutating these sites abolishes in vivo phosphorylation and shifts nuclear CDC34 to the cytoplasm.\",\n      \"method\": \"Yeast two-hybrid; co-immunoprecipitation in transfected cells; in vitro CK2 phosphorylation; site-directed mutagenesis; immunofluorescence\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods identifying writer kinase, phosphorylation sites, and functional consequence on localization\",\n      \"pmids\": [\"11546811\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Formation of the Cdc34-ubiquitin thioester increases the dissociation rate of Cdc34 from the SCF RING domain; release of ubiquitin-charged Cdc34 from the RING is essential for ubiquitination of the SCF(Cdc4)-bound substrate Sic1, as shown by F72V Cdc34 mutant with increased RING affinity that is unable to ubiquitinate Sic1.\",\n      \"method\": \"Kinetic binding measurements; mutagenesis (F72V Cdc34); in vitro ubiquitination assay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with engineered affinity mutant and kinetic measurements, defines a mechanistic step\",\n      \"pmids\": [\"13678584\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Cdc34 self-association in vivo depends on integrity of the Cdc34-ubiquitin thioester and key catalytic domain residues (active-site C95, S73, S97, and the catalytic domain insertion); self-association is required for multi-ubiquitin chain assembly and cell cycle function.\",\n      \"method\": \"Co-immunoprecipitation; site-directed mutagenesis of catalytic residues; in vivo complementation\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP with systematic mutagenesis, mechanistically links thioester formation to self-association and chain assembly\",\n      \"pmids\": [\"12861024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Sic1 ubiquitination by SCF(Cdc4)/Cdc34 occurs in two mechanistically distinct steps: slow rate-limiting attachment of the first ubiquitin, followed by rapid processive elongation of K48-linked chains; an acidic loop conserved in Cdc34 orthologs is required for processivity and K48-linkage specificity during chain elongation but not for first ubiquitin attachment.\",\n      \"method\": \"In vitro ubiquitination assay with purified recombinant proteins; acidic loop mutagenesis; kinetic analysis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution + mutagenesis separating two mechanistic steps, >230 citations\",\n      \"pmids\": [\"16360039\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Proximity-induced dimerization of human Cdc34 (via GST fusion or FKBP-AP20187 chemical dimerization) constitutively activates K48-linked polyubiquitin chain synthesis independently of SCF, suggesting SCF may activate Cdc34 by converting it from an inactive monomer to an active dimer.\",\n      \"method\": \"GST-fusion dimerization; chemical inducer of dimerization (FKBP-AP20187); in vitro ubiquitin chain synthesis assay\",\n      \"journal\": \"PNAS\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with two dimerization strategies, single lab\",\n      \"pmids\": [\"16210246\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Human Cdc34 employs distinct sites for mono- vs. polyubiquitination: the conserved charged stretch (residues 143–153) and acidic loop residues D102/D103 are required for Ub-Ub ligation (polychain assembly) but not for substrate monoubiquitination; N85 and S138 coordinate the attacking lysine for catalysis in both steps.\",\n      \"method\": \"Site-directed mutagenesis; in vitro ubiquitination assay with IκBα as substrate; GST-Cdc34 dimerization rescue experiments\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis with in vitro reconstitution, multiple distinct functional sites mapped\",\n      \"pmids\": [\"17698585\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"CK2 phosphorylates yeast Cdc34 C-terminal tail at S207 and S216 (and human Cdc34 at S203, S222, S231) in vitro; this phosphorylation stimulates Cdc34 ubiquitination activity toward Sic1 in SCFCdc4-dependent reactions; phosphosite alanine mutants with altered activity show corresponding changes in Sic1 degradation and cell cycle progression speed.\",\n      \"method\": \"In vitro CK2 phosphorylation; in vitro ubiquitination assay; yeast cell cycle synchronization and progression assay with cdc34 phosphosite mutants\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro kinase assay + in vitro ubiquitination + in vivo cell cycle phenotype, single lab but multiple methods\",\n      \"pmids\": [\"17461777\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"CK2 phosphorylates the catalytic domain of Cdc34 at S130 and S167 in vitro and in vivo; these phosphorylation events strongly stimulate Cdc34 ubiquitin-charging activity; the S130A/S167A double mutant fails to complement a cdc34-2ts strain.\",\n      \"method\": \"Mass spectrometry; in vitro CK2 phosphorylation; ubiquitin charging assay; yeast complementation in cka1Δcka2-8ts background\",\n      \"journal\": \"Cell cycle\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — MS-confirmed phosphosites, in vitro activity assay, in vivo complementation, single lab\",\n      \"pmids\": [\"18418079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The Cdc34 acidic C-terminal tail contributes both to submicromolar binding affinity for SCF(Cdc4) and to catalysis; the functional requirement for the tail is its acidity; Cdc34 retains partial function when fused to the C-terminus of Cul1 even without the tail.\",\n      \"method\": \"Binding affinity measurements; in vitro ubiquitination assay; tail deletion and charge mutants; Cdc34-Cul1 fusion proteins\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with multiple mutants and fusion proteins separating binding vs. catalytic roles\",\n      \"pmids\": [\"19875449\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Polyubiquitination of IκBα by SCF(βTrCP2) begins with rapid monoubiquitination by UbcH5 at K21/K22, followed by Cdc34-mediated K48-linked polyubiquitin chain elongation using the substrate-linked ubiquitin as a receptor; the IκBα K21-linked ubiquitin makes direct contacts with Cdc34 and the SCF RING subcomplex.\",\n      \"method\": \"Biochemical reconstitution with UbcH5 and Cdc34; ubiquitin-fused substrate constructs; in vitro ubiquitination assay\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution defining mechanistic handoff between two E2 enzymes\",\n      \"pmids\": [\"20347421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The human Cdc34 C-terminus contains two non-covalent ubiquitin-binding sites (UBS1: residues 206–215; UBS2: 216–225) mapped by NMR; UBS1 aromatic residues (F206, Y207, Y210, Y211) interact near ubiquitin Lys48 and C-terminus; Y210 is specifically required for donor ubiquitin transfer in SCF-dependent reactions.\",\n      \"method\": \"NMR chemical shift perturbation; mutagenesis; in vitro IκBα ubiquitination reconstitution\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structural mapping plus in vitro functional reconstitution with mutagenesis\",\n      \"pmids\": [\"20353940\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Residues surrounding acceptor lysines on Sic1 and ubiquitin are critical for Cdc34-mediated ubiquitination independent of SCF; key residues composing the Cdc34 catalytic core alter lysine preference and determine whether Cdc34 monoubiquitinates or polyubiquitinates Sic1.\",\n      \"method\": \"In vitro ubiquitination assay; mutagenesis of Cdc34 catalytic core residues and substrate lysine-flanking residues\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — systematic mutagenesis combined with in vitro reconstitution\",\n      \"pmids\": [\"20194622\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The I44A mutation in ubiquitin profoundly inhibits donor ubiquitin discharge from Cdc34; computationally predicted compensatory mutations in Cdc34 rescue this defect; the Cdc34-ubiquitin interaction at I44 promotes efficient deprotonation of the acceptor lysine for ubiquitin transfer.\",\n      \"method\": \"In vitro ubiquitination assay; site-directed mutagenesis (ubiquitin I44A and Cdc34 compensatory mutants); hydroxylamine acceptor experiments at varying pH\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstitution with rationally designed compensatory mutations and pH-dependence experiments\",\n      \"pmids\": [\"21474069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"A small molecule CC0651 selectively inhibits human Cdc34 by inserting into a cryptic allosteric pocket distant from the catalytic site, causing conformational changes in E2 secondary structure; it does not affect E1 charging or E3 binding but blocks ubiquitin discharge to acceptor lysines; cells show p27(Kip1) accumulation.\",\n      \"method\": \"Crystal structure of Cdc34-CC0651 complex; in vitro ubiquitination assay; cell-based p27 accumulation assay; thioester formation assay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure + in vitro activity assay + cell-based validation, defines allosteric inhibition mechanism\",\n      \"pmids\": [\"21683433\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"The CDC34 C-terminus is intrinsically disordered but intramolecularly interacts with the catalytically bound ubiquitin in the Cdc34-Ub thioester complex; the C-terminus contacts a lysine-rich face of ubiquitin (K6, K11, K29, K33) in a two-state equilibrium, suggesting a role in directing K48-linked chain formation.\",\n      \"method\": \"NMR spectroscopy; CDC34-Ub disulfide mimetic; chemical shift perturbation analysis\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structural characterization of thioester mimetic with functional implications\",\n      \"pmids\": [\"21296085\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"The Cdc34 acidic loop promotes SCF-Cdc34 interaction and suppresses the pKa of an ionizing species on ubiquitin or Cdc34; two glutamic acid residues on the distal side collaborate with a conserved histidine on the proximal side to promote catalysis.\",\n      \"method\": \"In vitro ubiquitination assay; mutagenesis; pKa measurements; quantification of E2-E3 binding\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis with in vitro reconstitution defining two distinct mechanistic roles for the loop\",\n      \"pmids\": [\"24129577\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"The Cdc34-SCF interaction occurs in multiple conformations where several residues from the Cdc34 acidic C-terminal tail contact a broad region of the SCF basic canyon; similar contacts occur with Cul2, implicating a conserved mechanism across cullin-RING ligases.\",\n      \"method\": \"Protein cross-linking followed by mass spectrometry; binding interface mapping\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cross-linking mass spectrometry, single lab\",\n      \"pmids\": [\"25425648\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"let-7 microRNA directly down-regulates Cdc34 via its 3′-UTR, reducing Cdc34 protein levels, stabilizing the SCF substrate Wee1 kinase, and causing G2/M accumulation in primary human fibroblasts; siRNA knockdown of Wee1 reverses the G2/M phenotype.\",\n      \"method\": \"Luciferase reporter assay (3′-UTR); microarray; siRNA knockdown; Western blot; flow cytometry\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct reporter assay for miRNA targeting + epistasis with Wee1 siRNA, single lab\",\n      \"pmids\": [\"19126550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Crystal structures of Cdc34 alone, in complex with E1 (Uba1), and as a Cdc34~Ub thioester mimetic reveal: unique E1-binding mode requiring conformational changes in both Uba1 and Cdc34 for transthiolation; the C-terminal extension contacts the catalytically bound ubiquitin, stabilizing a closed conformation critical for ubiquitin discharge.\",\n      \"method\": \"X-ray crystallography (Cdc34 alone, Cdc34-E1 complex, Cdc34~Ub mimetic); mutagenesis; cell-based assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple crystal structures with functional validation by mutagenesis and cell assays\",\n      \"pmids\": [\"31341161\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"CDC34 competes with the E3 ligase c-Cbl to bind EGFR at Y1045, thereby inhibiting EGFR polyubiquitination and degradation and promoting lung carcinogenesis; knockdown of CDC34 inhibits NSCLC tumor formation in mouse models.\",\n      \"method\": \"siRNA library screen; co-immunoprecipitation; overexpression/knockdown in vitro and in vivo xenograft model\",\n      \"journal\": \"EBioMedicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — co-IP defining competition mechanism plus in vivo mouse model, single lab\",\n      \"pmids\": [\"32114396\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"COP9 signalosome (CSN) protects UBC3/Cdc34 from SCF(βTrCP)-mediated proteasomal degradation; downregulation of CSN4 or CSN5 induces ubiquitination and degradation of Cdc34; this requires the acidic C-terminal extension of Cdc34 which is sufficient to impart SCF(βTrCP)-sensitivity to UBCH5 enzymes.\",\n      \"method\": \"RNAi knockdown of CSN subunits; domain swap (acidic tail transplant onto UBCH5); proteasome inhibitor experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — domain swap and RNAi defining specific interaction, single lab\",\n      \"pmids\": [\"20378537\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"CDC34/Ube2R1 is a Lys48-specific ubiquitin-conjugating enzyme (E2) that functions as part of SCF cullin-RING E3 complexes to drive proteasomal degradation of key cell cycle regulators (Sic1, Cln2, Wee1, p27, IκBα, Met4, and others) by catalyzing processive K48-linked polyubiquitin chain synthesis via a two-step mechanism (rate-limiting first-Ub attachment followed by rapid chain elongation), with its unique acidic C-terminal tail mediating electrostatic binding to the SCF basic canyon and promoting catalysis, its acidic loop controlling K48-linkage specificity and polyubiquitin processivity, CK2-mediated phosphorylation of both the catalytic domain and tail stimulating E2 charging and activity, and release of ubiquitin-charged Cdc34 from the RING subunit being an essential step that permits multisite substrate ubiquitination.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"CDC34 (Ube2R1) is a ubiquitin-conjugating enzyme (E2) that functions as the dedicated E2 partner of SCF (Skp1–Cullin–F-box) cullin-RING E3 ligase complexes to drive Lys48-linked polyubiquitination and proteasomal degradation of cell-cycle regulators and signaling proteins, thereby controlling the G1/S transition, S-phase entry, and mitotic timing [PMID:2842867, PMID:9285816, PMID:9836638]. CDC34 catalyzes ubiquitin transfer in two mechanistically distinct steps—rate-limiting attachment of the first ubiquitin to substrate, followed by rapid processive K48-linked chain elongation—with the conserved acidic loop conferring linkage specificity and processivity, and an intrinsically disordered acidic C-terminal tail mediating electrostatic docking to the SCF basic canyon and intramolecular contact with the thioester-linked ubiquitin to orient chain assembly [PMID:16360039, PMID:19875449, PMID:21296085]. CK2 phosphorylation of both the catalytic domain (S130, S167) and the C-terminal tail stimulates ubiquitin charging and SCF-dependent ubiquitination activity, coupling CDC34 function to proliferative signaling [PMID:11546811, PMID:18418079]. Through partnership with distinct F-box substrate receptors, CDC34/SCF targets Sic1, p27, Wee1, Swe1, IκBα, Met4, and other substrates for ubiquitin-dependent regulation, and a selective allosteric inhibitor (CC0651) blocks ubiquitin discharge and stabilizes p27 in cells [PMID:10385629, PMID:21683433].\",\n  \"teleology\": [\n    {\n      \"year\": 1988,\n      \"claim\": \"Established that CDC34 encodes a ubiquitin-conjugating enzyme required for the G1/S transition, answering the fundamental question of what biochemical activity underlies CDC34 cell-cycle function.\",\n      \"evidence\": \"Bacterially expressed Cdc34 catalyzed ubiquitin attachment to histones in vitro; sequence homology to RAD6 E2\",\n      \"pmids\": [\"2842867\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological substrates unknown\", \"No E3 ligase partner identified\", \"Mechanism of cell cycle arrest not defined\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Demonstrated that Cdc34 preferentially assembles K48-linked polyubiquitin chains processively, distinguishing its linkage specificity from other E2s like RAD6.\",\n      \"evidence\": \"In vitro conjugation assays with wild-type and K48R ubiquitin variants; kinetic analysis\",\n      \"pmids\": [\"1848239\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for K48 specificity unknown\", \"No physiological substrate tested\"]\n    },\n    {\n      \"year\": 1992,\n      \"claim\": \"Revealed that the unique 125-residue C-terminal tail is a portable functional determinant sufficient to confer CDC34 cell-cycle activity when grafted onto RAD6, separating catalytic core from regulatory tail functions.\",\n      \"evidence\": \"Chimeric E2 constructs complementing cdc34 and rad6 mutants in yeast\",\n      \"pmids\": [\"1639076\", \"1639075\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Biochemical role of the tail (binding vs. catalysis) not resolved\", \"Tail binding partners unidentified\"]\n    },\n    {\n      \"year\": 1994,\n      \"claim\": \"Identified Cdc34 self-association (dimerization) mediated by the C-terminal tail as a requirement for cell-cycle function, raising the question of whether oligomerization activates chain assembly.\",\n      \"evidence\": \"Chemical cross-linking and in vivo complementation of cdc34 mutants with tail deletions\",\n      \"pmids\": [\"7929378\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether dimerization is required for catalysis or SCF recruitment unclear\", \"Stoichiometry at SCF not determined\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Connected CDC34 to its first well-defined physiological substrate: Sic1 degradation requires Cdc34 and Cdc4, with phosphorylation by Cln/Cdc28 kinases triggering ubiquitination of the Sic1 N-terminus.\",\n      \"evidence\": \"In vitro reconstitution with DEAE-fractionated yeast extracts and cdc4-ts mutant extracts\",\n      \"pmids\": [\"9285816\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SCF complex composition not fully defined\", \"Mechanism of phosphodegron recognition unclear\"]\n    },\n    {\n      \"year\": 1998,\n      \"claim\": \"Defined the SCF architecture: Cdc53 scaffolds independent binding sites for Cdc34 and Skp1, and Skp1 bridges to F-box proteins (Cdc4, Met30, Grr1), establishing the modular E3 complex through which Cdc34 targets multiple substrates including Swe1.\",\n      \"evidence\": \"Reciprocal co-IP, two-hybrid, and genetic epistasis in yeast; Swe1 ubiquitination assays with mutant extracts\",\n      \"pmids\": [\"9499404\", \"9716410\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"RING subunit not yet identified\", \"How Cdc34 is recruited to cullin not structurally resolved\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identified the RING protein Hrt1/Rbx1 as a critical SCF subunit that directly binds Cdc34 and stimulates E3 activity, completing the four-subunit SCF model and enabling full reconstitution of substrate ubiquitination.\",\n      \"evidence\": \"Mass spectrometry identification of Hrt1; recombinant reconstitution of Cln2 ubiquitination\",\n      \"pmids\": [\"10385629\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of RING-Cdc34 interaction unknown\", \"Whether RING activates Cdc34 allosterically or by proximity not resolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Expanded the functional repertoire of Cdc34/SCF to include non-proteolytic ubiquitination: SCF(Met30)-mediated ubiquitination of Met4 represses transcription without triggering degradation, and Cdc34/SCF(βTrCP) catalyzes phosphorylation-dependent ubiquitination of IκBα.\",\n      \"evidence\": \"ChIP and metabolic labeling showing stable ubiquitinated Met4 at promoters; reconstitution of IκBα ubiquitination with recombinant components\",\n      \"pmids\": [\"10975521\", \"10918611\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism distinguishing proteolytic from non-proteolytic ubiquitination unclear\", \"Chain length determinants for proteasomal targeting unknown\"]\n    },\n    {\n      \"year\": 2001,\n      \"claim\": \"Identified CK2 as the kinase phosphorylating CDC34 at five C-terminal sites, with phosphorylation controlling nuclear localization and linking proliferative signaling to E2 regulation.\",\n      \"evidence\": \"Yeast two-hybrid, co-IP with CK2β, in vitro kinase assay, phosphosite mutagenesis altering nuclear localization\",\n      \"pmids\": [\"11546811\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether phosphorylation affects catalytic activity not yet tested\", \"Phosphorylation dynamics during cell cycle unknown\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrated that ubiquitin charging triggers release of Cdc34 from the SCF RING domain, and that this dissociation is essential for substrate ubiquitination—explaining why Cdc34 must cycle on and off SCF during multisite ubiquitination.\",\n      \"evidence\": \"Kinetic binding measurements and F72V Cdc34 mutant with increased RING affinity that fails to ubiquitinate Sic1\",\n      \"pmids\": [\"13678584\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How released Cdc34~Ub reaches the substrate lysine not visualized\", \"Whether multiple Cdc34 molecules act simultaneously unclear\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Separated Cdc34 catalysis into two kinetically distinct steps—slow first-ubiquitin attachment and rapid processive chain elongation—and identified the acidic loop as selectively required for processivity and K48-linkage specificity during elongation.\",\n      \"evidence\": \"In vitro reconstitution with purified recombinant SCF(Cdc4)/Cdc34; acidic loop mutagenesis; kinetic analysis\",\n      \"pmids\": [\"16360039\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for how the acidic loop orients acceptor ubiquitin not determined\", \"Whether the two-step model applies to all SCF substrates unclear\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Mapped distinct catalytic determinants for monoubiquitination versus polyubiquitination on Cdc34, and showed that CK2 phosphorylation of the C-terminal tail stimulates SCF-dependent ubiquitination activity in vitro and cell-cycle progression in vivo.\",\n      \"evidence\": \"Systematic mutagenesis of Cdc34 catalytic core and charged stretch with IκBα reconstitution; CK2 phosphorylation of tail sites and Sic1 ubiquitination assays; yeast cell-cycle synchronization\",\n      \"pmids\": [\"17698585\", \"17461777\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether tail phosphorylation affects SCF binding affinity not quantified\", \"Structural model of phosphorylated tail at SCF lacking\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Extended CK2 regulation to the Cdc34 catalytic domain itself: phosphorylation at S130/S167 stimulates ubiquitin charging (E1→E2 transfer), and these sites are essential for cell viability.\",\n      \"evidence\": \"MS-confirmed phosphosites; ubiquitin-charging assay; complementation of cdc34-2ts yeast\",\n      \"pmids\": [\"18418079\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism by which catalytic-domain phosphorylation enhances charging unknown\", \"Interplay between tail and core phosphorylation not dissected\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrated that the acidic C-terminal tail contributes both sub-micromolar SCF binding affinity and catalytic function through its net negative charge, with the tail contacting the SCF basic canyon—and that let-7 miRNA post-transcriptionally regulates CDC34 levels to control Wee1 stability and G2/M progression.\",\n      \"evidence\": \"Binding affinity measurements and tail charge mutants; Cdc34-Cul1 fusion proteins; let-7 3′-UTR reporter assays and Wee1 epistasis by siRNA\",\n      \"pmids\": [\"19875449\", \"19126550\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic resolution structure of tail–canyon interaction not available\", \"Physiological significance of let-7 regulation in vivo not confirmed in animal models\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Revealed a two-E2 relay for IκBα ubiquitination—UbcH5 primes monoubiquitination, then Cdc34 elongates K48-linked chains—and mapped two non-covalent ubiquitin-binding sites in the Cdc34 C-terminus by NMR, with Y210 required for donor ubiquitin transfer.\",\n      \"evidence\": \"Reconstitution with UbcH5 and Cdc34 on IκBα; NMR chemical shift perturbation mapping of UBS1/UBS2; mutagenesis\",\n      \"pmids\": [\"20347421\", \"20353940\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether two-E2 relay is general for all SCF substrates unknown\", \"How UBS sites coordinate with acidic loop during chain elongation not structurally resolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Provided structural and pharmacological insights: NMR showed the disordered C-terminus contacts thioester-bound ubiquitin in a closed conformation directing K48 specificity; ubiquitin I44 promotes acceptor lysine deprotonation; and the allosteric inhibitor CC0651 blocks ubiquitin discharge from a cryptic pocket, stabilizing p27 in cells.\",\n      \"evidence\": \"NMR of Cdc34~Ub disulfide mimetic; compensatory mutagenesis with pH-dependence; crystal structure of Cdc34-CC0651 complex; cell-based p27 accumulation\",\n      \"pmids\": [\"21296085\", \"21474069\", \"21683433\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"CC0651 selectivity mechanism across E2 family not fully explained\", \"Full closed-state structure of Cdc34~Ub at SCF not determined\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Crystal structures of Cdc34 alone, with E1, and as a Ub thioester mimetic revealed a unique E1-binding mode requiring conformational changes in both enzymes for transthiolation, and confirmed the C-terminal extension stabilizes a closed conformation critical for ubiquitin discharge.\",\n      \"evidence\": \"X-ray crystallography of three Cdc34 states; mutagenesis; cell-based assays\",\n      \"pmids\": [\"31341161\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of Cdc34 bound to an intact SCF-substrate complex\", \"Dynamics of conformational switching during catalytic cycle not captured\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A complete structural view of Cdc34 engaged with an SCF-substrate complex during catalysis, the mechanistic basis for how CK2 phosphorylation at both core and tail sites coordinately regulate activity, and the generality of the two-E2 priming/elongation relay across SCF substrates remain open questions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No cryo-EM or crystal structure of Cdc34~Ub bound to SCF with substrate\", \"Relative contributions of core vs. tail CK2 phosphorylation not quantitatively dissected in vivo\", \"Whether two-E2 relay (UbcH5 priming → Cdc34 elongation) applies to substrates beyond IκBα not established\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 3, 25, 27, 31]},\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 25, 34]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [5, 20, 22]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [20]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 9, 14, 39]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [1, 25, 27, 31, 35]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [18, 41]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [18]}\n    ],\n    \"complexes\": [\n      \"SCF (Skp1-Cul1-F-box-Rbx1)\"\n    ],\n    \"partners\": [\n      \"CUL1\",\n      \"SKP1\",\n      \"RBX1\",\n      \"CDC4\",\n      \"MET30\",\n      \"CSNK2B\",\n      \"BTRC\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}