Phosphorylation of the Cdc42 exchange factor Cdc24 by the PAK-like kinase Cla4 may regulate polarized growth in yeast

Rho-trpe GTPases control many cytoskeletal rearrangements, but their regula tion remains poorly understood. Here, we show that in S. cerevisiae, activa tion of the CDK Cdc28-Cln2 at bud emergence triggers relocalization of Cdc2 4, the GEF for Cdc42, from the nucleus to the polarization site, where it i s stably maintained by binding to the adaptor Bem1. Locally activated Cdc42 then polarizes the cytoskeleton in a manner dependent on its effecters Bni 1 and the PAK-like kinase Cla4. In addition, Cla4 induces phosphorylation o f Cdc24, leading to its dissociation from Bem1 at bud tips, thereby ending polarized bud growth in vivo. Our results thus suggest a dynamic temporal a nd spatial regulation of the Cdc42 module: Cdc28-Cln triggers actin polariz ation by activating Cdc42, which in turn restricts its own activation via a negative feedback loop acting on its GEF Cdc24.