RMI2

RMI2 is an OB-fold-containing scaffolding subunit of the BLM-Topo IIIα-RMI1-RMI2 (BTRR/BTR) dissolvasome complex that is essential for genome stability through its roles in double Holliday junction dissolution, DNA end resection, D-loop disruption, and ultrafine anaphase bridge resolution. RMI2 stabilizes the BTR complex, targets BLM helicase to chromatin and ultrafine DNA bridges, and—together with RMI1—potentiates the Topo IIIα-mediated strand-passage activity that forms a size-adjustable ssDNA gate capable of processive relaxation of negatively supercoiled DNA (PMID:18923083, PMID:35102151, PMID:41576078). The Topo IIIα-RMI1-RMI2 sub-complex orients BLM to efficiently disrupt D-loops, thereby directing homologous recombination toward non-crossover outcomes, and stimulates 5′ end resection processivity (PMID:35115525, PMID:25200081). During mitosis, MPS1 phosphorylates RMI2 at Ser112 to regulate its nuclear matrix redistribution and spindle assembly checkpoint maintenance, while CDK1-mediated destabilization of the BTRR complex at centromeres prevents inappropriate DNA unwinding (PMID:24108125, PMID:27977684).

1. 2008 High

   Identification of RMI2 as a core BTR complex subunit resolved how Topo IIIα-RMI1 achieves stable association with BLM and efficient dHJ dissolution, establishing that RMI2 stabilizes the complex, recruits BLM to chromatin, and is required for dissolvasome function.

   Evidence Co-immunoprecipitation, siRNA depletion, dHJ dissolution assays, chromatin fractionation, and immunofluorescence in human cells

   PMID:18923083

   Open questions at the time

   • Structural basis of RMI2 OB-fold interactions within the complex not resolved
   • Whether RMI2 has catalytic activity or is purely scaffolding was unclear
   • In vivo phenotype of complete RMI2 loss not yet characterized
2. 2013 High

   Three studies collectively established that the BTR complex functions beyond dHJ dissolution: RMI1-RMI2 potentiate Topo IIIα stimulation of BLM-mediated DNA unwinding and 5′ end resection processivity, RPA interaction with RMI1 is required for efficient dissolution, and MPS1 phosphorylation of RMI2 at Ser112 regulates spindle assembly checkpoint maintenance independent of BLM.

   Evidence Reconstituted in vitro resection/unwinding/dissolution assays with purified proteins; phospho-specific antibodies, S112A mutagenesis, rescue of RMI2-depleted cells, and cellular fractionation

   PMID:23543748 PMID:24108125 PMID:25200081

   Open questions at the time

   • How Ser112 phosphorylation mechanistically links to SAC signaling components was undefined
   • Whether resection stimulation is relevant in vivo was not tested
   • Direct structural role of RMI2 OB-fold in DNA binding during resection not determined
3. 2016 High

   Loss-of-function analysis in patient-derived and CRISPR knockout cells demonstrated that RMI2 is required in vivo for BLM localization to ultrafine DNA bridges, suppression of sister chromatid exchanges, and prevention of anaphase bridges and micronuclei—phenocopying partial Bloom syndrome.

   Evidence Patient cells with homozygous RMI2 deletion, CRISPR knockout, SCE assays, immunofluorescence for BLM and FANCD2

   PMID:27977684

   Open questions at the time

   • Whether RMI2 loss causes the full clinical spectrum of Bloom syndrome was not established
   • Mechanism by which RMI2 promotes FANCD2 localization to bridges was not defined
   • Contribution of RMI2 to other DNA repair pathways beyond HR/dissolution not tested
4. 2022 High

   Single-molecule studies revealed two new mechanistic functions: the TRR complex forms a size-adjustable ssDNA gate (~8.5 nm) capable of dsDNA strand passage, and TRR orients BLM to efficiently disrupt D-loops, establishing how the complex controls HR pathway choice at early recombination intermediates.

   Evidence Single-molecule optical tweezers, fluorescence microscopy, single-molecule FRET, and magnetic tweezers with purified TRR and BLM

   PMID:35102151 PMID:35115525

   Open questions at the time

   • Whether dsDNA passage through the gate occurs in vivo is unconfirmed
   • Structural basis of how TRR reorients BLM on D-loops not resolved
   • Role of RMI2 specifically vs. RMI1 in gate mechanics not dissected
5. 2025 Medium

   Discovery of RAD54L2 as a BTRR-interacting SNF2-family ATPase revealed a new upstream regulator that promotes BLM chromatin recruitment and non-crossover recombination through its ATPase activity.

   Evidence BioID proximal proteomics, co-immunoprecipitation, ATPase domain mutagenesis, SCE assays, chromatin recruitment assays

   PMID:39870965

   Open questions at the time

   • Whether RAD54L2 contacts RMI2 directly or interacts via BLM is unknown
   • Single-lab finding not yet independently replicated
   • Mechanism by which ATPase activity facilitates BLM recruitment is undefined
6. 2026 High

   Demonstration that the TRR complex processively relaxes negatively supercoiled DNA on a timescale compatible with PICH-generated supercoiling provided direct biophysical evidence supporting TRR's role in ultrafine anaphase bridge resolution.

   Evidence Single-molecule optical tweezers with fluorescence imaging and real-time supercoiling density measurement

   PMID:41576078

   Open questions at the time

   • How BLM and PICH coordinate with TRR processivity in vivo is not established
   • Whether processive relaxation requires RMI2 specifically or is driven by Topo IIIα catalysis alone is not dissected
   • Structural basis of stable post-relaxation DNA binding is unknown

• The individual structural and catalytic contributions of RMI2 within the TRR gate, its precise role in mitotic checkpoint signaling downstream of Ser112 phosphorylation, and whether RMI2 deficiency constitutes a bona fide Bloom syndrome spectrum disorder remain unresolved.
• No high-resolution structure of the full BTRR complex on a DNA substrate
• Downstream effectors of pSer112-RMI2 in SAC signaling unknown
• Clinical significance of RMI2 mutations in human disease not fully defined