AIGR Deep Research Report — *plo1* (S. pombe, UniProt P50528) OpenScientist openscientist-autonomous 1 citations 2 artifacts 2026-08-31T23:46:29.834855 citations file

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AIGR Deep Research Report — plo1 (S. pombe, UniProt P50528)

Hypothesis slug: spb-insertion-mechanism
Seed hypothesis: Plo1 directly phosphorylates a spindle-pole-body (SPB) ring component to drive localized nuclear-envelope breakdown and mitotic SPB insertion.
Focus type: free_text (core-function / mechanistic)

Executive Judgment

Verdict: Partially supported — the process is supported and already curated; the specific "direct phosphorylation of a ring component" molecular step is unresolved (inferred, not demonstrated).

Supported (process/BP level): Plo1 kinase activity is genetically required for the SPB-ring protein redistribution, the localized (partial) nuclear-envelope remodeling, and the insertion of the SPB into the nuclear envelope that build the mitotic spindle. This is exactly the process already captured by GO:0140480 (IMP).
Not established (molecular/MF level): No primary study demonstrates a direct kinase→substrate reaction in which Plo1 phosphorylates a named SPB ring component to trigger NEBD/insertion. The directness and substrate identity are inferred from loss-of-activity phenotypes, not from an in-vitro/in-vivo phospho-site assay.
Wording caveat: S. pombe undergoes closed mitosis. "Nuclear-envelope breakdown" should be qualified as partial/local fenestration ("insertion into the nuclear envelope"), as the seed's "localized" wording and the GO term already reflect. Global NEBD does not occur.
Notable constraint: Plo1 is not required for the founding Sad1 ring redistribution (Bestul 2021), so any direct-phosphorylation model cannot be a single upstream Plo1→Sad1 event.

Overall: The hypothesis is a reasonable, literature-consistent mechanistic model, but the "directly phosphorylates a SPB ring component" clause exceeds the current evidence. Treat the process claim as retained/justified; treat the direct-substrate claim as a hypothesis requiring a discriminating phospho-assay before any substrate-specific MF annotation.

Evidence Matrix

Citation	Evidence type	Direction	Claim tested	Key finding	Context	Confidence / limitations
PMID 34133218 (Bestul et al. 2021, Mol Biol Cell)	Mutant phenotype + super-res localization (SIM)	Supports process; qualifies directness	Plo1 drives localized NEBD & SPB-ring formation for insertion	SPB proteins redistribute into a ring around the SPB (precursor for localized NEBD); Plo1 activity is vital for redistribution of the other SPB ring proteins and for complete NEBD enabling SPB insertion; Plo1 is NOT required for Sad1 redistribution	S. pombe mitosis; plo1 loss-of-activity	Functional necessity, not a direct kinase–substrate assay; no phospho-site mapped
PMID 24963130 (Wälde & King 2014, J Cell Biol)	Mutant phenotype + interaction	Supports process; qualifies directness	Plo1 promotes SPB remodeling/insertion into NE	Plo1 activity drives a burst of Cut12/Pcp1 incorporation; delayed Plo1 recruitment (Kms2 depletion) → SPB-insertion defects; Kms2 binds Cut12/Pcp1/Plo1	S. pombe SUN-KASH (Kms2/Sad1)	Effect partly via recruitment timing; incorporation ≠ demonstrated direct phosphorylation
PMID 7744248 (Ohkura, Hagan & Glover 1995, Genes Dev)	Mutant phenotype	Supports essential role	plo1 essential for bipolar spindle	plo1 loss → monopolar spindle / mitotic arrest; overexpression → monopolar spindles + ectopic septa	S. pombe disruption/overexpression	Pleiotropic (also septation/actin ring); silent on direct SPB-ring phospho-mechanism
PMID 23222840 (Grallert et al. 2013, Nature)	Mutant phenotype + localization	Qualifies (context)	Local MPF activates Plo1 at G2 SPB	MPF activation of Plo1 on the SPB dictates timing of mitotic commitment & NETO	S. pombe SPB	Places Plo1 upstream in commitment; not a direct NEBD substrate mechanism
QuickGO record P50528 (76 annotations)	Database	Supports (process already curated)	Existing GO capture	GO:0140480 "mitotic spindle pole body insertion into the nuclear envelope" (IMP); CC: mitotic SPB, half bridge, inner plaque (IDA); MF only generic Ser/Thr kinase activity	GO database	IMP = mutant-based; no substrate-level MF annotated

(Provenance CSVs saved during the run: plo1_evidence_matrix.csv, plo1_go_decision_table.csv.)

GO Curation Implications (leads — require curator verification)

Term	Aspect	Action	Rationale
GO:0140480 mitotic spindle pole body insertion into the nuclear envelope	BP	RETAIN (IMP)	Directly matches the seed's process claim; supported by PMID 34133218 and PMID 24963130 loss-of-activity phenotypes.
GO:0004674 protein serine/threonine kinase activity	MF	RETAIN	Core catalytic activity (EXP/IBA/IDA).
Proposed substrate-specific "phosphorylates SPB ring component" MF/BP	MF	DO NOT ADD (yet)	No direct kinase–substrate assay; adding a specific substrate MF would over-annotate beyond the evidence.
GO:0007052 mitotic spindle organization	BP	RETAIN	Consistent with monopolar-spindle phenotype.

Annotation provenance (verified this run): the plo1 GO:0140480 annotation is a single record — qualifier=involved_in, evidence=IMP, reference=PMID:34133218, assignedBy=PomBase. PomBase itself curated this at mutant-phenotype (IMP) / involved_in strength — i.e., functional involvement, deliberately stopping short of a direct molecular-activity claim. The seed hypothesis's "directly phosphorylates a specific ring component" therefore over-specifies the mechanism relative to both the primary paper and the existing annotation.
The BP process in the hypothesis is already annotated and justified — the hypothesis does not warrant a new/stronger BP term.
The MF layer should stay at generic kinase activity. Do not promote to a substrate-specific molecular function, and do not fall back to "protein binding" as the informative term.
If any curator statement asserts Plo1 directly phosphorylates a specific ring component, it should be flagged as mechanistic inference (IMP-level), not IDA/direct-assay.

Mechanistic Scope

Immediate molecular activity (direct): ATP-dependent protein serine/threonine kinase (Polo-like kinase) — well established.
Directly demonstrated at the SPB: Plo1 localizes to the SPB (IDA) and its catalytic activity is required for mitotic SPB remodeling/insertion (loss-of-activity phenotype).
Inferred (not directly shown): that a specific SPB ring protein is a direct Plo1 phosphosubstrate, and that this phospho-event is the trigger for local fenestration/insertion.
Downstream/pleiotropic (do not conflate with the core insertion mechanism): mitotic commitment (Cdc25/Wee1 axis), septation-initiation-network activation, cytokinetic ring, NETO/polarity, meiotic SPB remodeling. Plo1's essentiality phenotypes are broad, so an SPB-insertion defect in a plo1 mutant is not, by itself, proof of a direct ring-substrate event.

Conflicts and Alternatives

Directness gap / alternative = indirect action: Plo1 could enable insertion by controlling recruitment/incorporation timing of SPB components (Wälde & King: "burst of incorporation") rather than by directly phosphorylating a ring protein — the phenotype is consistent with either.
Sad1 independence: Plo1 is dispensable for Sad1 ring redistribution (Bestul 2021), so the "ring component" in the hypothesis cannot be Sad1; the responsible substrate (if any) is unidentified.
Wording/organism biology: "nuclear-envelope breakdown" risks importing an open-mitosis concept; S. pombe uses partial/local fenestration in an otherwise closed mitosis (cf. Brr6 fenestra work, PMID 22042620). The term should stay "insertion into the nuclear envelope."
Pleiotropy / phenotype attribution: plo1 null is pleiotropic (spindle + septation), so SPB-insertion phenotypes must be isolated from indirect commitment/SIN effects.
No paralog-overannotation issue detected: Plo1 is the single S. pombe Polo kinase; the concern here is directness/substrate identity, not paralog confusion.

Knowledge Gaps

Gap	What was checked	Why it matters	What would resolve it
Identity of the direct Plo1 substrate among SPB ring proteins	Searched primary literature + QuickGO MF annotations	Determines whether a substrate-specific MF/mechanism can be curated	In-vitro kinase assay + in-vivo phospho-site mapping (phosphoproteomics) on candidate ring proteins
Whether phosphorylation causes insertion vs. correlates	Bestul/Wälde phenotypes	Distinguishes direct trigger from permissive/recruitment role	Non-phosphorylatable (S/T→A) and phosphomimetic (S/T→D/E) substrate alleles rescuing/blocking insertion
Kinase-dead vs. localization-only separation	Grallert 2013 (activity timing)	Separates catalytic requirement from scaffolding	Analog-sensitive plo1 (as-plo1) acute inhibition during SPB insertion window
Exact NE-remodeling target (lipid/pore/fenestra machinery)	Brr6/Tts1 literature noted	Connects Plo1 activity to membrane fenestration	Epistasis of plo1 with brr6/apq12/tts1 for insertion

Discriminating Tests

Analog-sensitive plo1 (as-plo1) acute inhibition timed to the SPB-insertion window + live imaging of SPB/NE markers — tests catalytic necessity for insertion specifically (vs. earlier commitment).
Candidate-substrate phospho-mapping: IP–MS phosphoproteomics of SPB ring proteins (e.g., Cut12, Pcp1, Cut11, other ring proteins from Bestul 2021) ± active Plo1; confirm sites by in-vitro kinase assay with recombinant Plo1.
Phosphosite allele swap: non-phosphorylatable vs. phosphomimetic alleles of the top candidate; score SPB insertion/fenestration and bipolar spindle formation.
Epistasis of plo1 activity with brr6/apq12/tts1 fenestration mutants to place Plo1 relative to NE-membrane remodeling.
Comparative check with budding-yeast Polo (Cdc5)/Mps1 SPB-insertion pathways to test conservation of a direct ring-substrate mechanism.

Curation Leads (require curator verification)

Action: Retain BP GO:0140480 (IMP) and MF GO:0004674; do not add a substrate-specific "phosphorylates SPB ring component" MF/BP term at this time; do not downgrade to "protein binding."
Candidate references + snippets to verify:
PMID 34133218: "its activity is vital for redistribution of other SPB ring proteins and for complete NEBD at the SPB to allow for SPB insertion" and "the Polo kinase Plo1 is not necessary for Sad1 redistribution".
PMID 24963130: "Plo1 activity supports mitotic SPB remodeling by driving a burst of incorporation of Cut12 and Pcp1".
PMID 7744248 (essentiality / monopolar spindle); PMID 23222840 (SPB-local Plo1 activation timing).
Suggested curator questions:
Is any statement in the review implying direct phosphorylation of a specific SPB ring component? If so, mark as mechanistic inference, not direct assay.
Should the review text replace "nuclear-envelope breakdown" with "localized/partial nuclear-envelope fenestration / SPB insertion into the nuclear envelope" to match closed-mitosis biology?
Suggested experiments: the discriminating tests above (as-plo1 timing; phospho-site mapping + S/T→A/D alleles).

Limitations of this analysis

Based on abstract-level literature synthesis and the public GO/QuickGO record; full-text methods were not parsed.
No local sequence/structure computation was needed to adjudicate the core (directness) question; the decisive gap is an unavailable direct kinase-substrate assay, which cannot be substituted by bioinformatics.
One citation-snippet auto-validation failed only because the stored abstract copy for PMID 34133218 was truncated; the verbatim text above is from the full NCBI abstract retrieved during the run.