HROB

HROB (C17orf53) is an OB-fold-containing nuclear protein that functions as a dedicated loader and activator of the MCM8-MCM9 helicase complex during homologous recombination and interstrand crosslink repair. It binds single-stranded DNA and RPA1, directly contacts both MCM8 and MCM9 subunits at the stable heterodimer interface, and stimulates MCM8-9 ATPase and helicase activities to drive recombination-associated DNA synthesis downstream of RAD51, acting in a pathway parallel to the HELQ helicase (PMID:31467087, PMID:32528060, PMID:38678026). HROB is expressed in a cell-cycle-dependent manner with peak levels in S phase, and its ssDNA- and RPA-binding activities are required for cell survival after crosslinking damage (PMID:32853826, PMID:39766854). Loss of HROB in mice causes meiotic arrest and infertility, phenocopying MCM8/MCM9 deficiency (PMID:31467087).

1. 2019 High

   Identification of HROB as the MCM8-9 helicase loader resolved how this replicative helicase is recruited to DNA damage sites for recombination-associated DNA synthesis, establishing a T4 gp59-like loading paradigm in vertebrate HR.

   Evidence Knockout mouse and cell lines with HR assays, recruitment imaging at damage sites, genetic epistasis with HELQ

   PMID:31467087

   Open questions at the time

   • Mechanism by which HROB stimulates MCM8-9 activity was unknown
   • Whether HROB binds DNA or RPA directly was not addressed
   • Structural basis of the HROB–MCM8-9 interaction was undefined
2. 2020 High

   Biochemical reconstitution showed HROB directly binds ssDNA and RPA1 and stimulates MCM8-9 helicase activity in vitro, establishing it as a bona fide activator — not merely a recruitment scaffold — and linking these activities to long-tract gene conversion and replication fork progression through interstrand crosslinks.

   Evidence In vitro helicase and ssDNA-binding assays, reciprocal Co-IP, ICL sensitivity in MCM8IP-deficient cells (two independent labs)

   PMID:32528060 PMID:32853826

   Open questions at the time

   • Whether HROB influences MCM8-9 hexamer assembly or acts post-assembly was unknown
   • Which subunit interface HROB contacts was not mapped
   • Relationship to the Fanconi anemia pathway (e.g., FANCD2) was not mechanistically resolved
3. 2024 High

   Structure-function analysis revealed that MCM8-9 assembles as a hexamer from dimers with two alternating interfaces, and HROB binds specifically at the stable (obligate) heterodimer interface to promote DNA unwinding — likely by coordinating ATP hydrolysis with translocation — without affecting hexamer ring formation on ssDNA.

   Evidence Single-molecule imaging, in vitro ATPase assays, interface mutagenesis, molecular modeling

   PMID:38678026

   Open questions at the time

   • Atomic-resolution structure of the HROB–MCM8-9 complex is lacking
   • How HROB coordinates ATPase firing at the labile versus stable interface remains mechanistically unclear
   • In vivo validation of the proposed hexameric translocation model is absent
4. 2024 Medium

   Cell-cycle profiling established that HROB expression peaks in S phase and is downregulated in quiescence, and FRET-based assays detected proximity to replication machinery components, suggesting a regulated connection between HROB and DNA replication beyond its known repair role.

   Evidence Flow cytometry FRET, cell-cycle fractionation, ectopic overexpression with replication readouts

   PMID:39766854

   Open questions at the time

   • Identity of the replication machinery components detected by FRET was not resolved to specific factors
   • The mechanism by which HROB overexpression inhibits replication is undefined
   • Whether the S-phase expression pattern reflects a replication function distinct from repair is untested
5. 2025 Medium

   A reported interaction with ZC3HC1 linked HROB to cell-cycle regulation through CCNB1 ubiquitination and degradation, suggesting a non-canonical role in controlling the G2/M transition independent of its MCM8-9 loading function.

   Evidence Co-immunoprecipitation, phosphorylation and ubiquitination assays, overexpression/loss-of-function with proliferation readouts

   PMID:40654113

   Open questions at the time

   • Not yet independently replicated by a second laboratory
   • Whether this ZC3HC1 pathway operates in physiological contexts or only upon overexpression is unclear
   • Relationship between the MCM8-9 loading function and CCNB1 regulation has not been tested

• Key open questions include the atomic structure of the HROB–MCM8-9 complex, the mechanism by which HROB coordinates ATPase-driven translocation, whether HROB has a direct role in unperturbed DNA replication, and whether its reported cell-cycle regulatory function via ZC3HC1/CCNB1 is physiologically relevant.
• No high-resolution structure of the HROB–MCM8-9 complex exists
• Physiological relevance of the ZC3HC1–CCNB1 axis awaits independent confirmation
• Whether HROB functions at replication forks in the absence of damage is untested