Mst27D is a spermatid-specific microtubule-binding protein related to the EB1 family. Its N-terminal calponin-homology domain binds microtubules, while its C-terminal region mediates dimerization and binding to the nuclear pore protein Nup358. This links the nuclear envelope to the spermatid dense complex and promotes progressive microtubule bundling during nuclear elongation. Unlike canonical EB1, intact Mst27D binds along microtubules rather than preferentially tracking their growing plus ends.
Summary: Mst27D organizes spermatid microtubule bundles, but membership of the GO-defined microtubule organizing center is unresolved.
Reason: PTHR10623 places the MTOC IBD at eukaryotic ancestor PTN000065701. GO:0005815 includes structures that nucleate and anchor microtubules and is not restricted to mitotic centrosomes. Mst27D functions postmeiotically near the dense complex and basal-body region; this timing therefore cannot alone reject MTOC localization. The direct study establishes microtubule and nuclear-envelope association, while distinguishing dense-complex scaffolding from actual MTOC membership requires focused adjudication.
Propagation Review
Root cause:UNRESOLVED
Sources checked:
PANTHER:PTN000065701 · PTN000065701 UNRESOLVED
The conserved MTOC node cannot be rejected merely because Mst27D acts after meiosis.
Summary: Mst27D binds the cytoplasmic microtubules of the spermatid dense complex and couples them to nuclear pores. Direct microscopy and domain experiments support this compartment.
Reason: Mst27D binds the cytoplasmic microtubules of the spermatid dense complex and couples them to nuclear pores. Direct microscopy and domain experiments support this compartment.
Summary: The Mst27D CH domain mediates microtubule association in a dimerization-dependent context. Full-length protein binds along microtubules, and the null mutant disrupts their bundling.
Reason: The Mst27D CH domain mediates microtubule association in a dimerization-dependent context. Full-length protein binds along microtubules, and the null mutant disrupts their bundling.
In Mst27D null mutants, this bundling process does not occur and nuclear elongation is abnormal.
GO:0031110 regulation of microtubule polymerization or depolymerization
IBA GO_REF:0000033
UNDECIDED
Summary: Mst27D directly promotes microtubule bundling, while its effect on polymerization/depolymerization remains unresolved.
Reason: The current PTN000065701 IBD retains this process. PMID:37428798 demonstrates bundling and discusses stabilization or growth as possible consequences, but the analyzed experiments do not isolate polymerization/depolymerization kinetics. Bundling could contribute structurally to regulation, so an absence of catalytic or direct kinetic evidence is not a demonstration of loss. A focused assessment of dynamic effects is pending.
Propagation Review
Root cause:UNRESOLVED
Sources checked:
PANTHER:PTN000065701 · PTN000065701 UNRESOLVED
Current PAINT retains the process; target bundling evidence neither proves nor refutes the dynamics-specific claim.
Summary: Full-length Mst27D binds along microtubules without EB1-like plus-end preference; the narrower plus-end assertion remains unresolved.
Reason: PTN000065701 carries the ancestral plus-end localization and binding assertions. GO:0051010 requires binding to the plus end, and GO:0035371 identifies the growing plus end; neither definition requires preferential comet-like tracking. PMID:37428798 establishes lattice-wide binding and loss of Eb1-like preference, which does not automatically exclude any end binding. The comet-forming construct is the isolated C-terminal fragment lacking the N-terminal CH domain, not a C-terminal truncation retaining that domain; the authors propose heterodimerization with endogenous EB-family proteins. Neither the fragment result nor the lack of preference settles full-length inherited capacity.
Propagation Review
Root cause:UNRESOLVED
Sources checked:
PANTHER:PTN000065701 · PTN000065701 UNRESOLVED
Target divergence in binding preference is established, but complete loss of plus-end binding/localization is not yet demonstrated.
Summary: Mst27D links Nup358-containing nuclear pores with dense-complex microtubules; its C-terminal region recruits Nup358 into microtubule cables. This provides target-specific support for protein localization to microtubules.
Reason: Mst27D links Nup358-containing nuclear pores with dense-complex microtubules; its C-terminal region recruits Nup358 into microtubule cables. This provides target-specific support for protein localization to microtubules.
Summary: Full-length Mst27D binds along microtubules without EB1-like plus-end preference; the narrower plus-end assertion remains unresolved.
Reason: PTN000065701 carries the ancestral plus-end localization and binding assertions. GO:0051010 requires binding to the plus end, and GO:0035371 identifies the growing plus end; neither definition requires preferential comet-like tracking. PMID:37428798 establishes lattice-wide binding and loss of Eb1-like preference, which does not automatically exclude any end binding. The comet-forming construct is the isolated C-terminal fragment lacking the N-terminal CH domain, not a C-terminal truncation retaining that domain; the authors propose heterodimerization with endogenous EB-family proteins. Neither the fragment result nor the lack of preference settles full-length inherited capacity.
Propagation Review
Root cause:UNRESOLVED
Sources checked:
PANTHER:PTN000065701 · PTN000065701 UNRESOLVED
Target divergence in binding preference is established, but complete loss of plus-end binding/localization is not yet demonstrated.
Summary: The characterized native Mst27D role begins after meiosis and supports spermatid microtubule bundling rather than spindle assembly.
Reason: The spindle-assembly IBD at PTN000065701 describes ancestral EB-family biology. PMID:37428798 provides target-specific developmental divergence: endogenous Mst27D accumulation begins after meiosis, and null-mutant analysis detects no earlier abnormalities but severe spermatid nuclear-elongation defects. This combination of native timing and stage-resolved perturbation challenges retention of the ancestral spindle-assembly role, rather than simply relying on absent target experiments or distant experimental descendants.
Propagation Review
Root cause:PROPAGATION BAD
Failure modes:CONTEXT OR TISSUE MISMATCH
Sources checked:
PANTHER:PTN000065701 · PTN000065701 SUPPORTS SOURCE BUT NOT TARGET
Endogenous postmeiotic expression and stage-resolved null phenotype support divergence from the ancestral spindle-assembly role.
Summary: Spindle-midzone localization is poorly matched to the characterized native postmeiotic Mst27D distribution.
Reason: PTN000065701 carries the spindle-midzone IBD. Endogenous Mst27D begins accumulating after meiosis and localizes to the spermatid nuclear envelope/dense complex (PMID:37428798). This target-specific timing and distribution challenge a native spindle-midzone assignment; a microtubule-organizing center is a different and broader structure and is being assessed separately. Retain a cautious over-annotation call rather than claiming that all possible ectopic or low-level spindle association has been excluded.
Propagation Review
Root cause:PROPAGATION BAD
Failure modes:CONTEXT OR TISSUE MISMATCH
Sources checked:
PANTHER:PTN000065701 · PTN000065701 SUPPORTS SOURCE BUT NOT TARGET
Native postmeiotic distribution diverges from the spindle-midzone context of the ancestral assertion.
IMP PMID:37428798 Nuclear elongation during spermiogenesis depends on physical...
NEW
Summary: Mst27D is required for progressive bundling of dense-complex microtubules during spermatid nuclear elongation.
Reason: Loss of Mst27D prevents normal progressive bundling and disrupts nuclear elongation; overexpression also promotes microtubule cables. This is the direct cytoskeletal assembly role.
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Microtubule binding is directly supported by CH-domain experiments, full-length localization and the bundling phenotype; plus-end-specific and polymerization-dynamics claims are unsupported EB1-family carry-over.
Review rationale: Microtubule binding is directly supported by CH-domain experiments, full-length localization and the bundling phenotype. The same broad term already appears in GOA. Although GOA also contains plus-end binding, primary evidence shows that Mst27D binds along the lattice rather than sharing EB1’s preference, so the unsupported narrower annotation is not grounds for LSP. ProtNLM correctly captures the established broad activity. The OpenScientist audit also retained GO:0008017 while identifying plus-end, spindle and dynamics terms as EB1/MAPRE-family IBA carry-over.
Supporting Evidence:
PMID:37428798: "Moreover, Mst27D binds all along MTs, contrasting with Eb1’s preference for growing MT plus ends."
PMID:37428798: "In Mst27D null mutants, this bundling process does not occur and nuclear elongation is abnormal."