{"gene":"HPF1","run_date":"2026-06-10T01:55:22","timeline":{"discoveries":[{"year":2016,"finding":"HPF1 (C4orf27) forms a robust protein complex with PARP-1 in cells, is recruited to DNA lesions in a PARP-1-dependent but catalytic-activity-independent manner, promotes PARP-1-dependent in trans ADP-ribosylation of histones, and limits DNA damage-induced hyper-automodification of PARP-1. Loss of HPF1 sensitizes human cells to DNA damaging agents and PARP inhibition.","method":"Co-immunoprecipitation, laser micro-irradiation recruitment assays, in vitro ADP-ribosylation assays, HPF1 knockout cell lines with genotoxic sensitivity assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, in vitro functional assay, KO cell phenotype, multiple orthogonal methods in a focused study","pmids":["27067600"],"is_preprint":false},{"year":2017,"finding":"HPF1 is necessary and sufficient to redirect PARP-1 and PARP-2 ADP-ribosylation from aspartate/glutamate to serine residues. Adding HPF1 to in vitro PARP-1/PARP-2 reactions produces serine-linked ADP-ribosylation on histones and PARP-1 itself. Serine ADPr does not occur in cells lacking HPF1, and three endogenous serine ADPr sites were mapped to the PARP-1 automodification domain.","method":"Quantitative mass spectrometry-based proteomics of HPF1 knockout cells, in vitro PARP-1/PARP-2 ADP-ribosylation reconstitution with and without HPF1, site mapping by MS","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution showing necessity and sufficiency, complemented by quantitative proteomics in KO cells, replicated across multiple independent studies","pmids":["28190768"],"is_preprint":false},{"year":2020,"finding":"HPF1 forms a composite (joint) active site with the catalytic domain of PARP1 or PARP2 by contributing a catalytic glutamate residue (Glu284) that is essential for serine-specific ADP-ribosylation after DNA damage. The HPF1–PARP interaction is allosterically enhanced by occupancy of the NAD+-binding site and by DNA damage signals, providing an additional regulatory layer. This composite active site implicates HPF1 as a determinant of response to clinical PARP inhibitors.","method":"Crystal/co-structure of HPF1 bound to PARP2 catalytic domain, NMR, biochemical mutagenesis assays, cellular ADP-ribosylation assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus NMR plus mutagenesis plus cellular validation in a single rigorous study, independently replicated by Sun et al. 2021","pmids":["32028527"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structure of human PARP2–HPF1 bound to a nucleosome shows that PARP2–HPF1 bridges two nucleosomes with broken DNA aligned for ligation. DNA bridging induces structural changes in PARP2 that signal break recognition to the catalytic domain, licensing HPF1 binding and PARP2 activation. Active PARP2 cycles through different conformational states to exchange NAD+ and substrate.","method":"Cryo-electron microscopy, biochemical ADP-ribosylation activity assays with nucleosome substrates","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure with biochemical validation, high-resolution mechanistic detail from a single rigorous study","pmids":["32939087"],"is_preprint":false},{"year":2020,"finding":"HPF1 is required for a phospho-guided chemoenzymatic serine ADP-ribosylation reaction; the HPF1/PARP1 writer complex can be used to install authentic serine-linked ADP-ribose at defined positions on peptides in a scalable, precise manner.","method":"Chemoenzymatic in vitro reconstitution using HPF1/PARP1 complex, phage display antibody selection, mono-ADPr proteomics","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with biochemical validation, single lab, novel application confirming serine-specificity mechanism","pmids":["33186521"],"is_preprint":false},{"year":2020,"finding":"Cryo-EM structure of two nucleosomes bridged by PARP2 shows that the PARP2 conformation adopted upon damaged-chromatin binding provides a binding platform for HPF1, and the resulting HPF1•PARP2•nucleosome complex is enzymatically active.","method":"Cryo-electron microscopy, in vitro ADP-ribosylation activity assays","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure with biochemical activity confirmation, single lab study","pmids":["33141820"],"is_preprint":false},{"year":2021,"finding":"Crystal structures of human HPF1/PARP1-CAT complex and mutagenesis data confirm that HPF1 Arg239 salt-bridges to Glu284/Asp286 to position Glu284 as the catalytic base for serine ADP-ribosylation, maintains the local HPF1 conformation to limit PARP1 automodification, and facilitates HPF1/PARP1 binding by neutralizing negative charge at Glu284.","method":"X-ray crystallography (1.98 Å HPF1/PARP1-CAT; 1.57–1.71 Å HPF1 alone), site-directed mutagenesis, quantitative binding assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure plus mutagenesis of key residues, independent replication of Suskiewicz et al. 2020 mechanism for PARP1","pmids":["33589610"],"is_preprint":false},{"year":2021,"finding":"HPF1 efficiently regulates PARP1/2 at sub-stoichiometric ratios matching their relative cellular abundances via a 'hit-and-run' mechanism: HPF1 rapidly associates/dissociates from multiple PARP1 molecules, initiating serine modification before glutamate/aspartate modification initiates, and accelerating initiation to be more comparable to elongation. This ensures HPF1 contributions during initiation do not persist and interfere with PAR chain elongation.","method":"Biochemical kinetic assays (in vitro ADP-ribosylation with sub-stoichiometric HPF1), cryo-EM structural analysis of HPF1/PARP1 on DNA break, DNA retention assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with kinetic characterization, cryo-EM, and DNA retention assay; multiple orthogonal methods in one study","pmids":["34795260"],"is_preprint":false},{"year":2021,"finding":"HPF1 provides Glu284 as a catalytic base that substantially redirects PARylation by PARP1 such that histones in nucleosomes become primary recipients of PAR chains. Surprisingly, HPF1 partitions most reaction product to free ADP-ribose (ADPR), resulting in much shorter PAR chains — a switch from PARP1 polymerase to hydrolase activity.","method":"In vitro PARP1 reconstitution with nucleosomes as activators and substrates, product analysis by TLC and gel electrophoresis, mutagenesis","journal":"eLife","confidence":"High","confidence_rationale":"Tier 1 / Moderate — rigorous in vitro reconstitution with nucleosome substrates, mutagenesis, and quantitative product analysis, single lab but multiple orthogonal methods","pmids":["33683197"],"is_preprint":false},{"year":2021,"finding":"HPF1 exhibits dual function: it can stimulate DNA-dependent and DNA-independent autoPARylation of PARP1 and PARP2 as well as heteroPARylation of histones in a defined range of HPF1 and NAD+ concentrations, while at other concentrations it limits PARylation and stimulates NAD+-hydrolase activity. PARP2 is more efficiently stimulated by HPF1 in autoPARylation and is more active in heteroPARylation of histones than in automodification.","method":"In vitro biochemical PARylation assays with purified components at varying stoichiometries, nucleosome substrates","journal":"Communications biology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with quantitative titration, single lab, multiple assay conditions","pmids":["34732825"],"is_preprint":false},{"year":2021,"finding":"PARP1-HPF1-dependent ADP-ribosylation of histone H3 is required for recruitment of LIG3-XRCC1 onto chromatin for backup Okazaki fragment ligation when LIG1 is absent. Depletion of PARP1 or HPF1 in Xenopus egg extracts prevents LIG3 chromatin recruitment and Okazaki fragment joining in LIG1-depleted extracts.","method":"Cell-free system from Xenopus egg extracts, immunodepletion of PARP1 or HPF1, chromatin fractionation, in vitro ligation assays","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Moderate — genetic epistasis via depletion in cell-free system, defined molecular phenotype (LIG3 recruitment failure), multiple depletion conditions tested","pmids":["33872376"],"is_preprint":false},{"year":2023,"finding":"HPF1 controls prolonged histone ADP-ribosylation at DNA breaks by regulating both the number and length of ADP-ribose chains. HPF1-dependent histone ADP-ribosylation triggers rapid chromatin unfolding (relaxation) near damage sites, facilitating access to DNA and promoting assembly of both homologous recombination and non-homologous end joining repair machineries.","method":"Live-cell FRAP/fluorescence microscopy, laser micro-irradiation, HPF1 knockout cells, chromatin compaction assays, repair factor recruitment assays (HR and NHEJ markers)","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 2 / Strong — KO cells with defined chromatin phenotype (relaxation), repair factor recruitment assays for two distinct pathways, multiple orthogonal methods","pmids":["37106138"],"is_preprint":false},{"year":2023,"finding":"The rate of dissociation (koff) of PARP inhibitors from the PARP1–HPF1 complex, rather than from PARP1 alone, best correlates with inhibitor potency in cells. HPF1 slightly increases the affinity of certain inhibitors (e.g., fluzoparib, olaparib) for PARP1.","method":"Kinetic binding assays (kon and koff measurements) for 8 PARP inhibitors with PARP1 and PARP1-HPF1 complex, correlation with cellular potency data","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — rigorous in vitro kinetic measurements with multiple inhibitors, single lab, functional correlation with cellular data","pmids":["37531469"],"is_preprint":false},{"year":2022,"finding":"HPF1-dependent histone PARylation catalyzed by PARP2 is specifically and most significantly stimulated by an incised AP site-containing BER DNA intermediate (1-nucleotide gap with 5'-dRP) in the context of nucleosomes. PARP2 affinity for DNA strongly depends on gap presence, and HPF1-induced stimulation of histone modification is a peculiar feature of PARP2 with gapped nucleosome substrates.","method":"In vitro PARylation assays with purified PARP1, PARP2, HPF1, and nucleosome core particles bearing defined DNA damage intermediates; binding affinity measurements","journal":"DNA repair","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with defined substrates comparing PARP1 and PARP2, single lab, multiple substrate conditions","pmids":["36356486"],"is_preprint":false},{"year":2023,"finding":"HPF1 interacts with PARP1 (confirmed by reciprocal co-IP) and promotes PARP1-mediated poly-ADP ribosylation of the RNA-binding protein HuR. HPF1-mediated HuR PARylation reduces HuR binding to p16 and p21 mRNAs, decreasing their stability (half-life), thereby modulating tendon stem/progenitor cell senescence.","method":"Reciprocal co-immunoprecipitation (HPF1 and PARP1), RNP-IP (HuR-mRNA binding), IP-based PARylation assay for HuR, mRNA half-life measurements, siRNA knockdown and overexpression","journal":"Genes & genomics","confidence":"Medium","confidence_rationale":"Tier 2 / Weak — reciprocal Co-IP confirmed HPF1-PARP1 interaction, HuR PARylation by IP, functional readouts; single lab, single study","pmids":["37713069"],"is_preprint":false},{"year":2024,"finding":"HPF1 loss does not generally increase cellular sensitivity to SSB-inducing genotoxins and SSBR kinetics are largely unaffected in HPF1-deficient cells. Poly-ADP-ribose chains sufficient to recruit XRCC1 are maintained at SSB sites even without HPF1, likely reflecting PARP1 auto-poly-ADP-ribosylation at non-serine residues. HPF1 and histone serine ADP-ribosylation are largely dispensable for PARP1-dependent SSBR.","method":"HPF1-deficient cell lines, genotoxic sensitivity assays, SSBR kinetics assays (comet assay, XRCC1 recruitment imaging), ADP-ribosylation detection","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO cells with multiple genotoxins and repair kinetics readouts, two independent cell lines, mechanistic interpretation of negative SSBR finding","pmids":["39162207"],"is_preprint":false},{"year":2025,"finding":"HPF1 modulates efficiency of DNA polymerase β (pol β)-catalyzed DNA synthesis in nucleosomes by regulating total poly(ADP-ribosyl)ation by PARP1 and especially PARP2. HPF1-dependent PARylation results in more efficient short-patch BER DNA synthesis in nucleosomes and also positively regulates long-patch BER.","method":"In vitro BER reconstitution with nucleosome substrates, DNA synthesis assays with pol β, comparing reactions with/without HPF1","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution with defined substrates, single lab, single study","pmids":["40076422"],"is_preprint":false},{"year":2026,"finding":"Cryo-EM of full-length PARP1 on a DNA single-strand break with HPF1 and a Timeless fragment shows that PARP1 remains dynamic even when its multi-domain structure is organized on a DNA break, with the minimal catalytic region displaying high mobility relative to DNA-engaging domains. The organization of PARP1 domains on a DNA break releases a tethered, constitutively active catalytic region to modify molecules in a radius surrounding the break.","method":"Single-particle cryo-EM, single-molecule DNA dynamics, small-angle X-ray scattering (SAXS)","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structure of full-length PARP1-HPF1 complex, complemented by SAXS and single-molecule dynamics, multiple orthogonal methods","pmids":["41698892"],"is_preprint":false},{"year":2026,"finding":"HPF1 regulates the formation of FUS-dependent DNA-rich compartments (condensates) by modulating PARP1 and PARP2 PARylation. Excess HPF1 over PARP1 diminishes PARP1 activity and reduces compartment size, but excess HPF1 over PARP2 does not significantly affect PARP2 activity or compartment size. HPF1 stimulates heteroPARylation of FUS (more strongly via PARP2 than PARP1), and HPF1-dependent intensive PARylation of FUS impairs DNA-rich compartment assembly.","method":"Atomic force microscopy, biochemical ADP-ribosylation assays, in vitro condensate formation assays","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro reconstitution with AFM visualization, multiple substrate and stoichiometry conditions, single lab","pmids":["41773021"],"is_preprint":false},{"year":2025,"finding":"HPF1 enhances ADP-ribosylation of certain free ribosomal proteins from the 60S subunit (RPL4, RPL6, RPL13A/RPL15) by PARP1 and PARP2, and switches the modification preferentially to serine/tyrosine residues on these proteins. HPF1-dependent enhancement is selective for 60S RPs and does not occur for 40S RPs.","method":"In vitro ADP-ribosylation assays with radioactively labeled NAD+, ribosomal subunit proteins, and purified HPF1/PARP1/PARP2; SDS-PAGE analysis","journal":"bioRxiv","confidence":"Low","confidence_rationale":"Tier 1 / Weak — in vitro reconstitution, preprint not yet peer-reviewed, single lab","pmids":["bio_10.1101_2025.09.15.676193"],"is_preprint":true},{"year":2025,"finding":"In the presence of HPF1, PARP2 serine 8 and serine 73 in the N-terminus are the predominant automodification sites. Serine 8 (present in both human PARP2 isoforms) is the major automodification site, and PARylation at these N-terminal serines is required for HPF1-dependent release of PARP2 from DNA damage sites.","method":"Site-directed mutagenesis of PARP2 serine residues, gel-based PARylation assays, fluorescence polarization DNA-release assay","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — mutagenesis combined with in vitro PARylation assay and functional DNA-release assay, preprint but multiple orthogonal methods","pmids":["bio_10.1101_2025.07.01.662498"],"is_preprint":true}],"current_model":"HPF1 is an accessory factor that transiently engages the catalytic domains of PARP1 and PARP2 to form a composite active site — contributing its catalytic glutamate (Glu284) — that switches amino acid specificity from aspartate/glutamate to serine, enabling serine-linked ADP-ribosylation of histones, PARP1/2 themselves, and hundreds of other targets in response to DNA damage; it operates via a 'hit-and-run' sub-stoichiometric mechanism that initiates serine modification without interfering with subsequent PAR chain elongation, limits PARP1 hyper-automodification, promotes chromatin relaxation and repair factor recruitment at breaks, and is a key determinant of PARP inhibitor potency through its influence on inhibitor dissociation kinetics from the PARP1–HPF1 complex."},"narrative":{"mechanistic_narrative":"HPF1 is an accessory factor of the DNA-damage-induced ADP-ribosylation machinery that converts PARP1 and PARP2 into serine-specific ADP-ribosyltransferases [PMID:27067600, PMID:28190768]. It is recruited to DNA lesions in a PARP1-dependent, catalytic-activity-independent manner, forms a robust complex with PARP1, and limits PARP1 hyper-automodification while promoting in trans ADP-ribosylation of histones [PMID:27067600]. Mechanistically, HPF1 is both necessary and sufficient to redirect PARP1/PARP2 modification from aspartate/glutamate to serine residues, including endogenous serine sites in the PARP1 automodification domain [PMID:28190768]. Structural and mutagenesis work establishes that HPF1 contributes its own catalytic glutamate (Glu284), positioned by HPF1 Arg239, into a composite active site shared with the PARP catalytic domain; this interaction is allosterically enhanced by NAD+ occupancy and DNA damage [PMID:32028527, PMID:33589610]. Cryo-EM of PARP2–HPF1 on nucleosomes shows that DNA-break recognition and nucleosome bridging induce PARP conformational changes that license HPF1 binding and enzyme activation [PMID:32939087, PMID:41698892]. HPF1 acts sub-stoichiometrically through a 'hit-and-run' mechanism, rapidly cycling among PARP molecules to initiate serine modification without persisting to interfere with PAR chain elongation, and it partitions reaction product toward shorter chains and free ADP-ribose, effectively tuning polymerase versus hydrolase output [PMID:34795260, PMID:33683197]. Functionally, HPF1-dependent histone ADP-ribosylation drives rapid chromatin relaxation at breaks and promotes assembly of both homologous recombination and non-homologous end joining machineries [PMID:37106138], and is required for LIG3-XRCC1 recruitment supporting backup Okazaki fragment ligation [PMID:33872376]. HPF1 also shapes base excision repair, stimulating PARP2-dependent histone modification on gapped nucleosome intermediates and enhancing pol β-catalyzed DNA synthesis [PMID:36356486, PMID:40076422]. Through its formation of the PARP1–HPF1 complex, HPF1 is a determinant of clinical PARP inhibitor potency, where inhibitor dissociation kinetics from the complex best correlate with cellular efficacy [PMID:32028527, PMID:37531469].","teleology":[{"year":2016,"claim":"Established HPF1 as a genuine PARP1-associated factor and placed it functionally in the DNA damage response, answering whether C4orf27 has any role in ADP-ribosylation signaling.","evidence":"Co-IP, laser micro-irradiation recruitment, in vitro ADP-ribosylation, and HPF1 knockout sensitivity assays in human cells","pmids":["27067600"],"confidence":"High","gaps":["Molecular basis of how HPF1 redirects modification was unknown","Identity of the chemical linkage on histones not yet defined"]},{"year":2017,"claim":"Defined the core biochemical function: HPF1 is necessary and sufficient to switch PARP1/PARP2 amino-acid specificity from Asp/Glu to serine, explaining how histone serine ADP-ribosylation arises.","evidence":"Quantitative MS proteomics of HPF1 knockout cells plus in vitro PARP1/PARP2 reconstitution with site mapping","pmids":["28190768"],"confidence":"High","gaps":["Structural mechanism of specificity switch not resolved","How HPF1 contributes catalytically was unknown"]},{"year":2020,"claim":"Resolved the catalytic mechanism by showing HPF1 forms a composite active site with PARP1/PARP2, donating Glu284 and being allosterically regulated by NAD+ and DNA damage, and structurally licensed by nucleosome bridging.","evidence":"Crystal structure of HPF1–PARP2 catalytic domain, NMR, cryo-EM of PARP2–HPF1–nucleosome complexes, mutagenesis, and cellular assays","pmids":["32028527","32939087","33141820","33186521"],"confidence":"High","gaps":["Kinetics of HPF1 association/dissociation during catalysis unresolved","Effect on PAR chain length and product partitioning not quantified"]},{"year":2021,"claim":"Refined the catalytic model and kinetics: confirmed the Arg239–Glu284 salt bridge, established the sub-stoichiometric 'hit-and-run' mechanism, and revealed HPF1 shifts PARP1 toward shorter chains and hydrolase activity.","evidence":"Crystallography, mutagenesis, in vitro kinetic and product analyses (TLC/gel), cryo-EM, and titration assays with nucleosomes","pmids":["33589610","34795260","33683197","34732825"],"confidence":"High","gaps":["Concentration/stoichiometry regimes producing stimulation vs inhibition not mapped in cells","Physiological NAD+/HPF1 ratios in vivo uncertain"]},{"year":2021,"claim":"Linked HPF1-dependent histone ADP-ribosylation to a specific repair outcome, showing it is required for LIG3-XRCC1 recruitment during backup Okazaki fragment ligation.","evidence":"Xenopus egg extract cell-free system with PARP1/HPF1 immunodepletion, chromatin fractionation, and ligation assays","pmids":["33872376"],"confidence":"High","gaps":["Direct histone serine site driving LIG3 recruitment not pinpointed","Relevance to human replication not directly tested"]},{"year":2023,"claim":"Connected HPF1 enzymatic output to chromatin dynamics, demonstrating that HPF1-dependent histone ADP-ribosylation triggers chromatin relaxation enabling both HR and NHEJ factor assembly at breaks.","evidence":"Live-cell FRAP, laser micro-irradiation, HPF1 knockout cells, chromatin compaction and repair factor recruitment assays","pmids":["37106138"],"confidence":"High","gaps":["Which repair pathway choice is influenced not resolved","Chromatin reader of the serine mark not identified"]},{"year":2023,"claim":"Identified the PARP1–HPF1 complex, not PARP1 alone, as the pharmacologically relevant target whose inhibitor dissociation kinetics predict cellular potency.","evidence":"Kinetic kon/koff binding measurements for 8 PARP inhibitors with PARP1 and PARP1–HPF1, correlated with cellular potency","pmids":["37531469"],"confidence":"Medium","gaps":["Effects measured for a limited inhibitor panel","Structural basis of HPF1-modulated inhibitor binding not solved"]},{"year":2023,"claim":"Extended HPF1 function beyond chromatin by showing it promotes PARP1-mediated PARylation of the RNA-binding protein HuR, affecting mRNA stability and cellular senescence.","evidence":"Reciprocal co-IP, RNP-IP, IP-based PARylation assays, mRNA half-life measurements in tendon stem/progenitor cells","pmids":["37713069"],"confidence":"Medium","gaps":["Single lab, single cell system","Whether HuR PARylation is serine-linked not established"]},{"year":2024,"claim":"Delimited the repair contexts requiring HPF1, finding it largely dispensable for single-strand break repair where non-serine PARP1 automodification suffices for XRCC1 recruitment.","evidence":"HPF1-deficient cell lines, genotoxic sensitivity assays, comet assays, XRCC1 recruitment imaging across two cell lines","pmids":["39162207"],"confidence":"Medium","gaps":["Reconciliation with earlier sensitivity phenotypes not fully resolved","Lesion-type specificity of HPF1 dependence incompletely defined"]},{"year":2025,"claim":"Broadened the substrate landscape, showing HPF1 enhances and switches ADP-ribosylation of specific 60S ribosomal proteins and shapes base excision repair DNA synthesis by pol β.","evidence":"In vitro ADP-ribosylation with ribosomal proteins (preprint) and in vitro BER reconstitution with nucleosomes and pol β","pmids":["bio_10.1101_2025.09.15.676193","40076422"],"confidence":"Low","gaps":["Ribosomal protein finding is a non-peer-reviewed preprint from a single lab","Cellular relevance of these substrates untested"]},{"year":2026,"claim":"Captured the full-length PARP1–HPF1 complex on a DNA break and extended HPF1 regulation to PARylation-dependent biomolecular condensate assembly.","evidence":"Cryo-EM, SAXS, single-molecule DNA dynamics of full-length PARP1–HPF1; AFM and in vitro condensate assays with FUS","pmids":["41698892","41773021"],"confidence":"High","gaps":["Condensate regulation demonstrated in vitro only","In vivo consequences of HPF1-tuned condensate dynamics unknown"]},{"year":null,"claim":"How HPF1-imposed serine ADP-ribosylation is decoded by downstream readers and how its stoichiometric balance is controlled in vivo to dictate repair-pathway choice remain open.","evidence":"No direct experimental evidence in the available corpus","pmids":[],"confidence":"Low","gaps":["No reader of the histone serine ADPr mark identified","In-cell control of HPF1:PARP stoichiometry undefined","Mechanistic link between specific serine marks and pathway selection unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,2,8]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,7,9]}],"localization":[{"term_id":"GO:0000228","term_label":"nuclear chromosome","supporting_discovery_ids":[0,11]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-73894","term_label":"DNA Repair","supporting_discovery_ids":[0,10,11,13]},{"term_id":"R-HSA-4839726","term_label":"Chromatin organization","supporting_discovery_ids":[1,8,11]}],"complexes":["PARP1–HPF1 complex","PARP2–HPF1–nucleosome complex"],"partners":["PARP1","PARP2","HUR","FUS"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9NWY4","full_name":"Histone PARylation factor 1","aliases":[],"length_aa":346,"mass_kda":39.4,"function":"Cofactor for serine ADP-ribosylation that confers serine specificity on PARP1 and PARP2 and plays a key role in DNA damage response (PubMed:28190768, PubMed:29480802, PubMed:29954836, PubMed:32028527, PubMed:32939087, PubMed:33186521, PubMed:33589610, PubMed:33683197, PubMed:34108479, PubMed:34210965, PubMed:34486521, PubMed:34625544, PubMed:34732825, PubMed:34795260, PubMed:34874266). Initiates the repair of double-strand DNA breaks: recruited to DNA damage sites by PARP1 and PARP2 and switches the amino acid specificity of PARP1 and PARP2 from aspartate or glutamate to serine residues, licensing serine ADP-ribosylation of target proteins (PubMed:28190768, PubMed:29480802, PubMed:29954836, PubMed:32028527, PubMed:32939087, PubMed:33589610, PubMed:33683197, PubMed:34486521, PubMed:34625544, PubMed:34732825, PubMed:34795260, PubMed:34874266). Serine ADP-ribosylation of target proteins, such as histones, promotes decompaction of chromatin and the recruitment of repair factors leading to the reparation of DNA strand breaks (PubMed:27067600, PubMed:28190768, PubMed:32939087, PubMed:33589610). Serine ADP-ribosylation of proteins constitutes the primary form of ADP-ribosylation of proteins in response to DNA damage (PubMed:29480802). HPF1 acts by completing the active site of PARP1 and PARP2: forms a composite active site composed of residues from HPF1 and PARP1 or PARP2 (PubMed:32028527, PubMed:33589610). While HPF1 promotes the initiation of serine ADP-ribosylation, it restricts the polymerase activity of PARP1 and PARP2 in order to limit the length of poly-ADP-ribose chains (PubMed:33683197, PubMed:34732825, PubMed:34795260). HPF1 also promotes tyrosine ADP-ribosylation, probably by conferring tyrosine specificity on PARP1 (PubMed:29954836, PubMed:30257210)","subcellular_location":"Chromosome; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q9NWY4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HPF1","classification":"Not Classified","n_dependent_lines":5,"n_total_lines":1208,"dependency_fraction":0.0041390728476821195},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/HPF1","total_profiled":1310},"omim":[{"mim_id":"616614","title":"HISTONE PARYLATION FACTOR 1; HPF1","url":"https://www.omim.org/entry/616614"},{"mim_id":"607725","title":"POLY(ADP-RIBOSE) POLYMERASE 2; PARP2","url":"https://www.omim.org/entry/607725"},{"mim_id":"194558","title":"ZINC FINGER PROTEIN 83; ZNF83","url":"https://www.omim.org/entry/194558"},{"mim_id":"173870","title":"POLY(ADP-RIBOSE) POLYMERASE 1; PARP1","url":"https://www.omim.org/entry/173870"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cytosol","reliability":"Approved"},{"location":"Microtubules","reliability":"Additional"},{"location":"Cytokinetic bridge","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/HPF1"},"hgnc":{"alias_symbol":["FLJ20534"],"prev_symbol":["C4orf27"]},"alphafold":{"accession":"Q9NWY4","domains":[{"cath_id":"-","chopping":"30-214","consensus_level":"medium","plddt":94.9232,"start":30,"end":214},{"cath_id":"-","chopping":"217-337","consensus_level":"medium","plddt":97.8779,"start":217,"end":337}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NWY4","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NWY4-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q9NWY4-F1-predicted_aligned_error_v6.png","plddt_mean":90.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HPF1","jax_strain_url":"https://www.jax.org/strain/search?query=HPF1"},"sequence":{"accession":"Q9NWY4","fasta_url":"https://rest.uniprot.org/uniprotkb/Q9NWY4.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q9NWY4/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q9NWY4"}},"corpus_meta":[{"pmid":"28190768","id":"PMC_28190768","title":"Serine 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Loss of HPF1 sensitizes human cells to DNA damaging agents and PARP inhibition.\",\n      \"method\": \"Co-immunoprecipitation, laser micro-irradiation recruitment assays, in vitro ADP-ribosylation assays, HPF1 knockout cell lines with genotoxic sensitivity assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, in vitro functional assay, KO cell phenotype, multiple orthogonal methods in a focused study\",\n      \"pmids\": [\"27067600\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"HPF1 is necessary and sufficient to redirect PARP-1 and PARP-2 ADP-ribosylation from aspartate/glutamate to serine residues. Adding HPF1 to in vitro PARP-1/PARP-2 reactions produces serine-linked ADP-ribosylation on histones and PARP-1 itself. Serine ADPr does not occur in cells lacking HPF1, and three endogenous serine ADPr sites were mapped to the PARP-1 automodification domain.\",\n      \"method\": \"Quantitative mass spectrometry-based proteomics of HPF1 knockout cells, in vitro PARP-1/PARP-2 ADP-ribosylation reconstitution with and without HPF1, site mapping by MS\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution showing necessity and sufficiency, complemented by quantitative proteomics in KO cells, replicated across multiple independent studies\",\n      \"pmids\": [\"28190768\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"HPF1 forms a composite (joint) active site with the catalytic domain of PARP1 or PARP2 by contributing a catalytic glutamate residue (Glu284) that is essential for serine-specific ADP-ribosylation after DNA damage. The HPF1–PARP interaction is allosterically enhanced by occupancy of the NAD+-binding site and by DNA damage signals, providing an additional regulatory layer. This composite active site implicates HPF1 as a determinant of response to clinical PARP inhibitors.\",\n      \"method\": \"Crystal/co-structure of HPF1 bound to PARP2 catalytic domain, NMR, biochemical mutagenesis assays, cellular ADP-ribosylation assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus NMR plus mutagenesis plus cellular validation in a single rigorous study, independently replicated by Sun et al. 2021\",\n      \"pmids\": [\"32028527\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of human PARP2–HPF1 bound to a nucleosome shows that PARP2–HPF1 bridges two nucleosomes with broken DNA aligned for ligation. DNA bridging induces structural changes in PARP2 that signal break recognition to the catalytic domain, licensing HPF1 binding and PARP2 activation. Active PARP2 cycles through different conformational states to exchange NAD+ and substrate.\",\n      \"method\": \"Cryo-electron microscopy, biochemical ADP-ribosylation activity assays with nucleosome substrates\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure with biochemical validation, high-resolution mechanistic detail from a single rigorous study\",\n      \"pmids\": [\"32939087\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"HPF1 is required for a phospho-guided chemoenzymatic serine ADP-ribosylation reaction; the HPF1/PARP1 writer complex can be used to install authentic serine-linked ADP-ribose at defined positions on peptides in a scalable, precise manner.\",\n      \"method\": \"Chemoenzymatic in vitro reconstitution using HPF1/PARP1 complex, phage display antibody selection, mono-ADPr proteomics\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with biochemical validation, single lab, novel application confirming serine-specificity mechanism\",\n      \"pmids\": [\"33186521\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Cryo-EM structure of two nucleosomes bridged by PARP2 shows that the PARP2 conformation adopted upon damaged-chromatin binding provides a binding platform for HPF1, and the resulting HPF1•PARP2•nucleosome complex is enzymatically active.\",\n      \"method\": \"Cryo-electron microscopy, in vitro ADP-ribosylation activity assays\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure with biochemical activity confirmation, single lab study\",\n      \"pmids\": [\"33141820\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Crystal structures of human HPF1/PARP1-CAT complex and mutagenesis data confirm that HPF1 Arg239 salt-bridges to Glu284/Asp286 to position Glu284 as the catalytic base for serine ADP-ribosylation, maintains the local HPF1 conformation to limit PARP1 automodification, and facilitates HPF1/PARP1 binding by neutralizing negative charge at Glu284.\",\n      \"method\": \"X-ray crystallography (1.98 Å HPF1/PARP1-CAT; 1.57–1.71 Å HPF1 alone), site-directed mutagenesis, quantitative binding assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure plus mutagenesis of key residues, independent replication of Suskiewicz et al. 2020 mechanism for PARP1\",\n      \"pmids\": [\"33589610\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"HPF1 efficiently regulates PARP1/2 at sub-stoichiometric ratios matching their relative cellular abundances via a 'hit-and-run' mechanism: HPF1 rapidly associates/dissociates from multiple PARP1 molecules, initiating serine modification before glutamate/aspartate modification initiates, and accelerating initiation to be more comparable to elongation. This ensures HPF1 contributions during initiation do not persist and interfere with PAR chain elongation.\",\n      \"method\": \"Biochemical kinetic assays (in vitro ADP-ribosylation with sub-stoichiometric HPF1), cryo-EM structural analysis of HPF1/PARP1 on DNA break, DNA retention assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with kinetic characterization, cryo-EM, and DNA retention assay; multiple orthogonal methods in one study\",\n      \"pmids\": [\"34795260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"HPF1 provides Glu284 as a catalytic base that substantially redirects PARylation by PARP1 such that histones in nucleosomes become primary recipients of PAR chains. Surprisingly, HPF1 partitions most reaction product to free ADP-ribose (ADPR), resulting in much shorter PAR chains — a switch from PARP1 polymerase to hydrolase activity.\",\n      \"method\": \"In vitro PARP1 reconstitution with nucleosomes as activators and substrates, product analysis by TLC and gel electrophoresis, mutagenesis\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous in vitro reconstitution with nucleosome substrates, mutagenesis, and quantitative product analysis, single lab but multiple orthogonal methods\",\n      \"pmids\": [\"33683197\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"HPF1 exhibits dual function: it can stimulate DNA-dependent and DNA-independent autoPARylation of PARP1 and PARP2 as well as heteroPARylation of histones in a defined range of HPF1 and NAD+ concentrations, while at other concentrations it limits PARylation and stimulates NAD+-hydrolase activity. PARP2 is more efficiently stimulated by HPF1 in autoPARylation and is more active in heteroPARylation of histones than in automodification.\",\n      \"method\": \"In vitro biochemical PARylation assays with purified components at varying stoichiometries, nucleosome substrates\",\n      \"journal\": \"Communications biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with quantitative titration, single lab, multiple assay conditions\",\n      \"pmids\": [\"34732825\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"PARP1-HPF1-dependent ADP-ribosylation of histone H3 is required for recruitment of LIG3-XRCC1 onto chromatin for backup Okazaki fragment ligation when LIG1 is absent. Depletion of PARP1 or HPF1 in Xenopus egg extracts prevents LIG3 chromatin recruitment and Okazaki fragment joining in LIG1-depleted extracts.\",\n      \"method\": \"Cell-free system from Xenopus egg extracts, immunodepletion of PARP1 or HPF1, chromatin fractionation, in vitro ligation assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis via depletion in cell-free system, defined molecular phenotype (LIG3 recruitment failure), multiple depletion conditions tested\",\n      \"pmids\": [\"33872376\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"HPF1 controls prolonged histone ADP-ribosylation at DNA breaks by regulating both the number and length of ADP-ribose chains. HPF1-dependent histone ADP-ribosylation triggers rapid chromatin unfolding (relaxation) near damage sites, facilitating access to DNA and promoting assembly of both homologous recombination and non-homologous end joining repair machineries.\",\n      \"method\": \"Live-cell FRAP/fluorescence microscopy, laser micro-irradiation, HPF1 knockout cells, chromatin compaction assays, repair factor recruitment assays (HR and NHEJ markers)\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — KO cells with defined chromatin phenotype (relaxation), repair factor recruitment assays for two distinct pathways, multiple orthogonal methods\",\n      \"pmids\": [\"37106138\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"The rate of dissociation (koff) of PARP inhibitors from the PARP1–HPF1 complex, rather than from PARP1 alone, best correlates with inhibitor potency in cells. HPF1 slightly increases the affinity of certain inhibitors (e.g., fluzoparib, olaparib) for PARP1.\",\n      \"method\": \"Kinetic binding assays (kon and koff measurements) for 8 PARP inhibitors with PARP1 and PARP1-HPF1 complex, correlation with cellular potency data\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — rigorous in vitro kinetic measurements with multiple inhibitors, single lab, functional correlation with cellular data\",\n      \"pmids\": [\"37531469\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HPF1-dependent histone PARylation catalyzed by PARP2 is specifically and most significantly stimulated by an incised AP site-containing BER DNA intermediate (1-nucleotide gap with 5'-dRP) in the context of nucleosomes. PARP2 affinity for DNA strongly depends on gap presence, and HPF1-induced stimulation of histone modification is a peculiar feature of PARP2 with gapped nucleosome substrates.\",\n      \"method\": \"In vitro PARylation assays with purified PARP1, PARP2, HPF1, and nucleosome core particles bearing defined DNA damage intermediates; binding affinity measurements\",\n      \"journal\": \"DNA repair\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with defined substrates comparing PARP1 and PARP2, single lab, multiple substrate conditions\",\n      \"pmids\": [\"36356486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"HPF1 interacts with PARP1 (confirmed by reciprocal co-IP) and promotes PARP1-mediated poly-ADP ribosylation of the RNA-binding protein HuR. HPF1-mediated HuR PARylation reduces HuR binding to p16 and p21 mRNAs, decreasing their stability (half-life), thereby modulating tendon stem/progenitor cell senescence.\",\n      \"method\": \"Reciprocal co-immunoprecipitation (HPF1 and PARP1), RNP-IP (HuR-mRNA binding), IP-based PARylation assay for HuR, mRNA half-life measurements, siRNA knockdown and overexpression\",\n      \"journal\": \"Genes & genomics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Weak — reciprocal Co-IP confirmed HPF1-PARP1 interaction, HuR PARylation by IP, functional readouts; single lab, single study\",\n      \"pmids\": [\"37713069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"HPF1 loss does not generally increase cellular sensitivity to SSB-inducing genotoxins and SSBR kinetics are largely unaffected in HPF1-deficient cells. Poly-ADP-ribose chains sufficient to recruit XRCC1 are maintained at SSB sites even without HPF1, likely reflecting PARP1 auto-poly-ADP-ribosylation at non-serine residues. HPF1 and histone serine ADP-ribosylation are largely dispensable for PARP1-dependent SSBR.\",\n      \"method\": \"HPF1-deficient cell lines, genotoxic sensitivity assays, SSBR kinetics assays (comet assay, XRCC1 recruitment imaging), ADP-ribosylation detection\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO cells with multiple genotoxins and repair kinetics readouts, two independent cell lines, mechanistic interpretation of negative SSBR finding\",\n      \"pmids\": [\"39162207\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"HPF1 modulates efficiency of DNA polymerase β (pol β)-catalyzed DNA synthesis in nucleosomes by regulating total poly(ADP-ribosyl)ation by PARP1 and especially PARP2. HPF1-dependent PARylation results in more efficient short-patch BER DNA synthesis in nucleosomes and also positively regulates long-patch BER.\",\n      \"method\": \"In vitro BER reconstitution with nucleosome substrates, DNA synthesis assays with pol β, comparing reactions with/without HPF1\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution with defined substrates, single lab, single study\",\n      \"pmids\": [\"40076422\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"Cryo-EM of full-length PARP1 on a DNA single-strand break with HPF1 and a Timeless fragment shows that PARP1 remains dynamic even when its multi-domain structure is organized on a DNA break, with the minimal catalytic region displaying high mobility relative to DNA-engaging domains. The organization of PARP1 domains on a DNA break releases a tethered, constitutively active catalytic region to modify molecules in a radius surrounding the break.\",\n      \"method\": \"Single-particle cryo-EM, single-molecule DNA dynamics, small-angle X-ray scattering (SAXS)\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structure of full-length PARP1-HPF1 complex, complemented by SAXS and single-molecule dynamics, multiple orthogonal methods\",\n      \"pmids\": [\"41698892\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"HPF1 regulates the formation of FUS-dependent DNA-rich compartments (condensates) by modulating PARP1 and PARP2 PARylation. Excess HPF1 over PARP1 diminishes PARP1 activity and reduces compartment size, but excess HPF1 over PARP2 does not significantly affect PARP2 activity or compartment size. HPF1 stimulates heteroPARylation of FUS (more strongly via PARP2 than PARP1), and HPF1-dependent intensive PARylation of FUS impairs DNA-rich compartment assembly.\",\n      \"method\": \"Atomic force microscopy, biochemical ADP-ribosylation assays, in vitro condensate formation assays\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro reconstitution with AFM visualization, multiple substrate and stoichiometry conditions, single lab\",\n      \"pmids\": [\"41773021\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"HPF1 enhances ADP-ribosylation of certain free ribosomal proteins from the 60S subunit (RPL4, RPL6, RPL13A/RPL15) by PARP1 and PARP2, and switches the modification preferentially to serine/tyrosine residues on these proteins. HPF1-dependent enhancement is selective for 60S RPs and does not occur for 40S RPs.\",\n      \"method\": \"In vitro ADP-ribosylation assays with radioactively labeled NAD+, ribosomal subunit proteins, and purified HPF1/PARP1/PARP2; SDS-PAGE analysis\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro reconstitution, preprint not yet peer-reviewed, single lab\",\n      \"pmids\": [\"bio_10.1101_2025.09.15.676193\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In the presence of HPF1, PARP2 serine 8 and serine 73 in the N-terminus are the predominant automodification sites. Serine 8 (present in both human PARP2 isoforms) is the major automodification site, and PARylation at these N-terminal serines is required for HPF1-dependent release of PARP2 from DNA damage sites.\",\n      \"method\": \"Site-directed mutagenesis of PARP2 serine residues, gel-based PARylation assays, fluorescence polarization DNA-release assay\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — mutagenesis combined with in vitro PARylation assay and functional DNA-release assay, preprint but multiple orthogonal methods\",\n      \"pmids\": [\"bio_10.1101_2025.07.01.662498\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"HPF1 is an accessory factor that transiently engages the catalytic domains of PARP1 and PARP2 to form a composite active site — contributing its catalytic glutamate (Glu284) — that switches amino acid specificity from aspartate/glutamate to serine, enabling serine-linked ADP-ribosylation of histones, PARP1/2 themselves, and hundreds of other targets in response to DNA damage; it operates via a 'hit-and-run' sub-stoichiometric mechanism that initiates serine modification without interfering with subsequent PAR chain elongation, limits PARP1 hyper-automodification, promotes chromatin relaxation and repair factor recruitment at breaks, and is a key determinant of PARP inhibitor potency through its influence on inhibitor dissociation kinetics from the PARP1–HPF1 complex.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"HPF1 is an accessory factor of the DNA-damage-induced ADP-ribosylation machinery that converts PARP1 and PARP2 into serine-specific ADP-ribosyltransferases [#0, #1]. It is recruited to DNA lesions in a PARP1-dependent, catalytic-activity-independent manner, forms a robust complex with PARP1, and limits PARP1 hyper-automodification while promoting in trans ADP-ribosylation of histones [#0]. Mechanistically, HPF1 is both necessary and sufficient to redirect PARP1/PARP2 modification from aspartate/glutamate to serine residues, including endogenous serine sites in the PARP1 automodification domain [#1]. Structural and mutagenesis work establishes that HPF1 contributes its own catalytic glutamate (Glu284), positioned by HPF1 Arg239, into a composite active site shared with the PARP catalytic domain; this interaction is allosterically enhanced by NAD+ occupancy and DNA damage [#2, #6]. Cryo-EM of PARP2\\u2013HPF1 on nucleosomes shows that DNA-break recognition and nucleosome bridging induce PARP conformational changes that license HPF1 binding and enzyme activation [#3, #17]. HPF1 acts sub-stoichiometrically through a 'hit-and-run' mechanism, rapidly cycling among PARP molecules to initiate serine modification without persisting to interfere with PAR chain elongation, and it partitions reaction product toward shorter chains and free ADP-ribose, effectively tuning polymerase versus hydrolase output [#7, #8]. Functionally, HPF1-dependent histone ADP-ribosylation drives rapid chromatin relaxation at breaks and promotes assembly of both homologous recombination and non-homologous end joining machineries [#11], and is required for LIG3-XRCC1 recruitment supporting backup Okazaki fragment ligation [#10]. HPF1 also shapes base excision repair, stimulating PARP2-dependent histone modification on gapped nucleosome intermediates and enhancing pol \\u03b2-catalyzed DNA synthesis [#13, #16]. Through its formation of the PARP1\\u2013HPF1 complex, HPF1 is a determinant of clinical PARP inhibitor potency, where inhibitor dissociation kinetics from the complex best correlate with cellular efficacy [#2, #12].\",\n  \"teleology\": [\n    {\n      \"year\": 2016,\n      \"claim\": \"Established HPF1 as a genuine PARP1-associated factor and placed it functionally in the DNA damage response, answering whether C4orf27 has any role in ADP-ribosylation signaling.\",\n      \"evidence\": \"Co-IP, laser micro-irradiation recruitment, in vitro ADP-ribosylation, and HPF1 knockout sensitivity assays in human cells\",\n      \"pmids\": [\"27067600\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of how HPF1 redirects modification was unknown\", \"Identity of the chemical linkage on histones not yet defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Defined the core biochemical function: HPF1 is necessary and sufficient to switch PARP1/PARP2 amino-acid specificity from Asp/Glu to serine, explaining how histone serine ADP-ribosylation arises.\",\n      \"evidence\": \"Quantitative MS proteomics of HPF1 knockout cells plus in vitro PARP1/PARP2 reconstitution with site mapping\",\n      \"pmids\": [\"28190768\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural mechanism of specificity switch not resolved\", \"How HPF1 contributes catalytically was unknown\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Resolved the catalytic mechanism by showing HPF1 forms a composite active site with PARP1/PARP2, donating Glu284 and being allosterically regulated by NAD+ and DNA damage, and structurally licensed by nucleosome bridging.\",\n      \"evidence\": \"Crystal structure of HPF1\\u2013PARP2 catalytic domain, NMR, cryo-EM of PARP2\\u2013HPF1\\u2013nucleosome complexes, mutagenesis, and cellular assays\",\n      \"pmids\": [\"32028527\", \"32939087\", \"33141820\", \"33186521\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinetics of HPF1 association/dissociation during catalysis unresolved\", \"Effect on PAR chain length and product partitioning not quantified\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Refined the catalytic model and kinetics: confirmed the Arg239\\u2013Glu284 salt bridge, established the sub-stoichiometric 'hit-and-run' mechanism, and revealed HPF1 shifts PARP1 toward shorter chains and hydrolase activity.\",\n      \"evidence\": \"Crystallography, mutagenesis, in vitro kinetic and product analyses (TLC/gel), cryo-EM, and titration assays with nucleosomes\",\n      \"pmids\": [\"33589610\", \"34795260\", \"33683197\", \"34732825\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Concentration/stoichiometry regimes producing stimulation vs inhibition not mapped in cells\", \"Physiological NAD+/HPF1 ratios in vivo uncertain\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Linked HPF1-dependent histone ADP-ribosylation to a specific repair outcome, showing it is required for LIG3-XRCC1 recruitment during backup Okazaki fragment ligation.\",\n      \"evidence\": \"Xenopus egg extract cell-free system with PARP1/HPF1 immunodepletion, chromatin fractionation, and ligation assays\",\n      \"pmids\": [\"33872376\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct histone serine site driving LIG3 recruitment not pinpointed\", \"Relevance to human replication not directly tested\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Connected HPF1 enzymatic output to chromatin dynamics, demonstrating that HPF1-dependent histone ADP-ribosylation triggers chromatin relaxation enabling both HR and NHEJ factor assembly at breaks.\",\n      \"evidence\": \"Live-cell FRAP, laser micro-irradiation, HPF1 knockout cells, chromatin compaction and repair factor recruitment assays\",\n      \"pmids\": [\"37106138\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Which repair pathway choice is influenced not resolved\", \"Chromatin reader of the serine mark not identified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identified the PARP1\\u2013HPF1 complex, not PARP1 alone, as the pharmacologically relevant target whose inhibitor dissociation kinetics predict cellular potency.\",\n      \"evidence\": \"Kinetic kon/koff binding measurements for 8 PARP inhibitors with PARP1 and PARP1\\u2013HPF1, correlated with cellular potency\",\n      \"pmids\": [\"37531469\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Effects measured for a limited inhibitor panel\", \"Structural basis of HPF1-modulated inhibitor binding not solved\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Extended HPF1 function beyond chromatin by showing it promotes PARP1-mediated PARylation of the RNA-binding protein HuR, affecting mRNA stability and cellular senescence.\",\n      \"evidence\": \"Reciprocal co-IP, RNP-IP, IP-based PARylation assays, mRNA half-life measurements in tendon stem/progenitor cells\",\n      \"pmids\": [\"37713069\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, single cell system\", \"Whether HuR PARylation is serine-linked not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Delimited the repair contexts requiring HPF1, finding it largely dispensable for single-strand break repair where non-serine PARP1 automodification suffices for XRCC1 recruitment.\",\n      \"evidence\": \"HPF1-deficient cell lines, genotoxic sensitivity assays, comet assays, XRCC1 recruitment imaging across two cell lines\",\n      \"pmids\": [\"39162207\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Reconciliation with earlier sensitivity phenotypes not fully resolved\", \"Lesion-type specificity of HPF1 dependence incompletely defined\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Broadened the substrate landscape, showing HPF1 enhances and switches ADP-ribosylation of specific 60S ribosomal proteins and shapes base excision repair DNA synthesis by pol \\u03b2.\",\n      \"evidence\": \"In vitro ADP-ribosylation with ribosomal proteins (preprint) and in vitro BER reconstitution with nucleosomes and pol \\u03b2\",\n      \"pmids\": [\"bio_10.1101_2025.09.15.676193\", \"40076422\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Ribosomal protein finding is a non-peer-reviewed preprint from a single lab\", \"Cellular relevance of these substrates untested\"]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Captured the full-length PARP1\\u2013HPF1 complex on a DNA break and extended HPF1 regulation to PARylation-dependent biomolecular condensate assembly.\",\n      \"evidence\": \"Cryo-EM, SAXS, single-molecule DNA dynamics of full-length PARP1\\u2013HPF1; AFM and in vitro condensate assays with FUS\",\n      \"pmids\": [\"41698892\", \"41773021\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Condensate regulation demonstrated in vitro only\", \"In vivo consequences of HPF1-tuned condensate dynamics unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How HPF1-imposed serine ADP-ribosylation is decoded by downstream readers and how its stoichiometric balance is controlled in vivo to dictate repair-pathway choice remain open.\",\n      \"evidence\": \"No direct experimental evidence in the available corpus\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No reader of the histone serine ADPr mark identified\", \"In-cell control of HPF1:PARP stoichiometry undefined\", \"Mechanistic link between specific serine marks and pathway selection unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 2, 8]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 7, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0000228\", \"supporting_discovery_ids\": [0, 11]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-73894\", \"supporting_discovery_ids\": [0, 10, 11, 13]},\n      {\"term_id\": \"R-HSA-4839726\", \"supporting_discovery_ids\": [1, 8, 11]}\n    ],\n    \"complexes\": [\"PARP1\\u2013HPF1 complex\", \"PARP2\\u2013HPF1\\u2013nucleosome complex\"],\n    \"partners\": [\"PARP1\", \"PARP2\", \"HuR\", \"FUS\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":9,"faith_total":9,"faith_pct":100.0}}