The Schizosaccharomyces pombe Cdc42 and Rho1 GTPases were tested for their ability to complement the cwg2‐1 mutant phenotype of a decrease in (1‐3)beta‐D‐glucan synthase activity when grown at the non‐permissive temperature. Only Rho1 is able to partly complement the defect in glucan synthase associated with the cwg2–1 mutation. Moreover, overexpression of the rho1 gene in wild‐type S.pombe cells causes aberrant morphology with loss of polarity and cells with several septa. Under this condition (1–3)beta‐D‐glucan synthase activity is increased four times, but is still dependent on GTP. When S.pombe is transformed with constitutively active rho1 mutant alleles (rho1‐G15V or rho1‐Q64L), cells stop growing and show a very thick cell wall with hardly any septum. Under this condition the level of (1–3)beta‐D‐glucan synthase activity is at least 20 times higher than wild‐type and is independent of GTP. Neither cdc42+ nor the cdc42‐V12G or cdc42‐Q61L constitutively active mutant alleles affect (1–3)beta‐D‐glucan synthase activity when overexpressed in S.pombe. Cells overproducing Rho1 are hypersensitive to inhibitors of cell wall biosynthesis or to cell wall degrading enzymes. We conclude that Rho1 GTPase directly activates (1–3)beta‐D‐glucan synthase and regulates S.pombe morphogenesis.

In fission yeast protein kinase C homologues (Pck1 and Pck2) are essential for cell morphogenesis. We have isolated mok1 + in a genetic screen to identify downstream effectors for Pck1/2. mok1 + is essential for viability and encodes a protein that has several membrane-spanning domains and regions homologous to glucan metabolic enzymes. mok1 mutant shows abnormal cell shape, randomization of F-actin and weak cell wall. Biochemical analysis shows that Mok1 appears to have α-glucan synthase activity. Mok1 localization undergoes dramatic alteration during the cell cycle. It localizes to the growing tips in interphase, the medial ring upon mitosis, a double ring before and dense dot during cytokinesis. Double immunofluorescence staining shows that Mok1 exists in close proximity to actin. The subcellular localization of Mok1 is dependent upon the integrity of the F-actin cytoskeleton. Conversely, overexpression of mok1 + blocks the translocation of cortical actin from one end of the cell to the other. pck2 mutant is synthetically lethal with mok1 mutant, delocalizes Mok1 and shows a lower level of α-glucan. These results indicate that Mok1 plays a crucial role in cell morphogenesis interdependently of the actin cytoskeleton and works as one of downstream effectors for Pck1/2.