RHO1 GTPASE ACTIVATES THE (1-3)BETA-D-GLUCAN SYNTHASE AND IS INVOLVEDIN SCHIZOSACCHAROMYCES-POMBE MORPHOGENESIS

The Schizosaccharomyces pombe Cdc42 and Rho1 GTPases were tested for t heir ability to complement the cwg2-1 mutant phenotype of a decrease i n (1-3)beta-D-glucan synthase activity when grown at the nonpermissive temperature, Only Rho1 is able to partly complement the defect in glu can synthase associated with the cwg2-1 mutation. Moreover overexpress ion of the rho1(+) gene in wild-type S.pombe cells causes aberrant mor phology with loss of polarity and cells with several septa, Under this condition (1-3)beta-D-glucan synthase activity is increased four time s, but is still dependent on GTP. When S.pombe is transformed with con stitutively 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 acti ve mutant alleles affect (1-3)beta-D-glucan synthase activity when ove rexpressed 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 s ynthase and regulates S.pombe morphogenesis.