cig2 (SPAPB2B4.03, P36630) review notes

Identity / overview

Cig2 (synonym Cyc17) is the major S-phase (G1/S) B-type cyclin of fission yeast, cyclin A/B
subfamily (UniProt). Non-essential: Cig1, Puc1 and above all the mitotic cyclin Cdc13 can
substitute for it. UniProt still calls it "G2/mitotic-specific cyclin cig2" (the name comes from
the 1993 Bueno & Russell paper), but every subsequent study places its physiological role at
G1/S. Deep research (falcon provider) was present and was used; it draws mainly on
Martín-Castellanos et al. 1996 (EMBO J, not in the local publication cache), Fisher & Nurse 1996,
Pickering et al. 2017 (bioRxiv) and the Kawamukai 2024 review.

Interactor identities used in the GOA WITH/FROM columns (resolved from the repo's UniProt files):
- PomBase:SPBC11B10.09 / UniProtKB:P04551 = cdc2 (CDK1)
- PomBase:SPCC18B5.03 = rum1 locus id used by PomBase for the IGI; UniProtKB:P40380 = rum1
- PomBase:SPBC582.03 = cdc13
- PomBase:SPAC22F3.09c / UniProtKB:P41412 = res2; UniProtKB:P33520 = res1
- UniProtKB:P87060 = pop1 (F-box protein, SCF-Pop1)
- PomBase:SPBC2G2.09c (in the IBA MTOC donor list) = crs1, the meiosis-specific cyclin, NOT cig1

Core function: S-phase cyclin that activates Cdc2 for Start / S-phase onset

• PMID:8657126
• PMID:8657126
• PMID:8657126
• PMID:8631306
• PMID:9552380
• PMID:9614176
• Deep research: Cig2-HA immunoprecipitates carry histone H1 kinase activity that is lost in a
  cdc2-33 background, so the catalytic activity is Cdc2's (Martín-Castellanos et al. 1996; not cached).

Regulation by Rum1 and cell size (Start control)

• PMID:9552380
• PMID:9614176
• PMID:9614176
• Deep research: cig2 deletion delays G1 exit particularly in small cells and in sensitized cdc2
  backgrounds (cdc2-56 cig2delta ~40% G1 arrest at 36.5C), consistent with the size-control term.

Pheromone-induced G1 arrest acts on Cig2-Cdc2

• PMID:9034336
• PMID:9034336

MBF transcription and the Cig2 feedback loop

• PMID:11781565
• PMID:11781565
• PMID:11781565
• PMID:7909513

Proteolysis (APC/C and SCF-Pop1/Pop2)

• PMID:14970237
• PMID:14970237
• UniProt: destruction box 51-60 (R51A/L54A stabilises), Cdc2-binding residues R169/E170/I171
  (PMID:11163211, not cached).

Sexual differentiation

• PMID:7909513
• PMID:7909513
• Cyc17 was isolated as an extragenic suppressor of pat1-114 (overexpression blocks the Pat1-inactivation-driven entry into meiosis).

Meiosis

• PMID:26804917
• PMID:30640914
• PMID:30640914
• The DSB reduction in cig2 alone is not statistically significant (p 0.21); crossovers are unchanged.

Localization

• [PMID:16823372 ORFeome YFP screen; PomBase HDA: nucleus and mitotic spindle pole body.]
• PMID:12419251 The cached record is abstract-only and does not
  mention Cig2; PomBase's IDA chromatin annotation for cig2 rests on the full text (origin ChIP of
  Cig2), which is consistent with Cig2-Cdc2 acting at origins at S phase. Deferred to the curator.
• Deep research notes that direct high-resolution localization data for Cig2 are limited.

Mitosis?

• PMID:8455610 and PMID:8455610 - the 1993 interpretation;
  later work (Obara-Ishihara 1994; Mondesert 1996; Fisher & Nurse 1996) showed no mitotic cyclin
  activity in wild-type cells, but deregulated Cig2-Cdc2 can drive (catastrophic) mitosis when
  Wee1/Mik1 inhibitory phosphorylation is removed (Pickering et al. 2017, deep research).

GO annotation decisions (summary)

• MF: GO:0061575 activator activity (IDA x2, IGI) and GO:0016538 regulator (IBA, IEA): ACCEPT, core.
• GO:0005515 protein binding (6 IPI rows): res1/res2 rows (PMID:11781565) and cdc2 row
  (PMID:8455610) MODIFY -> GO:0016538 (cyclin docking the kinase on Cdc2 / on its MBF substrate);
  pop1 row (PMID:14970237, Cig2 is the SCF substrate) and rum1 row (PMID:9614176, Cig2-Cdc2 is the
  CKI target) REMOVE as uninformative; the interactions themselves are not disputed.
• BP: G1/S transition (IBA), positive regulation of G1/S (IMP x3, IGI), traversing Start (IBA, IGI),
  G1 cell size control checkpoint signaling (IGI x2): ACCEPT, core. Mitotic cell cycle phase
  transition (IEA): ACCEPT (general). Negative regulation of conjugation (IMP), regulation of
  mitotic-to-meiotic switching (IMP), regulation of reciprocal meiotic recombination (IMP):
  KEEP_AS_NON_CORE.
• CC: CDK holoenzyme complex (EXP, IBA), nucleus (HDA, IBA, IEA), chromatin (IDA): ACCEPT.
  Mitotic SPB (HDA), SPB (IEA), MTOC (IBA): KEEP_AS_NON_CORE. Cytoplasm (IBA): KEEP_AS_NON_CORE
  (no cig2-specific cytoplasmic activity; only the SPB pool).
• No NEW terms proposed. The MBF feedback inhibition is described in core_functions (under the
  Start control point term GO:0007089) but no transcription-regulation process term is asserted:
  a QuickGO check (2026-09-26) shows that PomBase annotates neither cig2 nor cdc2 nor cdc13 to any
  transcription term, so the absence is a curation convention (the CDK regulates the transcription
  factor) rather than a gap; raised in suggested_questions instead.