GENETIC-ANALYSIS OF THE SACCHAROMYCES-CEREVISIAE RHO3 GENE, ENCODING - A RHO-TYPE SMALL GTPASE, PROVIDES EVIDENCE FOR A ROLE IN BUD FORMATION

RHO3 encodes a Rho-type small GTPase of the yeast Saccharomyces cerevi siae. We isolated temperature-sensitive alleles and a dominant active allele of RHO3. Ts(-) rho3 cells lost cell polarity during bud formati on and grew more isotropically than wild-type cells at nonpermissive t emperatures. In contrast, cells carrying a dominant active mutant RHO3 displayed cold sensitivity, and the cells became elongated and bent, often at the position where actin patches were concentrated. These phe notypes of the rho3 mutants strongly suggest that RHO3 is involved in directing the growing points during bud formation. In addition, we fou nd that SRO6, previously isolated as a multicopy suppressor of rho3 is the same as SEC4. The sec4-2 mutation was synthetic lethal with tempe ratures-sensitive rho3 mutations and suppressed the cold sensitivity c aused by a dominant active mutant RHO3. The genetic interactions betwe en RHO3 and SEC4, taken together with the fact that the Rab-type GTPas e Sec4p is required to fuse secretory vesicles together with plasma me mbrane for exocytosis, support a model in which the Rho3p pathway modu lates morphogenesis during bud growth via directing organization of th e actin cytoskeleton and the position of the secretory machinery for e xocytosis.