MCMBP

MCMBP (MCM-BP) is a dedicated chaperone and regulator of the MCM2-7 replicative helicase that governs the assembly, nuclear delivery, and turnover of MCM subcomplexes to maintain replication licensing fidelity and genome stability (PMID:35438632, PMID:21196493). It associates preferentially with MCM3 and MCM3-7 subcomplexes — interacting with individual MCM subunits but not the intact MCM2-7 hexamer — and uses nascent MCM3 to template hexamer assembly, such that acute MCMBP depletion lowers MCM2-7 levels, reduces licensing, and triggers p53-dependent G1 arrest or, in p53-null cells, DNA damage and loss of viability (PMID:35438632, PMID:21196493, PMID:22540012). During biogenesis MCMBP facilitates assembly and nuclear transport of MCM3-7 subcomplexes and is then removed by the CRL4DCAF12 ubiquitin ligase to permit MCM2 incorporation; loss of this ligase reduces chromatin-bound nascent MCMs and produces accelerated forks and replication stress (PMID:41145411). In addition to its biogenesis role, MCMBP accumulates in nuclei in late S phase and promotes the replication-dependent disassembly of MCM2-7 from chromatin, and it inhibits DDK (via direct binding to its Dbf4 subunit) phosphorylation of MCM4,6,7 without itself being a DDK substrate (PMID:21196493, PMID:22540012). Loss of MCMBP function causes abnormal nuclear morphology, centrosome amplification, replication stress, and DNA-damage checkpoint activation across human cells and yeast orthologs (PMID:22250201, PMID:21813639, PMID:23322785), establishing it as a balance point between nascent and parental MCM pools.

1. 2010 High

   Established that MCM-BP is a late-S-phase factor that actively disassembles the MCM2-7 helicase from chromatin, defining a regulated unloading step distinct from licensing.

   Evidence Xenopus egg extract immunodepletion, recombinant protein on isolated chromatin, and human cell siRNA knockdown

   PMID:21196493

   Open questions at the time

   • Did not define how MCM-BP recognizes replicated versus unreplicated chromatin
   • Mechanism of helicase disassembly not resolved at the molecular level
2. 2010 Medium

   Showed MCM-BP forms a stable complex with MCM7 but not the intact MCM2-7 hexamer, indicating it engages MCM subunits in a subcomplex rather than the assembled helicase.

   Evidence Reciprocal co-immunoprecipitation from Xenopus egg extracts

   PMID:21196493

   Open questions at the time

   • Single organism/condition
   • Did not establish whether the MCM7 interaction reflects assembly or disassembly intermediates
3. 2011 Medium

   Defined the binding selectivity of the ortholog for Mcm3-7 (excluding Mcm2) and showed that excess MCM-BP disrupts Mcm2 association, links the protein to replication and checkpoint control.

   Evidence Co-IP and overexpression with checkpoint/cell-cycle assays in fission yeast; protein complex purification with temperature-sensitive alleles

   PMID:21813639 PMID:22036784

   Open questions at the time

   • Whether the Mcm2-excluding subcomplex is an assembly or disassembly intermediate not resolved
   • Meiotic role only preliminarily characterized
4. 2012 High

   Identified a direct mechanism by which MCM-BP restrains helicase activation: it binds DDK's Dbf4 subunit and inhibits DDK phosphorylation of MCM4,6,7 without being a substrate itself.

   Evidence Yeast 2-hybrid, insect-cell co-expression, endogenous co-IP, and in vitro kinase assays

   PMID:22540012

   Open questions at the time

   • Whether DDK inhibition occurs on chromatin in vivo not established
   • Functional consequence of MCM2 escaping inhibition unclear
5. 2012 Medium

   Demonstrated that loss of MCM-BP produces genome-instability phenotypes — abnormal nuclei, centrosome amplification, and replication stress — specific to MCM-BP rather than other MCM subunits.

   Evidence shRNA knockdown with shRNA-resistant rescue, flow cytometry, and immunofluorescence in human cells

   PMID:22250201

   Open questions at the time

   • Did not separate replication-licensing defects from disassembly defects as the cause of phenotypes
   • Centrosome amplification mechanism unexplained
6. 2013 High

   Placed the ortholog upstream in pre-RC formation, showing it interacts with MCMs throughout the cycle, supports Mcm7 origin loading, and controls MCM nuclear retention.

   Evidence Co-IP, in vitro binding, temperature-sensitive genetics, multicopy suppressor screen, ChIP, and localization microscopy in fission yeast

   PMID:23322785

   Open questions at the time

   • Reconciliation of pre-RC (assembly) and disassembly roles not fully resolved
   • Direct nuclear-import mechanism not defined
7. 2014 Low

   Linked MCMBP dosage perturbation to abnormal nuclear morphology via aberrant sister chromatid cohesion in cancer cell lines.

   Evidence siRNA knockdown and overexpression with nuclear morphology analysis in breast and colorectal cells

   PMID:25246271

   Open questions at the time

   • Single method per condition with no mechanistic placement beyond morphology
   • Cohesion link not validated biochemically
8. 2022 High

   Resolved MCMBP's core function as an MCM3-anchored hexamer-assembly chaperone, showing acute loss collapses MCM2-7 levels and licensing with p53-dependent outcomes.

   Evidence Auxin-inducible degron depletion, co-IP, flow cytometry, DNA-damage markers, and viability assays in human cells

   PMID:35438632

   Open questions at the time

   • Structural basis of MCM3 recognition not defined
   • Relationship between assembly role and the earlier disassembly role not unified
9. 2025 High

   Defined MCMBP turnover control: CRL4DCAF12 degrades MCMBP to allow MCM2 incorporation into the MCM3-7 subcomplex, tying ligase loss to accelerated forks and replication stress.

   Evidence CRL4DCAF12 knockout/depletion, MCMBP stability assays, chromatin fractionation, and fork-speed measurement in human cells

   PMID:41145411

   Open questions at the time

   • Degron and recognition determinants on MCMBP not mapped
   • How removal timing is coupled to MCM2 availability unresolved
10. 2024 Medium

    Extended MCMBP function to an in vivo developmental context, linking its loss to fork acceleration, p53 activation, and microcephaly in neural progenitors.

    Evidence Conditional knockout in mouse neural progenitors, DNA fiber assay, immunofluorescence, p53 analysis (preprint)

    Open questions at the time

    • Preprint, not yet peer-reviewed
    • Direct molecular cause of fork acceleration not established
    • Whether centrosome/RGC attachment defects are MCM3-specific or MCMBP-dependent unclear

• How the assembly-chaperone and chromatin-disassembly activities are mechanistically partitioned, and the structural basis for MCMBP's subunit-selective binding, remain unresolved.
• No structure of MCMBP bound to MCM subunits
• No reconstitution distinguishing assembly versus disassembly modes
• Recruitment determinants to late-S chromatin undefined