CHARACTERIZATION OF SYNTHETIC-LETHAL MUTANTS REVEALS A ROLE FOR THE SACCHAROMYCES-CEREVISIAE GUANINE-NUCLEOTIDE EXCHANGE FACTOR CDC24P IN VACUOLE FUNCTION AND NA+ TOLERANCE

Cdc24p is the guanine-nucleotide exchange factor for the Cdc42p GTPase , which controls cell polarity in Saccharomyces cerevisiae. To identif y new genes that may affect cell polarity, we characterized six UV-ind uced csl (DC24 Synthetic-lethal) mutants that exhibited synthetic-leth ality with cdc24-4(ts) at 23 degrees. Five mutants were not complement ed by plasmid-borne CDC42, RSR1, BUD5, BEM1, BEM2, BEM3 or CLA4 genes, which are known to play a role in cell polarity. The csl3 mutant disp layed phenotypes similar to those observed with calcium-sensitive, Pet (-) vma mutants defective in vacuole function. CSL5 was allelic to VMA 5, the vacuolar H+-ATPase subunit C, and one third of csl5 cdc24-4(ts) cells were elongated or had misshapen buds. A cdc24-4(ts) Delta vma5: :LEU2 double mutant did not exhibit synthetic lethality, suggesting th at the csl5/vma5 cdc24-4(ts) synthetic-lethality was not simply due to altered vacuole function. The cdc24-4(ts) mutant, like Delta vma5::LE U2 and csl3 mutants, was sensitive to high levels of Ca2+ as well as N a+ in the growth media, which did not appear to be a result of a fragi le cell wall because the phenotypes were not remedied by 1 M sorbitol. Our results indicated that Cdc24p was required in one V-ATPase mutant and another mutant affecting vacuole morphology, and also implicated Cdc24p in Na+ tolerance.