interphase microtubule organizing center

GO:0031021 · Cytoskeleton · Multiprotein complex · Proposed

A microtubule organizing center found in interphase cells, which organize a longitudinal array of three to five MT bundles from the nuclear envelope during interphase. Each MT bundle is composed of two to seven MTs arranged in an antiparallel configuration, with the dynamic MT plus ends extending toward the cell tips and stable minus ends near the nucleus. [GO:0031021]

Nuclear-pore targeting and microtubule output in S. pombe. A diagrams the Nup146 FG5-12 deletion; B compares NE localization of the non-nucleating Mto1[9A1-NE]-GFP reporter. C shows anti-tubulin immunofluorescence during cold recovery with full-length wild-type Mto1. D-E show steady-state nucleation imaging and counts with Mto1[NE]-GFP; F compares crosslink-MS enrichment. Image bars are 5 micrometers. These distinct engineered and wild-type contexts must not be combined into a native stoichiometry or a resolved whole-iMTOC structure. FLUORESCENCE · *Schizosaccharomyces pombe* (`NCBITaxon:4896`) · Xun X Bao, Christos Spanos, Tomoko Kojidani, Eric M Lynch, Juri Rappsilber, Yasushi Hiraoka, Tokuko Haraguchi, Kenneth E Sawin; Figure 6, DOI:10.7554/eLife.33465; CC BY 4.0, via PubMed Central. Original image unchanged; caption paraphrased. · CC_BY_4_0 · source: PMC PMC6008054.1 fig6 · `DOI:10.7554/eLife.33465`

Synonyms: iMTOC; interphase MTOC; interphase microtubule organising center

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	Resident adaptor recruiting and activating gamma-tubulin machinery at non-SPB interphase sites; nuclear-envelope docking is one site-specific mechanism. `DOI:10.7554/eLife.33465` Nuclear-pore docking was dissected with engineered Mto1 reporters and tested functionally with separate full-length cold-regrowth assays. `DOI:10.1093/jmcb/mjz038` Mto1 and Alp7 show reciprocal NE localization dependencies during MBC recovery; mto1 deletion eliminates most non-SPB NE regrowth. `DOI:10.1016/j.cub.2014.03.006` Mto1 NE-targeting and bonsai constructs separate localization from gamma-tubulin-complex activation; engineered copy counts are not native stoichiometry.
Mto2-like microtubule nucleation adaptor	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	mto2	UNKNOWN	Mto1 partner present at interphase nucleation sites and supporting Mto1 association with the gamma-tubulin complex. `DOI:10.1091/mbc.E18-05-0279` Mto2 localizes with Mto1 on microtubules and redistributes toward SPB/NE sites in rsp1-deleted cells. `DOI:10.1016/j.cub.2014.03.006` Mto2 localizes to the NE in Mto1[NE] cells; association and activation experiments use engineered Mto1 constructs.
gamma-tubulin complex	Protein complex	part of	`GO:0000930`	alp4	UNKNOWN	Nucleating subcomplex represented experimentally by Alp4; no universal ring geometry, complete subunit inventory or copy number is assigned. `DOI:10.1016/j.cub.2014.03.006` Alp4 reports NE recruitment in Mto1[NE] cells and nucleation from engineered Mto1[bonsai] puncta; targeting and activation are separable. `DOI:10.1091/mbc.E18-05-0279` Alp4-containing particles occupy microtubule-associated sites and accumulate toward SPB/NE sites after rsp1 deletion.
Alp7-like microtubule-associated protein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	alp7	UNKNOWN	TACC-like resident supporting NE enrichment of full-length Mto1 and Alp14 during microtubule recovery. `DOI:10.1093/jmcb/mjz038` Alp7 localizes at NE regrowth foci, supports Mto1/Alp14 recruitment and itself depends on Mto1. Loss reduces, not universally abolishes, NE regrowth; lysate association is not purified direct binding. `DOI:10.7554/eLife.33465` Alp7 is not required for NE targeting of the engineered Mto1[9A1-NE] reporter, a distinct experimental context.
XMAP215	Protein	part of	`InterPro:IPR045110`	alp14	UNKNOWN	Polymerase-family resident recruited to NE sites through Alp7/Mto1, supporting efficient recovery and elongation. `DOI:10.1093/jmcb/mjz038` Alp14 localizes at the NE after MBC treatment in an Alp7/Mto1-dependent manner. alp14-deleted cells retain short regrowing microtubules, so initial regrowth is not absolutely Alp14-dependent.
Rsp1-like DnaJ-related cochaperone	Protein	acts on	Reviewed label only (reviewed name; no accession asserted)	rsp1	DISPENSABLE	Dynamically associated machinery regulating Mto1 distribution among interphase sites; effects depend on site and dosage. `DOI:10.1091/mbc.E18-05-0279` Endogenous Rsp1 partly colocalizes with Mto1 on microtubules. rsp1 deletion leaves residual newly formed non-SPB microtubules and NE-associated Mto1/Mto2/Alp4, despite major redistribution defects. `DOI:10.1126/sciadv.adq0670` Klp2-dependent lattice delivery and Rsp1-Ssa1 regulation of Mto1-Mto2/gamma-TuSC association are site-specific; Klp2 loss does not eliminate every cellular Rsp1 pool.

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 (4 references) Reference-sequence exemplar for species-level experiments; not an assertion that every experimental strain is this reference stock. `DOI:10.7554/eLife.33465` Nuclear-pore docking was dissected with engineered Mto1 reporters and tested functionally with separate full-length cold-regrowth assays. `DOI:10.1093/jmcb/mjz038` Mto1 and Alp7 show reciprocal NE localization dependencies during MBC recovery; mto1 deletion eliminates most non-SPB NE regrowth. `DOI:10.1016/j.cub.2014.03.006` Mto1 NE-targeting and bonsai constructs separate localization from gamma-tubulin-complex activation; engineered copy counts are not native stoichiometry. `https://www.uniprot.org/uniprotkb/O94488/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by 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; not an assertion that every experimental strain is this reference stock. `DOI:10.1091/mbc.E18-05-0279` Mto2 localizes with Mto1 on microtubules and redistributes toward SPB/NE sites in rsp1-deleted cells. `DOI:10.1016/j.cub.2014.03.006` Mto2 localizes to the NE in Mto1[NE] cells; association and activation experiments use engineered Mto1 constructs. `https://www.uniprot.org/uniprotkb/Q9USP7/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by 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) Reference-sequence exemplar for species-level experiments; not an assertion that every experimental strain is this reference stock. Alp4 reports one subunit, not the entire complex. `DOI:10.1016/j.cub.2014.03.006` Alp4 reports NE recruitment in Mto1[NE] cells and nucleation from engineered Mto1[bonsai] puncta; targeting and activation are separable. `DOI:10.1091/mbc.E18-05-0279` Alp4-containing particles occupy microtubule-associated sites and accumulate toward SPB/NE sites after rsp1 deletion. `https://www.uniprot.org/uniprotkb/Q9Y705/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by 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; not an assertion that every experimental strain is this reference stock. `DOI:10.1093/jmcb/mjz038` Alp7 localizes at NE regrowth foci, supports Mto1/Alp14 recruitment and itself depends on Mto1. Loss reduces, not universally abolishes, NE regrowth; lysate association is not purified direct binding. `DOI:10.7554/eLife.33465` Alp7 is not required for NE targeting of the engineered Mto1[9A1-NE] reporter, a distinct experimental context. `https://www.uniprot.org/uniprotkb/Q9URY2/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by 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; not an assertion that every experimental strain is this reference stock. `DOI:10.1093/jmcb/mjz038` Alp14 localizes at the NE after MBC treatment in an Alp7/Mto1-dependent manner. alp14-deleted cells retain short regrowing microtubules, so initial regrowth is not absolutely Alp14-dependent. `https://www.uniprot.org/uniprotkb/O94534/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by 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 (3 references) Reference-sequence exemplar for species-level experiments; not an assertion that every experimental strain is this reference stock. `DOI:10.1091/mbc.E18-05-0279` Endogenous Rsp1 partly colocalizes with Mto1 on microtubules. rsp1 deletion leaves residual newly formed non-SPB microtubules and NE-associated Mto1/Mto2/Alp4, despite major redistribution defects. `DOI:10.1126/sciadv.adq0670` Klp2-dependent lattice delivery and Rsp1-Ssa1 regulation of Mto1-Mto2/gamma-TuSC association are site-specific; Klp2 loss does not eliminate every cellular Rsp1 pool. `https://www.uniprot.org/uniprotkb/O13601/entry` Native reviewed accession, label, gene and source taxon verified on 2026-10-06; residence and activity are established by the cited experiments.

Canonical examples

NCBITaxon:4896 Schizosaccharomyces pombe — Non-SPB interphase nucleation sites studied by live imaging and perturbation. NE recovery, engineered docking reporters and steady-state lattice-associated sites are distinguished; no fungal-wide frequency or invariant stock is inferred. [DOI:10.7554/eLife.33465]

Functions

microtubule nucleation by interphase microtubule organizing center GO:0051415 — Organizes non-SPB interphase microtubule initiation; the contributions of NE and microtubule-associated sites vary with experimental conditions.
- DOI:10.7554/eLife.33465 Nup146 FG-repeat perturbation changes NE-region nucleation and full-length-Mto1 cold regrowth without abolishing cytoplasmic nucleation.
- DOI:10.1091/mbc.E18-05-0279 Time-lapse imaging measures new non-SPB microtubules on pre-existing microtubules, with residual events after rsp1 deletion.
microtubule cytoskeleton organization GO:0000226 — Spatially organizes the interphase microtubule system by positioning nucleation machinery and maintaining dynamic nucleation-site distributions.
- DOI:10.1091/mbc.E18-05-0279 Rsp1 perturbations redistribute Mto1/Mto2/Alp4 and alter the organization of interphase arrays.
- DOI:10.1093/jmcb/mjz038 NE enrichment and partial forced-targeting rescue link local Mto1 accumulation to regrowth organization.

Mechanism graphs

Nuclear-pore docking supports a nuclear-envelope iMTOC (ASSEMBLY)

Site-specific S. pombe dependencies, not universal iMTOC composition. The 9A1-NE reporter resolves docking but cannot nucleate. Separate active Mto1[NE] and full-length cold-regrowth assays connect docking to output. RanGTP dependence does not prove physical Ran membership, and crosslink-MS does not resolve direct interfaces.

Subject	Predicate	Object	Evidence
Mto1-like microtubule nucleation adaptor	supports	Mto1 docked at the cytoplasmic NPC face	`DOI:10.7554/eLife.33465` NES deletion disrupts localization and Crm1 association; GFP-Mto1[1-29]-GST targets NPCs whereas the monomeric comparison does not.
Crm1 exportin	supports	Mto1 docked at the cytoplasmic NPC face	`DOI:10.7554/eLife.33465` LMB displaces Mto1[9A1-NE]; the resistant crm1-C529A allele retains docking under LMB.
RanGTP-dependent state	supports	Mto1 docked at the cytoplasmic NPC face	`DOI:10.7554/eLife.33465` Dominant-negative Spi1[T23N], but not the tested active Spi1[Q68L], disrupts targeting; Ran was not enriched in the comparative interactome.
Nup146 with intact FG5-12 region	supports	Mto1 docked at the cytoplasmic NPC face	`DOI:10.7554/eLife.33465` Deleting the repeat segment abolishes 9A1-NE reporter docking while the mutant nucleoporin remains at NPCs.
Mto1 docked at the cytoplasmic NPC face	positions	interphase microtubule organizing center	`DOI:10.7554/eLife.33465` The model is supported by distinct docking and nucleation assays; an inactive 9A1-NE punctum alone is not a complete functional iMTOC.
interphase microtubule organizing center	supports	microtubule nucleation by interphase microtubule organizing center	`DOI:10.7554/eLife.33465` Nup146 FG5-12 deletion reduces steady-state NE-region nucleation with Mto1[NE] and redirects full-length-Mto1 cold regrowth into the cytoplasm.

Alp7-Mto1 recruitment and Alp14-supported recovery (FUNCTION)

Full-length proteins in S. pombe recovering from MBC, not a purified three-protein reaction or normal-state rate measurement. Residual stubs and SPB regrowth remain possible. Recruitment is reciprocal; partial Nup45-GBP rescue is not autonomous sufficiency, and Alp14 deletion does not abolish initial short regrowth.

Subject	Predicate	Object	Evidence
Alp7-like microtubule-associated protein	promotes_recruitment_of	Mto1-like microtubule nucleation adaptor	`DOI:10.1093/jmcb/mjz038` alp7 deletion strongly reduces NE Mto1 and shifts it toward cytoplasmic structures; the remaining large NE focus overlaps the SPB marker.
Mto1-like microtubule nucleation adaptor	promotes_recruitment_of	Alp7-like microtubule-associated protein	`DOI:10.1093/jmcb/mjz038` mto1 deletion removes the NE localization of Alp7 in the examined treated and untreated cells.
Alp7-like microtubule-associated protein	promotes_recruitment_of	XMAP215	`DOI:10.1093/jmcb/mjz038` Alp14 NE localization is lost in alp7-deleted cells, while Alp7 can remain NE-associated without Alp14.
Mto1-like microtubule nucleation adaptor	supports	interphase microtubule organizing center	`DOI:10.1093/jmcb/mjz038` Deleting mto1 removes most NE regrowth; Nup45-GBP tethering of Mto1 in alp7-deleted cells gives only partial rescue.
Alp7-like microtubule-associated protein	supports	interphase microtubule organizing center	`DOI:10.1093/jmcb/mjz038` alp7 deletion reduces NE output and delays regrowth; residual events remain.
interphase microtubule organizing center	supports	Microtubule regrowth after MBC washout	`DOI:10.1093/jmcb/mjz038` Live imaging follows growth from NE Mto1/Alp7 foci after MBC washout; not every event can be asserted to be de novo because stubs can remain.
XMAP215	promotes	Efficient elongation of recovering microtubules	`DOI:10.1093/jmcb/mjz038` alp14-deleted cells retain initial short NE microtubules with elongation defects and delayed first-visible growth.

Rsp1 regulates interphase nucleation-site distribution (REGULATION)

Endogenous colocalization is partial and dynamic. Deletion, truncation and overexpression are distinct perturbations; the graph does not turn the proposed chaperone-driven disassembly mechanism into an established reaction. The cited Mto1/2 activation experiment uses engineered bonsai puncta and is not a native NE stoichiometry measurement.

Subject	Predicate	Object	Evidence
Rsp1-like DnaJ-related cochaperone	supports_lattice_localization_of	Mto1-like microtubule nucleation adaptor	`DOI:10.1091/mbc.E18-05-0279` rsp1 deletion and the interaction-defective truncation reduce lattice Mto1 and enrich Mto1 toward SPB/NE sites without eliminating all nucleation.
Rsp1-like DnaJ-related cochaperone	supports_lattice_localization_of	Mto2-like microtubule nucleation adaptor	`DOI:10.1091/mbc.E18-05-0279` Mto2 particles become fewer and accumulate toward SPB/NE sites in rsp1-deleted cells.
Rsp1-like DnaJ-related cochaperone	supports_lattice_localization_of	gamma-tubulin complex	`DOI:10.1091/mbc.E18-05-0279` Alp4 particles redistribute in rsp1-deleted cells; the reporter does not resolve every gamma-tubulin-complex subunit.
Mto1-like microtubule nucleation adaptor	supports_lattice_localization_of	Rsp1-like DnaJ-related cochaperone	`DOI:10.1091/mbc.E18-05-0279` mto1 deletion compromises lattice Rsp1 while SPB Rsp1 remains.
Rsp1 overexpression	reduces	Mto1 association with gamma-tubulin machinery	`DOI:10.1091/mbc.E18-05-0279` Reciprocal co-IP under overexpression shows less associated Alp4/Mto1; this is not a purified dissociation-rate measurement.
Mto1-like microtubule nucleation adaptor	supports	Mto1 association with gamma-tubulin machinery	`DOI:10.1016/j.cub.2014.03.006` The 9A1 mutation disrupts gamma-TuC association/nucleation in engineered constructs while punctum formation and targeting are separately tested.
Mto2-like microtubule nucleation adaptor	supports	Mto1 association with gamma-tubulin machinery	`DOI:10.1016/j.cub.2014.03.006` Mto1[bonsai] co-immunoprecipitation with gamma-TuC depends on Mto2.
Mto1 association with gamma-tubulin machinery	supports	microtubule nucleation by interphase microtubule organizing center	`DOI:10.1016/j.cub.2014.03.006` Nucleation from free Mto1[bonsai], Mto2, Alp4 and Alp6 puncta separates activation from normal-site targeting; native site geometry remains unresolved.

Discussions

INTERPRETATION (RESOLVED): Distinguish the organizing center from its arrays and site-specific mechanisms. GO retains a historical NE-oriented definition with bundle counts. Modern S. pombe papers also call microtubule-associated sites iMTOCs. This record follows that named non-SPB interphase organizing entity, not every interphase centrosome, the whole output bundle, eMTOC or nuclear pore. No universal NPC/NE parthood or count is asserted.
KNOWLEDGE_GAP (OPEN): Resolve native site architecture and assembly essentiality. Co-localization and engineered nucleation puncta do not establish invariant stoichiometry or a universal gamma-tubulin-ring geometry. Core assembly essentialities remain UNKNOWN rather than being inferred from cell viability or activity. Rsp1 is DISPENSABLE associated machinery because deletion retains newly formed non-SPB microtubules and NE-associated organizing components, despite major defects.
CURATION_TODO (OPEN): Find exact source-neutral families for the label-only classes. Mto1 and Alp7 matches are domains; Mto2's native InterPro endpoint returns no content. Rsp1's broad family IPR051964 is not exactly the narrow Rsp1-like class. XMAP215 family membership alone cannot establish residence of every member, including Dis1.
INTERPRETATION (RESOLVED): Keep site-specific docking factors separate from invariant core composition. Crm1 and Nup146 are demonstrated NE docking machinery, not evidence for the same composition at all iMTOCs; they remain explicit graph context rather than universal components. RanGTP is functionally required in the docking assay but was not enriched with NE-targeted Mto1, so physical membership is unresolved. Nup82/Nsp1 mass-spectrometry enrichment does not establish universal residence. No direct purified interface, high-affinity NES constant or cargo-handover reaction is asserted.
INTERPRETATION (RESOLVED): Do not convert recovery assays into normal-state rates or strict de novo counts. High-dose MBC can leave residual stubs; NE-only time-course selection and drug-induced redistribution limit rate comparisons. Alp14 loss retains short regrowth, and Alp7 loss shifts rather than universally abolishes output. Nup45-GBP tethering is engineered and only partially restores numbers. An Alp7-Mto1-Alp14 assembly is a proposed working model, not a purified stoichiometric complex.
INTERPRETATION (RESOLVED): Retain the difference between full-length and 9A1-NE Mto1 targeting. Alp7 supports NE localization of full-length Mto1 during MBC recovery, but is not required for localization of Mto1[9A1-NE] in the docking study. Different-region binding is a proposed reconciliation, not an experimentally resolved combined pathway.
INTERPRETATION (RESOLVED): Separate whole-cell Rsp1 loss from lattice-specific delivery and biochemical models. Rsp1 deletion redistributes Mto1 and leaves residual nucleation. Klp2 deletion instead removes much lattice Rsp1 while retaining SPB Rsp1 and does not detectably change NE/cytoplasmic nucleation in the later study. Purified Klp2-Rsp1 transport and Rsp1-Ssa1 effects on Mto1-Mto2/gamma-TuSC association support site-specific regulation, not invariant Klp2/Ssa1 membership. The 2018 turnover/affinity/size-control alternatives are not all resolved by those later assays. Whole-cell concentrations are not MTOC stoichiometry.
KNOWLEDGE_GAP (OPEN): Extend taxonomic and native structural evidence without importing inaccessible details. The direct mechanistic exemplar is S. pombe, not a fungal-wide presence frequency. Reference-sequence taxa do not identify every experimental strain. Detailed supplementary files and raw PRIDE datasets were not analyzed. The 2004 definition-linked source was read only as an abstract; it does not support uninspected quantitative or direct-binding claims.

Evidence

DOI:10.7554/eLife.33465 PMID29809148; full main text read, with separate supplements/raw MS unread. Nuclear-pore docking and site-specific nucleation experiments.
DOI:10.1093/jmcb/mjz038 PMID31087092; full main text and captions read, supplementary files unread. Reciprocal recruitment and regrowth after MBC treatment.
DOI:10.1091/mbc.E18-05-0279 PMID30427751; full main text and captions read, supplementary files unread. Rsp1-dependent distribution and dosage effects.
DOI:10.1016/j.cub.2014.03.006 PMID24704079; main Results/Discussion and brief Experimental Procedures read. Detailed supplemental protocols unread. Engineered targeting/activation experiments.
DOI:10.1126/sciadv.adq0670 PMID39752482; full main text read; detailed supplementary purification protocols unread. Later site-specific Rsp1-Ssa1/Klp2 evidence.
DOI:10.1016/s1534-5807(04)00096-6 PMID15068790; primary abstract read, full Results/Methods not accessible through the attempted routes. Historical definition source; satellite/chaperone redistribution is a proposed model.

Curation history

2026-10-06T06:38:06Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-06T06:41:44Z codex · EXPANDED_RECORD — Added six source-bounded components with native protein groundings, two functions, three condition-specific causal graphs, eight discussions and an unchanged checksum-verified CC BY 4.0 figure; preserved PROPOSED status.
2026-10-06T06:45:07Z codex · CORRECTED_GROUNDED_NODE_LABELS — Resolved issue 2093: use canonical GO labels on four grounded graph nodes and preserve site/assay qualifiers in node descriptions; unchanged identifiers, topology and evidence.

Source: data/structures/cytoskeleton/interphase_microtubule_organizing_center.yaml