Budding yeast (S. cerevisiae) pheromone-response (Ste11-Ste7-Fus3) MAPK cascade module

A decomposition of the Saccharomyces cerevisiae pheromone-response (mating) MAP kinase cascade, the fungal realization of the generic three-tier MAP kinase relay (MODULE:mapk_relay). Mating pheromone (a-factor or alpha-factor) activates a G-protein-coupled receptor (Ste2/Ste3); the released G-protein beta-gamma dimer and the PAK kinase Ste20, organized on the scaffold Ste5, activate the MAP3K Ste11, which phosphorylates the MAP2K Ste7, which dually phosphorylates the MAPK Fus3 (and the partially redundant Kss1) on its TEY activation-loop motif. Active Fus3 phosphorylates the transcription factor Ste12 and the cell-cycle inhibitor Far1, driving the mating transcriptional program, G1 arrest, and polarized shmoo formation for conjugation. The kinase relay - Ste11 -> Ste7 -> Fus3 - is declared as an inner bundle that `conforms_to` mapk_relay, while the pheromone/GPCR input and the Ste12/Far1 output are this cascade's free extensions around the conforming core. Grounded in GO:0071507 (pheromone response MAPK cascade); the pre-composed term GO:0000750 (pheromone-dependent signal transduction involved in conjugation with cellular fusion) was obsoleted in GO release 2026-05-19 with replaced_by GO:0071507.

This module is the Saccharomyces cerevisiae mating-pathway realization of the generic three-tier MAP kinase relay (MODULE:mapk_relay): the inner `fus3_relay` node bundles the Ste11 -> Ste7 -> Fus3 tiers and declares conformance to that motif, while the pheromone/GPCR input and the downstream Ste12/Far1 output are this cascade's free extensions around the conforming core. The cascade is organized on the Ste5 scaffold. Representative UniProt members (Ste11 P23561, Ste7 P06784, Fus3 P16892, Kss1 P14681) were verified against UniProt. No matching UniProt-seed PAINT IBD rows were found in the local interpro/panther cache for these S. cerevisiae exemplars, so this species-specific cascade is not assigned a PTN ancestry anchor in this pass. PANTHER grounding: kinase-tier family descriptors carry family-level PTHR ids checked against interpro/panther membership (PTHR24055 for every MAPK tier, PTHR48013 for all MAP2Ks, including MEK1/MEK2, MKK3/4/5/6/7 and Pbs2/Ste7, per the current PANTHER classification, PTHR24361 for MEKK2/MEKK3). PANTHER subfamilies are deliberately not used: they are not clean paralog groups here (e.g. PTHR48013:SF9, named for MEK5, also contains yeast Ste7 and plant MKKs), so branch identity is carried by the representative members and the PTN evidence rather than by a subfamily id.