sty1 (Q09892, SPAC24B11.06c) review notes

Sty1 (aka Spc1, Phh1, Hog1-like) is the fission-yeast stress-activated MAP kinase (SAPK), the central effector of the core environmental stress response (CESR). It is the ortholog of S. cerevisiae Hog1 and mammalian p38/JNK.

Core identity and activation

• MAP kinase of the HOG1 sub-subfamily; dual phosphorylation of the TGY/TXY motif (Thr-171/Tyr-173) activates it. UniProt: "Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. MAP kinase subfamily. HOG1 sub-subfamily." MOD_RES Thr-171 phosphothreonine, Tyr-173 phosphotyrosine.
• Activated by the MAPKK Wis1, negatively regulated by Pyp1/Pyp2 tyrosine phosphatases and PP2C phosphatases. PMID:7657164
• Activated by multiple stresses (broad p38-like). PMID:8649397 PMID:9136929

Cascade / two-component upstream

• Mcs4 response regulator -> Wak1/Wis4 + Win1 MAPKKKs -> Wis1 MAPKK -> Sty1. PMID:9136929
• p38MAPK cascade (GO:0038066) annotations rest on this established cascade.

Downstream / transcription

• Phosphorylates bZIP TF Atf1; Atf1 is the key nuclear substrate. PMID:8824588 PMID:8824587
• Drives transcription of CESR genes (gpd1, tps1, ctt1/catalase, pyp2). PMID:8649397 PMID:8824588
• Regulates Pap1 oxidative-stress TF localization (nuclear accumulation upon oxidative stress, Crm1-dependent export). PMID:9585505
• Directly binds Rpb1-CTD and phosphorylates Ser5 of the Pol II CTD heptapeptide at stress promoters. PMID:33410907 -> grounds GO:0008353 RNA pol II CTD kinase activity AND chromatin localization.
• Phosphorylates Lsk1 (CTDK-I catalytic subunit) promoting Ser2 CTD phosphorylation for ste11 transcription during meiosis. PMID:22144909

Localization

• Cytoplasmic in unstressed cells; rapid transient nuclear accumulation on stress, requires Wis1 phosphorylation and Atf1 retention. PMID:9585506
• Active nucleocytoplasmic shuttling (Pim1 import, Crm1 export). PMID:10233152
• A fraction localizes to mitochondria (regulated by mitochondrial PP2C Ptc4). PMID:22139357 -> grounds GO:0005758 mitochondrial intermembrane space (IDA) and GO:1903715 aerobic respiration.
• Cytoplasmic stress granule localization under stress (large scale / Puf2 study). PMID:32071154

Cell-cycle control (G2/M)

• Promotes mitotic entry under normal/stress conditions; sty1 mutants have G2 delay. PMID:7501024
• Also negatively regulates G2/M via Srk1 -> Cdc25 inhibition, and via Rad24/14-3-3. PMID:18272791 PMID:29065217
• Dual role (both positive and negative regulation of G2/M) is genuine and context dependent.

Sexual differentiation / meiosis

• Required for sexual development under nitrogen starvation; needed for ste11 induction and mating. PMID:8824587
• Negatively regulates conjugation under non-starvation via Srk1. PMID:12080074 (Srk1 negatively regulates entry into sexual development; sty1 acts through Srk1.)
• Regulates ade6-M26 meiotic recombination hotspot via Atf1(Mts1) phosphorylation. PMID:9819443 -> grounds GO:0010520 regulation of reciprocal meiotic recombination and GO:0051445 regulation of meiotic cell cycle.

Other processes

• Translation adaptation: binds eEF2/eIF3a; sty1 mutants recover translation poorly after stress. PMID:18065650 -> grounds GO:1990625 neg reg of cytoplasmic translational initiation in response to stress.
• Actomyosin ring integrity: SAPK downregulates CAR assembly by reducing For3 under stress. PMID:32915139 -> grounds GO:1903500.
• Heterochromatin (mating-type silencing): MAPK pathway + Atf1/Pcr1 phosphorylation promote Swi6/HP1 heterochromatin assembly at mat locus. PMID:15292231 -> grounds GO:0090055.
• Positive regulation of protein import into nucleus: Sty1 required for stress-induced nuclear accumulation of Pap1. PMID:9585505 -> grounds GO:0042307.

Interaction partners (for protein binding IPI annotations)

• Atf1 (P52890), Srk1 (O94547), Sin1 (SPAC19D5.01), Cmk2, Hal4 (SPAC23A1.06c / Cmk2 SPAC2... ), Ptc4 (O14156), Pin1 (SPBC28F2.12), Rnc1 (SPCC757.09c), Cdc37 etc. UniProt SUBUNIT: "Interacts with cdc37, cmk2, hal4, sin1 and srk1."
• protein binding (GO:0005515) IPI annotations are uninformative per curation guidelines; recommend replacing/keeping non-core in favor of specific MF where the partner is a substrate.

Assessment summary

• Core MF: MAP kinase activity (GO:0004707) — strongly supported by many IDA papers. protein serine/threonine kinase activity acceptable parent. RNA pol II CTD kinase activity is a genuine specific MF (PMID:33410907).
• Core BP: stress-activated MAPK cascade / p38MAPK cascade; cellular response to osmotic and oxidative stress; regulation of G2/M.
• Over-general/uninformative: bare "protein binding" (many IPI) — KEEP_AS_NON_CORE / note guidance.
• IEA "protein kinase activity" (GO:0004672) is a less-informative parent of MAP kinase activity -> MODIFY/over-annotated relative to MAP kinase activity but acceptable as general.