ste9 (srw1) — curation notes

UniProt O13286 (SRW1_SCHPO), PomBase SPAC144.13c, PANTHER PTHR19918:SF1 (Fizzy-related
subfamily). 556 aa, seven C-terminal WD repeats (246-541), disordered N-terminus carrying
the Cdk sites S62, T98, T177, S214 (PMID:10921878) and a mass-spec phosphosite S187
(PMID:18257517). Cited as the fission yeast Cdh1 exemplar in
modules/metaphase_anaphase_transition_and_mitotic_exit.yaml; the yeast CDH1 and human
FZR1 reviews are the finished comparators.

Identity and history

• Isolated twice: as srw1 (suppressor of rad1-1 wee1-50), a multicopy suppressor of
  hyperactivated Cdc2 PMID:9398669,
  and as the classical sterile mutant ste9 PMID:9571240. Yamaguchi et al. note the
  identity: PMID:9398669.

What APC/C-Ste9 does

• G1-specific mitotic cyclin destruction. PMID:10921876 and PMID:10921876
• Independent confirmation: PMID:10921878. Mitotic-exit cyclin destruction is
  attributed to Slp1.
• Cig2 is NOT an APC/C-Ste9 substrate (contrary to the brief I was given, which said Ste9
  targets Cig2). Yamaguchi 1997: PMID:9398669 and Blanco 2000 as above. Ste9 restrains Cdc2-Cig2
  genetically (cig2 deletion suppresses ste9 sterility) but not by degrading Cig2. The
  description and core functions reflect Cdc13 + Cig1, not Cig2.
• Direct biochemistry (Kimata 2011, the PomBase IDA source): purified Lid1-TAP APC/C plus
  in vitro translated coactivators. PMID:21389117 (Cut2 and Cdc13). Cell-cycle regulation is
  reproduced in extracts: PMID:21389117 and PMID:21389117. Ste9 also ubiquitylates Mes1 in vitro.
• IMP arm of Kimata 2011: Ste9 repressed in meiosis (Puhp1-HA-ste9) — PMID:21389117. Ste9 is hyperphosphorylated (inactive) through most of
  meiosis and is not involved in the MI to MII transition.

Regulation

• Cdc2-Cdc13 phosphorylation inhibits Ste9 two ways: PMID:10921876 and PMID:10921876
• Four Cdk sites; unphosphorylatable mutant stabilised and hyperactive: PMID:10921878; PMID:10921878; the 4A mutant causes diploidisation, suppressed by extra Cdc13.
• Phosphorylation depends mainly on Cdc2-Cdc13, not Cig1/Cig2 PMID:10921878
• From the deep-research file (papers not in the cache, not used for any action):
  PP2A-B56/Par1 needed for Ste9 dephosphorylation and Cdc13 loss in pre-Start G1 (Stonyte
  2020, iScience 23:101063); Cds1 phosphorylates and inhibits Ste9 during HU arrest,
  protecting the MBF activator Rep2, an APC/C-Ste9 substrate (Chu 2009, MCB,
  doi:10.1128/MCB.00562-09). Both raised as suggested_questions rather than annotated.

Physiology (the G1 terms)

• Nitrogen-starvation G1 arrest, sterility: PMID:9398669; PMID:9571240; PMID:9398669
• Pre-Start G1 block to mitosis: PMID:9571240; synthetic lethality with wee1.
• Gain of function: PMID:9571240
• Size control framing (Blanco): PMID:10921876
• Redundancy with Rum1: the ste9 rum1 double deletion is viable with no mitotic defect
  PMID:10921878.
• Localisation: nucleus by ORFeome YFP screen (PMID:16823372, abstract-only cache; the
  Ste9 datum is in the supplementary data, cited by UniProt). No cell-cycle-resolved imaging
  of endogenous Ste9 in the dedicated literature; suggested as an experiment.

Annotation decisions

All 18 GOA rows reviewed; all ACCEPT. One NEW row added (GO:0005680 anaphase-promoting
complex, part_of, IDA from PMID:10921876). Rationale for the rows that needed thought:

• GO:0031568 mitotic G1 cell size control checkpoint signaling (IGI 9398669 with cig2;
  IMP 9571240). Definition: "A signal transduction process that contributes to a cell size
  control checkpoint during the G1/S transition". PomBase uses the same term for rum1, cig2,
  cdc2 and wee1, i.e. it is their term for the pre-Start G1 delay. Ste9 executes the
  cyclin-destruction arm of that delay, which is its core physiological role, so ACCEPT
  rather than KEEP_AS_NON_CORE (the cdc2 review used non-core because for the master kinase
  it is one of many downstream outputs; for Ste9 it is the main output). Raised the
  "signaling" wording as a question.
• GO:2000134 negative regulation of G1/S transition (IGI 9571240 with cig2). Definition is
  a pathway that inhibits cell-cycle CDK activity at G1/S; Ste9 lowers Cdc2-Cdc13 activity
  by destroying Cdc13, and cig2 mutation suppresses the sterility. ACCEPT.
• GO:0097027 ubiquitin-protein transferase activator activity (IEA InterPro). Parent of
  GO:1990757; accepted as in the CDH1 and FZR1 reviews (the slp1 draft used MODIFY; I
  followed the completed exemplars).
• GO:0031145 (IMP 21389117) rests on the meiotic Mes1 stabilisation, a partial phenotype
  shared with fzr2 and fzr3; still a genuine dependence of APC/C substrate destruction on
  Ste9, so ACCEPT.
• Kimata 2011's title is about Fzr1/Mfr1 and Slp1, but the cached full text contains the
  Ste9 assays, so the PomBase IDA/IMP rows are verified, not inferred.
• NEW GO:0005680 anaphase-promoting complex (part_of). Comparator check: yeast CDH1 (IPI
  with Cdc23/Cdc16 and IBA), human FZR1, and fission yeast slp1 (IBA) all carry part_of
  APC/C, and slp1's IBA comes from PTN000460086, the same PAINT node that supplies ste9's
  GO:0031145/GO:1905786/GO:1990757 IBAs. So this is a propagation gap, not a MOD-wide
  convention against annotating coactivators to the complex. Direct evidence in this
  species: Blanco 2000 co-association of dephosphorylated Ste9 with the APC/C in G1
  (cut9-HA strain) and Kimata 2011 reconstitution of the holoenzyme from purified APC/C
  plus in vitro translated Ste9. It is a CC assertion, not a BP participation claim, so the
  "do not add what curators declined" bar for process terms does not apply in the same way;
  still raised as a question for PomBase.
• proposed_new_terms empty.

Deep research

ste9-deep-research-falcon.md (Edison/falcon) was present and used for context. It is
accurate on the core biology and correctly warns that the 2024 Slp1/Pmk1 papers concern Slp1
not Ste9. Its statement that localisation is "unresolved" ignores the ORFeome HDA, which is
a legitimate (high-throughput) observation. Kominami 1998 (PMID:9736616), the Rep2 paper and
the Par1 paper are not in the publications cache; I did not fetch them and did not base any
action on them.