RHNO1

RHNO1 (RHINO) is a nuclear checkpoint and DNA-repair factor that amplifies ATR–Chk1 signaling by bridging the 9-1-1 (RAD9-RAD1-HUS1) clamp to the ATR activator TopBP1 (PMID:21659603). It assembles into a stable stoichiometric heterotetramer with the 9-1-1 clamp, contacting both RAD9 and RAD1, and is recruited to DNA damage sites by 9-1-1; tethering RHINO to chromatin is sufficient to drive Chk1 phosphorylation in a RAD9- and Claspin-dependent manner, and its loss attenuates ATR–Chk1 signaling after UV without disrupting 9-1-1 loading (PMID:21659603, PMID:25602520). Structurally, RHINO engages the edge and back face of the 9-1-1 ring through the RAD1 subunit, establishing 9-1-1 as a double-faced clamp, and it binds RAD1 via a motif shared with the clamp loader RAD17, with which it competes (PMID:31776186, PMID:36841485). Separately, RHINO confines mutagenic microhomology-mediated end joining to mitosis: it accumulates in M phase, is phosphorylated by PLK1, directly binds Polymerase theta (Polθ), and recruits Polθ to double-strand breaks for MMEJ repair (PMID:37440612, PMID:36993461). In cancer cells RHNO1 supports clonogenic growth, homologous recombination repair, and PARP-inhibitor resistance, and is co-regulated with FOXM1 through a shared bidirectional promoter (PMID:33890574).

1. 2011 High

   Established RHINO as a checkpoint signaling factor by showing it physically bridges the 9-1-1 clamp and TopBP1 and is required for full Chk1 activation, answering how 9-1-1 and the ATR activator are functionally linked.

   Evidence Co-IP, DNA-damage focus recruitment, and knockdown with phospho-Chk1 readout in human cells

   PMID:21659603

   Open questions at the time

   • Did not resolve the stoichiometry or structural basis of the 9-1-1 interaction
   • Mechanism by which RHINO stimulates ATR remained undefined
2. 2015 High

   Reconstituted RHINO into a defined heterotetrameric complex with 9-1-1 and demonstrated that chromatin-tethered RHINO is sufficient to trigger Chk1 phosphorylation, separating RHINO's signaling role from 9-1-1 loading.

   Evidence In vitro complex reconstitution, LacO/LacR tethering, chromatin fractionation, and siRNA with phospho-Chk1 readout

   PMID:25602520

   Open questions at the time

   • Atomic interface with the clamp unresolved
   • Why only a portion of RHINO is chromatin-bound was not explained
3. 2019 High

   Defined the atomic interface by crystallizing 9-1-1 with a RHINO peptide, revealing RHINO binds the edge/back face via RAD1 and that 9-1-1 is a functionally double-faced clamp.

   Evidence X-ray crystallography of the 9-1-1–RHINO peptide complex

   PMID:31776186

   Open questions at the time

   • Functional consequence of back-face binding not tested in cells
   • TopBP1-binding region of RHINO not structurally mapped
4. 2023 Medium

   Showed the RAD1 binding motif is shared between RHINO and the clamp loader RAD17 and that the two compete, implying RHINO participates in clamp loading/unloading dynamics.

   Evidence Crystal structure of 9-1-1–RAD17 peptide and competitive binding assay compared to RHINO binding

   PMID:36841485

   Open questions at the time

   • RHINO's role in loading/unloading inferred structurally, not directly tested with RHINO
   • Biological outcome of RHINO–RAD17 competition unmeasured
5. 2023 High

   Uncovered a checkpoint-independent function in which RHINO restricts mutagenic MMEJ to mitosis by recruiting Polθ to DSBs under PLK1 control, defining a cell-cycle-gated repair pathway choice.

   Evidence CRISPR synthetic-lethal screen, cell-cycle fractionation, RHINO–Polθ Co-IP, PLK1 phosphorylation assay, and repair assays

   PMID:36993461 PMID:37440612

   Open questions at the time

   • How PLK1 phosphorylation governs the Polθ interaction is undefined
   • Relationship between the MMEJ role and the 9-1-1/ATR role not mechanistically integrated
6. 2021 Medium

   Placed RHNO1 in an oncogenic context, linking it to HR repair, PARP-inhibitor resistance, and FOXM1 co-regulation via a shared bidirectional promoter in ovarian carcinoma cells.

   Evidence Promoter reporter assays, knockdown, clonogenic and DR-GFP HR assays, and PARP-inhibitor sensitivity assay

   PMID:33890574

   Open questions at the time

   • Biochemical mechanism by which RHNO1 promotes HR not reconstituted
   • Whether HR effects depend on the 9-1-1 or Polθ functions untested
7. 2010 Medium

   Provided early evidence that RHNO1 (C12orf32) is cell-cycle-regulated and proteolytically processed and supports proliferation by enabling G1/S transition.

   Evidence Immunocytochemistry, shRNA knockdown, western blot, and proliferation assay in breast cancer cells

   PMID:20811708

   Open questions at the time

   • Functional role of the 35→16 kDa processing unknown
   • No pathway placement beyond a G1/S phenotype
8. 2023 Low

   Reported that RHNO1 knockdown induces mitochondrial apoptosis via PI3K/Akt inactivation in hepatocellular carcinoma, suggesting a survival-signaling link.

   Evidence siRNA knockdown, apoptosis assay, PI3K/Akt western blots, and xenograft

   PMID:37364391

   Open questions at the time

   • Pathway placement inferred from western blots without epistasis or reconstitution
   • Connection to RHINO's checkpoint/repair functions not addressed

• How RHINO's distinct roles—ATR–Chk1 amplification via 9-1-1/TopBP1 and PLK1-gated Polθ recruitment for mitotic MMEJ—are coordinated within a single protein across the cell cycle remains unresolved.
• No structural or functional model integrating the 9-1-1 and Polθ interaction surfaces
• Regulatory logic switching RHINO between checkpoint and repair modes undefined