HAPSTR1

HAPSTR1 (C16orf72/TAPR1) is an evolutionarily conserved regulator of cellular stress responses that operates principally by controlling the ubiquitin ligase HUWE1 (PMID:35776542, PMID:37167062). HAPSTR1 oligomerizes through perfectly conserved residues and binds HUWE1, which serves as a required cofactor for HAPSTR1-dependent stress signaling; reciprocally, HUWE1 ubiquitinates and destabilizes HAPSTR1, establishing a feedback loop (PMID:35776542). Functionally, HAPSTR1 is required for nuclear localization of HUWE1, and its loss mislocalizes HUWE1 away from the nucleus, impairing nuclear substrate ubiquitination, cell proliferation, and modulation of the p53 and NF-κB stress pathways (PMID:37167062, PMID:38453366). Consistent with a role upstream of p53, HAPSTR1 constrains p53 levels in the telomere-erosion response (PMID:33660365), and its loss in mice causes perinatal lethality independent of p53 status (PMID:38453366). HAPSTR1 abundance is set by an opposing pair of enzymes — the ubiquitin ligase TRIP12 destabilizes it while the deubiquitinase USP7 stabilizes it (PMID:38453366). Beyond the HUWE1 axis, HAPSTR1 interacts with BRCA1 and the helicase Senataxin to recruit them to RNA:DNA hybrids at stalled replication forks, suppressing replication-associated R-loops in parallel to PARP1/2 (PMID:37591890), and it binds LRPPRC and PSMD14 to suppress LRPPRC ubiquitination (PMID:38643468).

1. 2021 High

   Established HAPSTR1/TAPR1 as a buffer against telomere attrition acting upstream of p53, addressing how cells restrain the p53 stress response during telomere erosion.

   Evidence Genome-wide CRISPR synthetic-sick screens with TERT loss, nutlin-3a treatment, and p53 loss-of-function rescue

   PMID:33660365

   Open questions at the time

   • Did not identify the molecular mechanism by which HAPSTR1 constrains p53
   • No physical partner of HAPSTR1 identified in this study
2. 2022 High

   Identified HUWE1 as the key HAPSTR1 partner and defined a reciprocal regulatory feedback loop, answering how HAPSTR1 executes stress signaling and how its own levels are controlled.

   Evidence Domain mutagenesis of conserved oligomerization residues, co-immunoprecipitation, and CRISPR co-dependency/epistasis analysis

   PMID:35776542

   Open questions at the time

   • Did not define which HUWE1 substrates mediate the downstream effects
   • Subcellular site of the HAPSTR1–HUWE1 interaction not resolved
3. 2023 High

   Showed HAPSTR1 is required for nuclear localization of HUWE1, providing the spatial mechanism linking HAPSTR1 to nuclear substrate ubiquitination and p53/NF-κB control.

   Evidence Unbiased quantitative proteomics, subcellular fractionation and imaging, and knockdown/knockout functional readouts across multiple cell lines

   PMID:37167062

   Open questions at the time

   • Mechanism of HUWE1 nuclear import via HAPSTR1 not structurally defined
   • Specific nuclear substrates not enumerated
4. 2023 High

   Revealed a HUWE1-independent genome-maintenance role, addressing how HAPSTR1 contributes to replication-fork stability.

   Evidence Genome-wide CRISPR synthetic-lethality screen with PARP1/2 disruption, Co-IP with BRCA1 and Senataxin, R-loop detection, and fork restart assays

   PMID:37591890

   Open questions at the time

   • Direct versus indirect nature of the BRCA1/Senataxin interactions not fully resolved
   • Whether this function depends on HUWE1 not tested
5. 2024 High

   Placed HAPSTR1 in a defined stability circuit (TRIP12/USP7) and established organismal essentiality independent of p53, clarifying the upstream control and physiological importance of HAPSTR1.

   Evidence Multiplexed proteomics, conditional Hapstr1 knockout mice with p53-null epistasis, and HUWE1 localization assays

   PMID:38453366

   Open questions at the time

   • The p53-independent essential function causing perinatal lethality not identified
   • Conditions regulating the TRIP12/USP7 balance unknown
6. 2024 Medium

   Extended the HAPSTR1 interactome to LRPPRC and PSMD14, addressing how HAPSTR1 may regulate substrate ubiquitination and tumor-cell phenotypes.

   Evidence Co-immunoprecipitation, overexpression/knockdown, and LRPPRC knockdown rescue in ovarian cancer cells

   PMID:38643468

   Open questions at the time

   • Single Co-IP study from one lab without reciprocal validation
   • Direct versus indirect binding not established
   • Relationship to the HUWE1 axis unclear

• The molecular basis of HAPSTR1's p53-independent essential function and the full set of HUWE1 substrates it directs remain unresolved.
• No structural model of HAPSTR1–HUWE1 or how it drives HUWE1 nuclear import
• The essential developmental substrate/pathway underlying perinatal lethality not defined