Cdk1 and control of bud morphogenesis. Landmark proteins select the bud site, which is followed by recruitment and activation of Bud1, which in turn recruits and activates the small GTPase Cdc42. Cdk1 reinforces activation of Cdc42 by inhibiting the activity of the GAPs Bem2/3 and Rga2, and by phosphorylating the adaptor proteins Bem1 and Boi1/2. Cdk1 may also activate Cdc42 by phosphorylating the GEF Cdc24. GTP-bound Cdc42 then recruits Cla4, which mediates polarization of the actin cytoskeleton, which is required for bud growth. In addition, Cdk1 promotes the activity of the small GTPase Rho1 by inhibiting Bem2 and by activating the GEF Tus1, which supports bud growth. The septins Shs1 and Cdc3 are also phosphorylated by Cdk1, which may affect the mobility of Cdc3, while phosphorylation of Shs1 may affect the activity of Cdk1 by negative feedback in a later stage of the cell cycle. See text for details.