THE CDC42 GTPASE-ASSOCIATED PROTEINS GIC1 AND GIC2 ARE REQUIRED FOR POLARIZED CELL-GROWTH IN SACCHAROMYCES-CEREVISIAE

BEM2 of Saccharomyces cerevisiae encodes a Rho-type GTPase-activating protein that is required for proper bud site selection at 26 degrees C and for bud emergence at elevated temperatures. We show here that the temperature-sensitive growth phenotype of bem2 mutant cells can be su ppressed by increased dosage of the GIC1 gene. The Gic1 protein, toget her with its structural homolog Gic2, are required for cell size and s hape control, bud site selection, bud emergence, actin cytoskeletal or ganization, mitotic spindle orientation/ positioning, and mating proje ction formation in response to mating pheromone. Each protein contains a CRIB (Cdc42/Rac-interactive binding) motif and each interacts in th e two-hybrid assay with the GTP-bound form of the Rho-type Cdc42 GTPas e, a key regulator of polarized growth in yeast. The CRIB motif of Gic 1 and the effector domain of Cdc42 are required for this association. Genetic experiments indicate that Gic1 and Gic2 play positive roles in the Cdc42 signal transduction pathway, probably as effecters of Cdc42 . Subcellular localization studies with a functional green fluorescent protein-Gic1 fusion protein indicate that this protein is concentrate d at the incipient bud site of unbudded cells, at the bud tip and moth er-bud neck of budded cells, and at cortical sites on large-budded cel ls that may delimit future bud sites in the two progeny cells. The abi lity of Gic1 to associate with Cdc42 is important for its function but is apparently not essential for its subcellular localization.