MIS18BP1

MIS18BP1 (M18BP1/KNL-2) is a central epigenetic licensing factor for centromere identity that couples recognition of existing CENP-A chromatin to the deposition of new CENP-A nucleosomes, restricting this assembly to a defined cell-cycle window (PMID:28743005, PMID:41629527). It is recruited to centromeres by directly binding the constitutive centromere protein CENP-C through its conserved SANT/SANTA domain, and loss of CENP-C abolishes both M18BP1 targeting and downstream CENP-A assembly (PMID:21911481, PMID:22540025). M18BP1 also reads existing centromeres directly by binding CENP-A nucleosomes via a conserved motif resembling that of CENP-C, with the two proteins competing for the same nucleosomal surface (PMID:28743005). Productive function requires M18BP1 to dimerize on a Mis18α:Mis18β 4:2 hexamer scaffold through its N-terminal 140 residues, an assembly that licenses recognition of multivalent CENP-A arrays and subsequent cooption of PLK1 and the CENP-A chaperone HJURP for new CENP-A deposition (PMID:28059702, PMID:41629527). This entire cycle is governed by CDK/cell-cycle phosphorylation: phosphorylation destabilizes M18BP1 binding to both the Mis18α:Mis18β scaffold and to CENP-A nucleosomes, and in metaphase M18BP1–CENP-C binding competitively blocks HJURP access, so that dephosphorylation in G1/interphase is required to permit CENP-A loading (PMID:28059702, PMID:37141119, PMID:41680291). Beyond centromere licensing, M18BP1 directly binds the CAP-G2 subunit of condensin II and activates it for mitotic chromosome condensation, displacing the condensin II antagonist MCPH1 at mitotic onset — a function genetically separable from CENP-A loading (PMID:34734636, PMID:40614722).

1. 2011 High

   Established how new CENP-A assembly machinery is templated onto pre-existing centromeres by identifying CENP-C as the direct recruitment factor for M18BP1.

   Evidence Co-IP, direct binding assays, and CENP-C depletion with centromere localization and CENP-A assembly readouts in human cells

   PMID:21911481

   Open questions at the time

   • Did not define the M18BP1 domain mediating the interaction
   • Did not resolve cell-cycle regulation of the interaction
2. 2012 High

   Mapped the M18BP1–CENP-C interaction to the central SANT domain of M18BP1 and the CENP-C C-terminus, confirming the recruitment axis in a second model system.

   Evidence Centromere interaction screen, domain mapping by Co-IP/pulldown, and CENP-C knockdown in mouse embryonic stem cells

   PMID:22540025

   Open questions at the time

   • Did not address how the interaction is restricted by the cell cycle
   • Did not reconstitute the interaction biochemically
3. 2017 High

   Defined the molecular architecture and cell-cycle gating of the Mis18 complex, showing M18BP1 dimerizes on a Mis18α:Mis18β hexamer and that CDK1 phosphorylation of its N-terminus times complex formation to G1.

   Evidence Biochemical reconstitution with defined stoichiometry, in vitro CDK1 phosphorylation, phosphomutant analysis, and cell-based centromere localization

   PMID:28059702

   Open questions at the time

   • Did not establish how the assembled complex recognizes centromeric chromatin
   • Did not link scaffold assembly to HJURP recruitment
4. 2017 High

   Revealed that M18BP1 directly reads CENP-A nucleosomes through a CENP-C-like motif and competes with CENP-C, providing a chromatin-recognition mechanism for centromere identity.

   Evidence Xenopus egg extract assembly assay, direct nucleosome binding, competition assay, and motif mutagenesis

   PMID:28743005

   Open questions at the time

   • Did not resolve how nucleosome binding and CENP-C binding are coordinated in vivo
   • Did not define the phosphoregulation of nucleosome binding
5. 2019 High

   Showed that CDK phosphorylation of M18BP1 is required for its metaphase SANTA-domain-mediated CENP-C binding and for its nuclear localization, integrating localization control with assembly competence.

   Evidence Xenopus egg extract and cell-based assays with phosphomutant and domain-deletion analysis

   PMID:30606714

   Open questions at the time

   • Did not reconcile metaphase CENP-C binding with the licensing of G1 assembly
   • Did not identify the responsible kinase sites in detail
6. 2021 High

   Demonstrated in C. elegans that the CENP-A N-tail directly binds KNL-2/M18BP1 and can partially substitute for the absent HJURP, illuminating an evolutionarily divergent CENP-A loading route.

   Evidence Direct pulldown, N-tail deletion analysis, and RNAi functional assays with kinetochore and condensation readouts

   PMID:33852350

   Open questions at the time

   • Relationship to the HJURP-dependent vertebrate pathway not addressed
   • Structural basis of the N-tail interaction unresolved
7. 2021 High

   Genetically separated M18BP1's two activities by showing CDK-1 phosphorylation of C-terminal KNL-2 sites controls condensin II loading and chromosome condensation independently of CENP-A loading.

   Evidence In vitro CDK-1 phosphorylation, phosphodeficient mutant worms, condensin II/CENP-A localization, and lethality quantification

   PMID:34734636

   Open questions at the time

   • Did not identify the direct condensin II contact
   • Mechanism of condensin II activation not resolved
8. 2024 Medium

   Identified a non-canonical KNL-2 function in meiotic kinetochore assembly via recruitment of the nucleoporin MEL-28/ELYS, expanding M18BP1's roles beyond CENP-A loading.

   Evidence RNAi/degron depletion, N-terminal domain mutants, and co-depletion epistasis in C. elegans oocytes

   PMID:39353426

   Open questions at the time

   • No direct binding reconstitution reported
   • Single-lab epistasis without structural validation
9. 2024 Medium

   Showed that CENP-C can recruit CENP-A via HJURP independently of Knl2/MIS18BP1 in chicken cells, defining the boundary of M18BP1's requirement in the pathway.

   Evidence Artificial tethering, auxin-inducible degron knockout, and Co-IP in DT40 cells

   PMID:38319136

   Open questions at the time

   • Physiological contribution of the Knl2-independent route unclear
   • Single-lab; potential species-specific behavior
10. 2023 High

    Established a metaphase inhibitory mechanism whereby M18BP1–CENP-C binding competitively blocks HJURP, requiring its relief plus HJURP dephosphorylation to permit CENP-A assembly.

    Evidence Xenopus cell-free assembly assay, competitive binding analysis, and phosphomutant HJURP rescue

    PMID:37141119

    Open questions at the time

    • How the inhibitory and recruitment roles of M18BP1 are temporally switched not fully resolved
    • Structural basis of HJURP exclusion not defined
11. 2025 High

    Identified M18BP1's direct condensin II activation mechanism: it binds CAP-G2 and displaces the antagonist MCPH1 at mitotic onset to license condensation.

    Evidence Genetic and proteomic approaches, direct M18BP1–CAP-G2 binding, MCPH1 competition, and condensation readouts

    PMID:40614722

    Open questions at the time

    • Structural basis of the M18BP1/MCPH1 switch on CAP-G2 unresolved
    • How this is coordinated with M18BP1's CENP-A licensing role unclear
12. 2026 High

    Provided the first SANTA domain crystal structure and showed that dimerization-driven M18BP1 recognizes multivalent CENP-A assemblies via multiple CCAN-contacting motifs before PLK1/HJURP cooption.

    Evidence Artificial dimerization constructs, SANTA domain crystal structure, CCAN interaction footprint mapping, and CENP-A deposition assays in human cells

    PMID:41629527

    Open questions at the time

    • Higher-order architecture of the M18BP1–CCAN–CENP-A assembly not resolved
    • Temporal order of PLK1 and HJURP cooption not fully defined
13. 2026 High

    Defined a phosphoregulatory switch on M18BP1's CENP-A nucleosome binding, showing metaphase phosphorylation disrupts binding while interphase dephosphorylation enables assembly.

    Evidence Xenopus egg extract assembly assay, phosphomutant analysis, and direct CENP-A nucleosome binding

    PMID:41680291

    Open questions at the time

    • Identity of all responsible kinases/phosphatases not fully resolved
    • Integration with the CENP-C-binding switch not detailed

• How M18BP1's dual roles in CENP-A licensing and condensin II activation are coordinated within a single mitotic timeline, and the structural basis of its multivalent CCAN/CENP-A recognition, remain open.
• No integrated structural model of the full M18BP1-centromere assembly
• Mechanistic coupling between condensation and CENP-A licensing functions undefined