The nucleotide exchange factor Cdc24p may be regulated by auto-inhibition

Site-specific activation of the Rho-type GTPase Cdc42p by its guanine-nucleotide exchange factor (GEF) Cdc24p is critical for the establishment of cell polarity. Here we show that binding of Cdc24p to the small GTPase Rsr1p/Bud1p is required for its recruitment to the incipient bud site. Rsr1p/Bud1p binds to the CH-domain of Cdc24p, which is essential for its function in vivo. We have identified a cdc24-mutant allele, which is specifically defective for bud-site selection. Our results suggest that Cdc24p is auto-inhibited by an intramolecular interaction with its carboxy-terminal PB1-domain. Rsr1p/Bud1p appears to activate the GEF activity of Cdc24p in vivo, possibly by triggering a conformational change that dissociates the PB1-domain from its intramolecular binding site. Genetic experiments suggest that Bem1p functions as a positive regulator of Cdc24p by binding to the PB1-domain of Cdc24p, thereby preventing its re-binding to the intramolecular inhibitory site. Taken together, our results support a two-step molecular mechanism for the site-specific activation of Cdc24p, which involves Rsr1p/Bud1p and the adaptor protein Bem1p.