THE SPA2-RELATED PROTEIN, SPH1P, IS IMPORTANT FOR POLARIZED GROWTH INYEAST

The Saccharomyces cerevisiae protein Sph1p is both structurally and fu nctionally related to the polarity protein, Spa2p, Sph1p and Spa2p are predicted to share three 100-amino acid domains each exceeding 30% se quence identity, and the amino-terminal domain of each protein contain s a direct repeat common to Homo sapiens and Caenorhabditis elegans pr otein sequences, sph1- and spa2-deleted cells possess defects in matin g projection morphology and pseudohyphal growth. sph1 Delta spa2 Delta double mutants also exhibit a strong haploid invasive growth defect a nd an exacerbated mating projection defect relative to either sph1 Del ta or spa2 Delta single mutants, Consistent with a role in polarized g rowth, Sph1p localizes to growth sites in a cell cycle-dependent manne r: Sph1p concentrates as a cortical patch at the presumptive bud site in unbudded cells, at the tip of small, medium and large buds, and at the bud neck prior to cytokinesis, In pheromone-treated cells, Sph1p l ocalizes to the tip of the mating projection. Proper localization of S ph1p to sites of active growth during budding and mating requires Spa2 p, Sph1p interacts in the two-hybrid system with three mitogen-activat ed protein (MAP) kinase kinases (MAPKKs): Mkk1p and Mkk2p, which funct ion in the cell wall integrity/cell polarization MAP kinase pathway, a nd Ste7p, which operates in the pheromone and pseudohyphal signaling r esponse pathways, Sph1p also interacts weakly with STE11, the MAPKKK k nown to activate STE7, Moreover, two-hybrid interactions between SPH1 and STE7 and STE11 occur independently of STE5, a proposed scaffolding protein which interacts with several members of this MAP kinase modul e, We speculate that Spa2p and Sph1p may function during pseudohyphal and haploid invasive growth to help tether this MAP kinase module to s ites of polarized growth, Our results indicate that Spa2p and Sph1p co mprise two related proteins important for the control of cell morphoge nesis in yeast.