Figure 6.

A model of how bud site selection couples to polarity establishment in late G1. Step 1: Bud5p exchanges GDP for GTP on Rsr1p. Step 2: Cdc24p and Cdc42p associate with GTP-bound Rsr1p at the bud site. Step 3: Bud2p activates Rsr1p to hydrolyze GTP bound to Rsr1p. Step 4: Dissociation of GDP-bound Rsr1p from Cdc24p may activate Cdc24p. Activated Cdc24p exchanges GDP for GTP on Cdc42p. GTP-bound Cdc42 then triggers actin assembly and exocyst localization to establish an axis of polarity. Bud5p at the bud site would then return Rsr1p to a GTP bound state (dashed line), allowing for another cycle of signal transduction. See DISCUSSION for details of the model.