Gic2p may link activated Cdc42p to components involved in actin polarization, including Bni1p and Bud6p (Aip3p)

Gic2p is a Cdc42p effector which functions during cytoskeletal organization at bud emergence and in response to pheromones, but it is not understood h ow Gic2p interacts with the actin cytoskeleton. Here we show that Gic2p dis played multiple genetic interactions with Bni1p, Bud6p (Aip3p), and Spa2p, suggesting that Gic2p may regulate their function in vivo. In support of th is idea, Gic2p cofractionated with Bud6p and Spa2p and interacted with Bud6 p by coimmunoprecipitation and two-hybrid analysis. Importantly, localizati on of Bni1p and Bud6p to the incipient bud site was dependent on active Cdc 42p and the Gic proteins but did not require an intact actin cytoskeleton. We identified a conserved domain in Gic2p which was necessary for its polar ization function but dispensable for binding to Cdc42p-GTP and its localiza tion to the site of polarization. Expression of a mutant Gic2p harboring a single-amino-acid substitution in this domain (Gic2p(W23A)) interfered with polarized growth in a dominant-negative manner and prevented recruitment o f Bni1p and Bud6p to the incipient bud site. We propose that at bud emergen ce, Gic2p functions as an adaptor which may link activated Cdc42p to compon ents involved in actin organization and polarized growth, including Bni1p, Spa2p, and Bud6p.