SPC24 (also MUN, MERISTEM UNSTRUCTURED; locus At3g08880) is the Arabidopsis thaliana homolog of kinetochore protein SPC24, one of the four subunits of the NDC80 outer-kinetochore complex (NDC80, NUF2, SPC24, SPC25). It is a small (201 aa) protein with a central coiled-coil and a C-terminal RWD-like globular domain; together with SPC25 it forms the inner-kinetochore-anchoring module of the NDC80 complex, while NDC80 and NUF2 form the microtubule-binding module. The NDC80 complex localizes to the outer kinetochore at centromeres and provides the principal end-on attachment site coupling spindle microtubules to chromosomes, and is required for accurate chromosome segregation and spindle-checkpoint signaling during cell division. AtSPC24/MUN localizes to the centromere/kinetochore, colocalizing with the centromeric histone variant CENH3, and physically interacts with the other three NDC80-complex subunits (AtNDC80, AtNUF2, AtSPC25). It is highly expressed in actively dividing tissues (shoot and root apical meristems, vasculature, emerging leaves). Loss of function is essential: null mun mutants arrest in early embryogenesis (division arrest before the globular stage), while weak alleles show stunted growth, DNA aneuploidy, reduced cell-division rate and chromosome-segregation defects, with secondary disorganization of the shoot apical meristem (ectopic WUSCHEL expression).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000775 chromosome, centromeric region | IEA GO_REF:0000044 | ACCEPT | Summary: Centromeric localization derived from the UniProt "Chromosome, centromere" subcellular-location term. Experimentally corroborated: AtSPC24/MUN colocalizes with CENH3 at the centromere [PMID:29356153]. Reason: Accurate. The protein is an NDC80-complex kinetochore subunit localizing to the centromeric region; AtSPC24/MUN was shown to colocalize with the centromere-specific histone variant CENH3. GO:0000775 is correct, and the more specific child term GO:0000776 "kinetochore" is also separately annotated (IDA). Supporting Evidence: PMID:29356153 The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. |
| GO:0051983 regulation of chromosome segregation | IEA GO_REF:0000002 | ACCEPT | Summary: Electronic InterPro2GO inference (IPR044951 SPC24-like to GO:0051983). Redundant with, and superseded by, the experimental IMP annotation to the same term from PMID:29356153. Reason: Correct in direction and consistent with the experimental annotation to the identical term. SPC24/NDC80-complex proteins are required for accurate chromosome segregation and contribute to spindle-checkpoint regulation [PMID:29356153]. Retained as a valid (if redundant) electronic annotation. Supporting Evidence: PMID:29356153 The mutant exhibits stunted growth, embryo arrest, DNA aneuploidy, and defects in chromosome segregation with a low cell division rate. |
| GO:0031262 Ndc80 complex | ISS GO_REF:0000024 | ACCEPT | Summary: Membership in the NDC80 complex, transferred by sequence similarity from human SPC24 (UniProtKB:Q8NBT2). Strongly confirmed experimentally in Arabidopsis: the UniProt SUBUNIT statement reports SPC24 as a component of the NDC80 complex, and AtSPC24/MUN physically interacts with AtNDC80 (AT3G54630), AtNUF2 (AT1G61000) and AtSPC25 (AT3G48210) [PMID:29356153]. Reason: Well supported and a core annotation. Beyond the ISS transfer, the NDC80-complex membership is directly demonstrated in Arabidopsis by yeast two-hybrid and in planta co-immunoprecipitation with all three other subunits [PMID:29356153]. This cellular-component annotation captures the gene's core structural role. Supporting Evidence: PMID:29356153 Interactions among the components of the NDC80 complex were confirmed in a yeast two-hybrid assay and in planta co-immunoprecipitation. PMID:29356153 The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. |
| GO:0005634 nucleus | ISM GO_REF:0000122 | ACCEPT | Summary: Computational (AtSubP/TAIR) prediction of nuclear localization. Consistent with the protein's centromere/kinetochore localization but far less specific, and redundant with the IDA nucleus annotation. Reason: Correct but uninformative. A centromere/kinetochore protein is nuclear, so the prediction is not wrong, but the experimentally supported kinetochore (GO:0000776) and centromeric-region (GO:0000775) annotations describe the localization at the appropriate granularity. Kept as a valid, generic, electronic supporting annotation. Supporting Evidence: PMID:29356153 The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. |
| GO:0000776 kinetochore | IDA PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | ACCEPT | Summary: Direct experimental localization to the kinetochore. AtSPC24/MUN is an NDC80-complex subunit that localizes to the centromere/kinetochore, colocalizing with CENH3 [PMID:29356153]. Reason: Strong experimental cellular-component annotation (IDA) and a core localization for this gene. The localization is directly observed (fluorescent reporter colocalizing with CENH3 at the centromere) and is exactly what is expected for an NDC80-complex subunit at the outer kinetochore, so the term is supported both experimentally and mechanistically. Supporting Evidence: PMID:29356153 The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. |
| GO:0009933 meristem structural organization | IMP PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | KEEP AS NON CORE | Summary: Phenotype-based annotation: mun mutants exhibit an unstructured shoot apical meristem with ectopic WUSCHEL expression [PMID:29356153]. This is a downstream developmental consequence of impaired kinetochore function and cell division. Reason: The annotation is valid (supported by the mun loss-of-function phenotype and correctly qualified acts_upstream_of_or_within), but meristem structural organization is a tissue-level developmental outcome secondary to the gene's molecular role as a kinetochore/NDC80-complex subunit. Impaired chromosome segregation and reduced cell division in the meristem lead to disorganized meristem architecture and ectopic stem-cell niche generation; it is not the core function and is retained as non-core. Supporting Evidence: PMID:29356153 In addition, we report the ectopic generations of stem cell niches by the malfunction of kinetochore components. |
| GO:0051781 positive regulation of cell division | IMP PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | KEEP AS NON CORE | Summary: Phenotype-based annotation: mun mutants show a reduced cell-division rate, and null mutants arrest cell division in early embryogenesis [PMID:29356153]. This is a secondary consequence of a defective kinetochore rather than a positive-regulatory activity on the cell cycle. Reason: The supporting phenotype is real (low cell-division rate; embryonic division arrest), but the term "positive regulation of cell division" over-interprets it. SPC24 is a structural subunit of the kinetochore/NDC80 complex; when it is lost, cells fail mitosis and accumulate aneuploidy, so a reduced division rate is the trivial downstream result of broken chromosome segregation, not evidence that the protein acts as a positive regulator of the decision to divide. The mechanistic, core annotations are the kinetochore (GO:0000776), Ndc80 complex (GO:0031262) and chromosome-segregation (GO:0051983) terms. The annotation is correctly qualified acts_upstream_of_or_within and is not wrong, so it is retained as a non-core, indirect phenotype annotation rather than removed. (A defensible alternative would be MARK_AS_OVER_ANNOTATED; KEEP_AS_NON_CORE is chosen because the upstream-of relationship to cell division is genuine.) Supporting Evidence: PMID:29356153 The mutant exhibits stunted growth, embryo arrest, DNA aneuploidy, and defects in chromosome segregation with a low cell division rate. |
| GO:0051983 regulation of chromosome segregation | IMP PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | ACCEPT | Summary: Experimental (IMP) annotation: loss of AtSPC24/MUN causes defects in chromosome segregation and DNA aneuploidy [PMID:29356153], consistent with its role as an essential NDC80-complex kinetochore subunit. Reason: A core biological-process annotation, directly supported by the mutant chromosome-segregation defect and aneuploidy. The NDC80 complex mediates kinetochore-microtubule attachment and chromosome segregation and contributes to spindle-checkpoint control, so involvement in (regulation of) chromosome segregation is well founded by the direct loss-of-function evidence (segregation defects, aneuploidy). Supporting Evidence: PMID:29356153 The mutant exhibits stunted growth, embryo arrest, DNA aneuploidy, and defects in chromosome segregation with a low cell division rate. |
| GO:0005515 protein binding | IPI PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | ACCEPT | Summary: Three IPI interaction annotations (collapsed here) recording physical binding of AtSPC24/MUN to the three other NDC80-complex subunits: AtNDC80 (AT3G54630), AtNUF2 (AT1G61000) and AtSPC25 (AT3G48210), shown by yeast two-hybrid and in planta co-immunoprecipitation [PMID:29356153]. Reason: The interactions are genuine and important experimental evidence — they establish assembly of the Arabidopsis NDC80 complex. The GO molecular function term "protein binding" (GO:0005515) is itself uninformative and should not be treated as a core molecular function; its biological meaning is fully captured by the GO:0031262 "Ndc80 complex" cellular-component annotation. Retained (per standard practice for IPI evidence to named partners) but not promoted to a core function. No more-specific MF term is warranted, as SPC24's contribution is structural (anchoring, via SPC25) rather than a distinct catalytic or binding activity. Supporting Evidence: PMID:29356153 Interactions among the components of the NDC80 complex were confirmed in a yeast two-hybrid assay and in planta co-immunoprecipitation. |
| GO:0005634 nucleus | IDA PMID:29356153 MUN (MERISTEM UNSTRUCTURED), encoding a SPC24 homolog of NDC... | ACCEPT | Summary: Direct experimental nuclear localization [PMID:29356153]. The protein localizes to the centromere/kinetochore, which is within the nucleus. Reason: Valid experimental localization, but generic relative to the centromere/kinetochore compartment where the protein actually resides (GO:0000775, GO:0000776). Kept as a correct but broad CC annotation that is subsumed by the more specific kinetochore/centromere terms; it is the least informative of the localization annotations for this gene. Supporting Evidence: PMID:29356153 The NDC80 complex, composed of four proteins, NDC80, NUF2, SPC24 and SPC25, is localized at the outer kinetochore and connects spindle fibers to the kinetochore. |
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Download this section (compressed HTML)Q: Does AtSPC24/MUN contribute to spindle-assembly-checkpoint signaling in Arabidopsis, as its NDC80-complex orthologs do in other eukaryotes, or is its role limited to kinetochore-microtubule attachment?
Q: Is the meristem disorganization in mun mutants entirely a downstream consequence of impaired cell division, or does ectopic WUSCHEL expression reflect a more direct link between kinetochore function and stem-cell-niche patterning?
Experiment: Map the AtSPC24-AtSPC25 and AtSPC24-(NDC80/NUF2) binding interfaces by truncation/point-mutation interaction assays and, if feasible, reconstitute the recombinant Arabidopsis NDC80 complex to test microtubule binding.
Hypothesis: AtSPC24 binds SPC25 via its C-terminal RWD domain to form the inner-kinetochore-anchoring module, while microtubule binding is contributed by the NDC80-NUF2 module.
Type: protein interaction / in vitro reconstitution
Experiment: Use live-cell imaging of fluorescently tagged AtSPC24/MUN through the cell cycle to quantify kinetochore loading dynamics and test for spindle-checkpoint-dependent behavior under spindle poisons.
Hypothesis: AtSPC24 occupies kinetochores throughout mitosis and participates in checkpoint-controlled kinetochore-microtubule attachment.
Type: live-cell imaging / cell biology
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