equatorial microtubule organizing center

GO:0000923 · Cytoskeleton · Multiprotein complex · Proposed

A microtubule organizing center formed by a band of gamma-tubulin that is recruited to a circumferential band of F-actin at the midpoint of a cell and which nucleates microtubules from the cell division site at the end of mitosis. [GO:0000923]

Multipanel Figure 1 follows equatorial Mto1, contractile-ring Myp2 and microtubules through cytokinesis. Fluorescence views and time series are accompanied by timing, intensity and position plots, including wild-type and increased-Myp2 comparisons. Microscopy scale bars are 5 or 2 micrometers as marked; plots are not micrographs or a purified-complex structure. FLUORESCENCE · *Schizosaccharomyces pombe* (`NCBITaxon:4896`) · Kimberly Bellingham-Johnstun, Zoe L. Tyree, Jessica Martinez-Baird, Annelise Thorn, Caroline Laplante; CC BY, via PubMed Central · CC_BY_4_0 · source: PMC PMC10047812.1 cells-12-00917-f001 · `DOI:10.3390/cells12060917`

Synonyms: EMTOC; equatorial microtubule organising centre

Part of: GO:0032153 — cell division site (no local record)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
Mto1-like microtubule nucleation adaptor	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	mto1	UNKNOWN	Equatorial adaptor supporting recruitment of the gamma-tubulin nucleation machinery. `DOI:10.1016/j.cub.2010.10.006` Mto1 localizes to the equator; the mto1-427 targeting mutant loses equatorial Mto1 and post-anaphase microtubules while retaining spindle-pole targeting.
Mto2-like microtubule nucleation adaptor	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	mto2	DISPENSABLE	Resident Mto1 partner supporting efficient gamma-tubulin-complex recruitment and sustained nucleation; residual equatorial activity can remain without it. `DOI:10.1091/mbc.E04-11-1003` Accessible primary Results and Figure 4 place endogenous Mto2 at the equator with Mto1; deletion permits an early post-anaphase array that is not maintained. `DOI:10.1242/jcs.038414` Very limited equatorial nucleation and occasional faint Alp4 localization remain in mto2-deleted cells. DISPENSABLE means initial assembly is possible, not normal maintenance or activity.
gamma-tubulin complex	Protein complex	part of	`GO:0000930`	alp4	UNKNOWN	Nucleating subcomplex recruited to equatorial sites; Alp4 is an experimentally imaged subunit. `DOI:10.1016/j.cub.2010.10.006` Alp4 reports equatorial gamma-tubulin-complex recruitment. Equatorial Alp4 is undetected after Myp2 deletion or truncation to Mto1(1-1051), while Mto1(1-1065) supports equatorial Alp4 localization. `DOI:10.1242/jcs.038414` Mto1 CM1 mutations separate gamma-tubulin-complex association from Mto1 localization; Mto2 loss can leave weak equatorial recruitment. `GO:0000930` Native authority label and scope verified; the family or complex match alone does not establish equatorial residence.
Alp7-like microtubule-associated protein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	alp7	DISPENSABLE	Equatorial TACC-like component supporting Alp14 recruitment to microtubule organizing centers. `DOI:10.1016/j.cub.2018.04.008` Figure 6D-E and Results localize Alp7 to eMTOCs and other MTOCs; Figure 6F-G tests Alp14 localization after alp7 deletion. Interphase bonsai nucleation rates are not native equatorial rates. `DOI:10.1091/mbc.E05-08-0811` Accessible abstract and Figure 5 caption support Mia1/Alp7 localization and maintenance defects; initial equatorial markers remain in the mutant. Full Methods were not accessible. Review update 2026-10-11: the equatorial Results section, main Methods, Discussion and Figure 5 were inspected directly. In S. pombe mia1-deletion cells, Figure 5C-E shows transient nucleation and retained equatorial Mto1, Alp4 and Rsp1 markers, but defective sustained use of nucleation sites and PAA maintenance. DISPENSABLE refers only to initial, possibly defective equatorial assembly under these conditions, not normal maintenance or all MTOCs. Separate supplements and movies were not inspected.
XMAP215	Protein	part of	`InterPro:IPR045110`	alp14	UNKNOWN	Resident microtubule polymerase-family component with a contribution to nucleation, represented here by Alp14. `DOI:10.1016/j.cub.2018.04.008` Figure 3A and Results explicitly place Alp14 at eMTOCs. Figure 3B co-immunoprecipitates Mto1 and gamma-tubulin-complex proteins from yeast lysates; it does not establish every direct contact or a native equatorial stoichiometry. `InterPro:IPR045110` Native authority label and scope verified; the family or complex match alone does not establish equatorial residence.
Rsp1-like DnaJ-related cochaperone	Protein	acts on	Reviewed label only (reviewed name; no accession asserted)	rsp1	UNKNOWN	MTOC-associated turnover machinery supporting equatorial-center disassembly after cytokinesis. `DOI:10.1016/s1534-5807(04)00096-6` The primary abstract reports persistent equatorial asters in rsp1 loss-of-function mutants and inhibited center formation after overexpression. Persistence in these mutants does not establish assembly in the complete absence of Rsp1; assembly essentiality remains unknown from the accessed evidence. The biochemical disassembly mechanism remains a proposal.

Organism-specific protein examples

Named proteins in which each component's role was established, with the organism the evidence comes from. The component above is the taxon-agnostic family; these are the specific entries behind it.

Mto1-like microtubule nucleation adaptor

Accession	Protein	Gene	Organism	Entry
`UniProtKB:O94488`	Gamma tubulin complex adapter mto1	mto1	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (2 references) Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1016/j.cub.2010.10.006` Mto1 localizes to the equator; the mto1-427 targeting mutant loses equatorial Mto1 and post-anaphase microtubules while retaining spindle-pole targeting. `https://www.uniprot.org/uniprotkb/O94488/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

Mto2-like microtubule nucleation adaptor

Accession	Protein	Gene	Organism	Entry
`UniProtKB:Q9USP7`	Gamma tubulin complex adapter mto2	mto2	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (3 references) Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1091/mbc.E04-11-1003` Accessible primary Results and Figure 4 place endogenous Mto2 at the equator with Mto1; deletion permits an early post-anaphase array that is not maintained. `DOI:10.1242/jcs.038414` Very limited equatorial nucleation and occasional faint Alp4 localization remain in mto2-deleted cells. DISPENSABLE means initial assembly is possible, not normal maintenance or activity. `https://www.uniprot.org/uniprotkb/Q9USP7/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

gamma-tubulin complex

Accession	Protein	Gene	Organism	Entry
`UniProtKB:Q9Y705`	Spindle pole body component alp4	alp4	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (3 references) Alp4 is one imaged subunit and reporter, not an accession for the entire complex. Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1016/j.cub.2010.10.006` Alp4 reports equatorial gamma-tubulin-complex recruitment. Equatorial Alp4 is undetected after Myp2 deletion or truncation to Mto1(1-1051), while Mto1(1-1065) supports equatorial Alp4 localization. `DOI:10.1242/jcs.038414` Mto1 CM1 mutations separate gamma-tubulin-complex association from Mto1 localization; Mto2 loss can leave weak equatorial recruitment. `https://www.uniprot.org/uniprotkb/Q9Y705/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

Alp7-like microtubule-associated protein

Accession	Protein	Gene	Organism	Entry
`UniProtKB:Q9URY2`	Microtubule protein alp7	alp7	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (3 references) Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1016/j.cub.2018.04.008` Figure 6D-E and Results localize Alp7 to eMTOCs and other MTOCs; Figure 6F-G tests Alp14 localization after alp7 deletion. Interphase bonsai nucleation rates are not native equatorial rates. `DOI:10.1091/mbc.E05-08-0811` Accessible abstract and Figure 5 caption support Mia1/Alp7 localization and maintenance defects; initial equatorial markers remain in the mutant. Full Methods were not accessible. Review update 2026-10-11: the equatorial Results section, main Methods, Discussion and Figure 5 were inspected directly. In S. pombe mia1-deletion cells, Figure 5C-E shows transient nucleation and retained equatorial Mto1, Alp4 and Rsp1 markers, but defective sustained use of nucleation sites and PAA maintenance. DISPENSABLE refers only to initial, possibly defective equatorial assembly under these conditions, not normal maintenance or all MTOCs. Separate supplements and movies were not inspected. `https://www.uniprot.org/uniprotkb/Q9URY2/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

XMAP215

Accession	Protein	Gene	Organism	Entry
`UniProtKB:O94534`	Spindle pole body component alp14	alp14	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (2 references) Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1016/j.cub.2018.04.008` Figure 3A and Results explicitly place Alp14 at eMTOCs. Figure 3B co-immunoprecipitates Mto1 and gamma-tubulin-complex proteins from yeast lysates; it does not establish every direct contact or a native equatorial stoichiometry. `https://www.uniprot.org/uniprotkb/O94534/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

Rsp1-like DnaJ-related cochaperone

Accession	Protein	Gene	Organism	Entry
`UniProtKB:O13601`	DnaJ-related protein rsp1	rsp1	Schizosaccharomyces pombe 972h- (`NCBITaxon:284812`)	REVIEWED
Role and evidence (2 references) Reference-sequence exemplar for species-level experiments; no assertion that this exact reference stock was imaged. `DOI:10.1016/s1534-5807(04)00096-6` The primary abstract reports persistent equatorial asters in rsp1 loss-of-function mutants and inhibited center formation after overexpression. Persistence in these mutants does not establish assembly in the complete absence of Rsp1; assembly essentiality remains unknown from the accessed evidence. The biochemical disassembly mechanism remains a proposal. `https://www.uniprot.org/uniprotkb/O13601/entry` Native accession, reviewed status, primary gene, protein label and source taxon verified on 2026-10-06. Residence and function come from the cited experiments.

Canonical examples

NCBITaxon:4896 Schizosaccharomyces pombe — Equatorial assembly in late mitosis/cytokinesis, distinct from the spindle pole body and interphase nucleation sites. No fungal-wide distribution is inferred. [DOI:10.1016/j.cub.2010.10.006]

Functions

microtubule nucleation by microtubule organizing center GO:0051418 — Nucleates microtubules of the post-anaphase array from the cell division site.
- DOI:10.1016/j.cub.2010.10.006 Equator-selective Mto1 targeting defects eliminate post-anaphase microtubules without eliminating spindle-pole nucleation.
- DOI:10.3390/cells12060917 Time-resolved imaging links equatorial Mto1 recruitment to post-anaphase microtubule production.

Mechanism graphs

Recruitment at the division site (ASSEMBLY)

Perturbation-defined recruitment dependencies in S. pombe, not a purified binding pathway. The F-actin/Myp2 ring context is external to the nucleation-center core; Mto2 loss permits weak residual assembly.

Subject	Predicate	Object	Evidence
Contractile-ring F-actin	supports	Mto1-like microtubule nucleation adaptor	`DOI:10.1016/j.cub.2010.10.006` Latrunculin B treatment abolishes equatorial Mto1 localization; no direct actin-Mto1 binding is inferred.
Myp2 in the contractile ring	supports	Mto1-like microtubule nucleation adaptor	`DOI:10.1016/j.cub.2010.10.006` Myp2 deletion removes equatorial Mto1 and Alp4 while retaining spindle-pole Mto1. Co-immunoprecipitation and two-hybrid association do not exclude intermediates. `DOI:10.3390/cells12060917` Mto1 occupies an inner layer offset from Myp2; increased Myp2 does not proportionally recruit Mto1. Simple direct coassembly is not established.
Mto1-like microtubule nucleation adaptor	supports	gamma-tubulin complex	`DOI:10.1016/j.cub.2010.10.006` The paper reports equatorial Alp4 with Mto1(1-1065), but not Mto1(1-1051). Separately, full-length mto1-427 loses equatorial Mto1 and PAA nucleation while retaining spindle-pole targeting; these are distinct readouts and constructs.
Mto2-like microtubule nucleation adaptor	supports	gamma-tubulin complex	`DOI:10.1242/jcs.038414` Occasional faint Alp4 and very limited nucleation persist after mto2 deletion; this is not an absolute-requirement edge.
gamma-tubulin complex	supports	equatorial microtubule organizing center	`DOI:10.1016/j.cub.2010.10.006` Localization and nucleation assays support this assembly interpretation, not sufficiency of isolated gamma-tubulin complex.

Production of the post-anaphase array (FUNCTION)

Equatorial nucleation and its microtubule output, not a claim that the output is the whole center or that it is universally required for cytokinesis.

Subject	Predicate	Object	Evidence
equatorial microtubule organizing center	supports	microtubule nucleation by microtubule organizing center	`DOI:10.1016/j.cub.2010.10.006` Selective disruption of equatorial Mto1 targeting prevents PAA nucleation.
microtubule nucleation by microtubule organizing center	generates	post-anaphase microtubule array	`DOI:10.3390/cells12060917` Live-cell imaging follows Mto1 recruitment and subsequent microtubule production and organization. `GO:1990295`

Rsp1-dependent center turnover (DISASSEMBLY)

Genetic dependence reported in the accessible primary abstract. Molecular interfaces and a complete chaperone reaction at the equator remain unresolved; no interphase lattice mechanism is transferred here.

Subject	Predicate	Object	Evidence
Rsp1-like DnaJ-related cochaperone	supports	equatorial microtubule organizing center disassembly	`DOI:10.1016/s1534-5807(04)00096-6` Loss-of-function cells retain an equatorial aster.
equatorial microtubule organizing center disassembly	disassembles	equatorial microtubule organizing center	`DOI:10.1016/s1534-5807(04)00096-6` `GO:0031025`

Discussions

CURATION_TODO (RESOLVED): Distinguish the equatorial assembly from other MTOCs and its microtubule output. Parsed identifiers, labels and synonyms from all 903 pre-existing records and hidden/ignored-inclusive name, accession and citation searches found no duplicate identity. GO:0000923 exactly names this center. GO:0005815 is its parent and GO:0032153 its division-site whole. GO:1990295 has-part this center but names the larger post-anaphase array; that class-level existential is not inverted into universal eMTOC part-of PAA. MULTIPROTEIN_COMPLEX describes the resident nucleation assembly, not a single purified stoichiometric complex.
CURATION_TODO (RESOLVED): Keep ring context, spindle-pole scaffolds and output microtubules separate from the core. F-actin and Myp2 supply upstream contractile-ring docking context. Their recruitment dependencies do not make the entire contractile ring an eMTOC component. Sid4/Cdc11 targeting evidence in the 2010 paper concerns spindle poles, not this equatorial assembly. Alpha/beta tubulin and Ase1 occur in the nucleated array rather than establishing core eMTOC membership. Mto1/Myp2 offset and dosage results limit a simple direct-coassembly model without excluding indirect association.
CURATION_TODO (OPEN): Find exact source-neutral whole-protein mappings for four label-only classes. Complete InterPro matches were inspected for Mto1 O94488, Mto2 Q9USP7, Alp7 Q9URY2 and Rsp1 O13601. Domains and superfamilies are not whole-protein family equivalents. Rsp1 has a broad family annotation, but no exact mapping to the narrower class is adopted. Mto2's empty API response is not absence of homologs; divergent fungal homologs are discussed in the 2015 primary study. Alp14 uses the verified XMAP215 family, not a TOG domain.
CURATION_TODO (RESOLVED): Separate assembly, recruitment, activity and maintenance phenotypes. Mto2 deletion can leave faint Alp4 recruitment, very limited nucleation or an early array that fails to persist. A failure to detect equatorial gamma-tubulin in another assay is not proof of zero molecules. Alp7/Mia1 mutants retain initial equatorial markers despite maintenance defects. The accessible Rsp1 abstract reports persistent asters in loss-of-function mutants, not demonstrated assembly in the complete absence of Rsp1. Rsp1 assembly essentiality therefore remains UNKNOWN, separately from its supported turnover role. Unknown assembly essentiality remains UNKNOWN for the other classes; no cell-viability phenotype is substituted for assembly necessity. Review update 2026-10-11: direct inspection of the 2006 equatorial Results and Figure 5C-E confirms residual equatorial assembly in mia1-deletion cells. Alp7 assembly essentiality is therefore now DISPENSABLE under the tested S. pombe conditions, superseding its earlier UNKNOWN classification; sustained nucleation-site use remains defective. The other unresolved essentiality values, including Rsp1 UNKNOWN, are unchanged.
CURATION_TODO (RESOLVED): Do not transfer interphase or engineered-system mechanisms to the equator. Alp7 and Alp14 equatorial localization is direct evidence, but bonsai construct nucleation frequencies, interphase satellite dwell times and forced GBP recruitment are not native eMTOC measurements. The 2025 Klp2/Rsp1/Ssa1 study resolves a microtubule-lattice pathway; it does not directly establish the same reaction at an equatorial center. Klp2 and Ssa1 are therefore not imported as universal eMTOC members. Mto2 phosphorylation quantification in the 2015 paper excluded eMTOC-bearing cells. No equator-specific phosphoregulation graph or native copy number is inferred.
KNOWLEDGE_GAP (OPEN): Resolve the molecular disassembly reaction at equatorial centers. The rsp1 loss-of-function phenotype supports turnover dependence. Satellite redistribution and chaperone-based breakup remain models in the accessible 2004 account; the 2025 lattice experiments do not by themselves close this site-specific gap.
CURATION_TODO (RESOLVED): Keep inferred ring mechanics distinct from universal cytokinesis requirements. The 2023 selective mto1-427 perturbation changes inferred contractile-ring stiffness without a measurable cytokinesis or anchoring defect under its tested conditions. Stiffness estimation assumes unchanged initial tension; mutant recoil did not plateau, limiting fitted parameters. Earlier anchoring experiments used cytokinesis delay or actin perturbation. Those contexts are retained rather than declaring a universal requirement or an unexplained contradiction. Array counts and offsets are not universal eMTOC dimensions.
CURATION_TODO (RESOLVED): Preserve organism, source-access and measurement limits. Only S. pombe is a canonical example. Protein accessions identify the reference strain, not every experimental stock. The 2004 Rsp1 paper and 2006 Mia1 paper were not read in full; two 2005 papers were accessible as indexed primary Results, not complete Methods. No unverified trait, exact Complex Portal equivalent, direct structural dataset, exhaustive composition or fixed copy number is supplied. The native GO definition is an identity definition, not an independent new quantitative experiment. Review update 2026-10-11: both 2005 studies were subsequently read through primary PMC HTML, including full main Results, Discussion and Methods. The 2006 paper was assessed through its equatorial Results section, main Methods, Discussion, abstract and Figure 5, not all Results or separate supplements/movies. The 2004 Rsp1 study remains abstract-only. Earlier access statements above describe the initial curation, not current access availability.

Evidence

GO:0000923 Native non-obsolete cellular-component identity, definition, synonyms, is-a MTOC and part-of cell division site verified through OAK on 2026-10-06.
DOI:10.1016/j.cub.2010.10.006 PMID 20970338; native PMC2989437.1 Results and Discussion read. Selective Mto1 targeting and Myp2/actin dependencies are the principal assembly evidence.
DOI:10.1242/jcs.038414 PMID 19001497; native PMC2743986.1 Results and relevant Methods read. Residual equatorial activity limits all-or-none interpretations of Mto2 dependence.
DOI:10.1091/mbc.E04-11-1003 PMID 15659644, PMC1142446; accessible indexed primary Results and captions read, not complete Methods or supplements.
DOI:10.1091/mbc.E04-12-1043 PMID 15800064, PMC1142447; accessible indexed primary Results read. Detection and cytokinesis-delay contexts differ from later selective-targeting experiments.
DOI:10.1016/j.cub.2018.04.008 PMID 29779879; native PMC7263743.1 Results, Discussion, STAR Methods and Figures 3/6 captions read. Direct equatorial localization is distinct from interphase bonsai nucleation quantification.
DOI:10.1016/s1534-5807(04)00096-6 PMID 15068790; primary abstract accessible, full Results and Methods not read. Used for the genetic turnover dependency, not a fully resolved biochemical mechanism.
DOI:10.3390/cells12060917 PMID 36980258; native PMC10047812.1 Results, Discussion, Methods and Figure 1 caption read; original Figure 1 inspected after native checksum verification.

Curation history

2026-10-06T03:09:14Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-06T03:11:17Z codex · EXPANDED_RECORD — Added six source-bounded component classes with verified protein examples, three causal graphs, a checksum-verified CC BY 4.0 primary figure, and explicit identity, access, essentiality and mechanism limits.
2026-10-06T03:13:21Z codex · CORRECTED_EVIDENCE_ATTRIBUTION — Separated the reported Alp4 localization readouts for Mto1 truncations from mto1-427 targeting/nucleation results; removed shared serialization aliases without changing other claims.
2026-10-06T03:16:15Z codex · EXPOSED_RENDERED_EVIDENCE_LIMITS — Exposed the existing three graph scope paragraphs and eight discussion rationales through renderer-supported fields, without changing claims, citations, graph edges or the shared template.
2026-10-06T03:42:24Z codex · BOUNDED_ASSEMBLY_ESSENTIALITY — Addressed PR 2088 review issue 2089: set Rsp1 assembly essentiality to UNKNOWN and distinguish loss-of-function persistence from assembly without the protein; retained the supported turnover dependency.
2026-10-11T22:42:47Z codex · ADDRESS_SAVED_REVIEW — Addressed #2263: recorded deletion-backed residual equatorial assembly without Alp7, retaining defective maintenance and site-specific scope; appended updated source-access accounting.

Source: data/structures/cytoskeleton/equatorial_microtubule_organizing_center.yaml