THE FISSION YEAST STS5(-KINASE-C AND AN OSMOSENSING MAP-KINASE PATHWAY() GENE IS REQUIRED FOR MAINTENANCE OF GROWTH POLARITY AND FUNCTIONALLY INTERACTS WITH PROTEIN)

Cell morphogenesis is a fundamental phenomenon that involves understan ding a number of biological processes including the developmental prog ram, polarity and cell division, Fission yeast sts5 mutant cells are r ound rather than cylindrical with cortical actin randomly dispersed, G enetic analyses demonstrate that the sts5(+) gene is required for main tenance of cell shape during interphase when the cell normally exhibit s polarised growth, The sts5 mutant is not defective in cell wall inte grity, Deletion of ppe1(+), which encodes a type 2A-like protein phosp hatase, shows similar phenotypes to the sts5 mutant and these two muta tions are synthetically lethal, Multicopy plasmids containing either t he protein kinase C-like gene pck1(+) or the protein tyrosine phosphat ase pyp1(+), an inhibitor of an osmosensing Sty1/Spc1 MAP-kinase, are capable of suppressing the sts5 mutation, Consistent with this, we hav e found that the wis1 mutation, which is defective in a MAP-kinase kin ase of the pathway, suppresses the sts5 mutation, The predicted sts5() gene product exhibits sequence similarity to two yeast proteins, Dis 3 and Ssd1 and a nematode protein, F46E8.6, where the former two yeast proteins have been shown to be involved in cell cycle control and cel l morphogenesis, The sts5(+) gene is not essential for cell viability, but is absolutely required for polarised growth as the gene disruptio n showed the same phenotypes as those of the original mutants, Overexp ression of the sts5(+) gene resulted in altered cell morphology and, c ortical actin in these overproducing cells was also abnormal, fainter and often dispersed, Anti-Sts5 antibody specifically detected a 130 kD a protein by western blotting, A green fluorescent protein-Sts5 fusion protein localised in the cytoplasm with a discrete punctate pattern, suggesting that the Sts5 protein is a component of a novel structure, These results have indicated that the Sts5 protein is a crucial determ inant of polarised growth and that it functionally interacts with the serine/threonine phosphatase, protein kinase C, and an osmosensing MAP -kinase to maintain cell morphology.