Figure 5.

Scheme for function of a GTPase cascade in determination of yeast cell polarity. In this model, the GTPase Bud1 determines the localization to the bud site of proteins that organize the actin cytoskeleton—Bem1, Cdc24, and the GTPase Cdc42. First, a tetrapartite complex is formed at the bud site, at which the Bud2, the GTPase-activating protein for Bud1, appears to be located (H.-O.P., unpublished results). Bud2 stimulates conversion of Bud1–GTP to Bud1–GDP, which allows a reconfiguration of components at the bud site and activation of Cdc24. This guanine nucleotide exchange factor, Cdc24, then stimulates formation of Cdc42–GTP, which is presumably the active species for organizing the actin cytoskeleton. Bud1–GDP is converted to Bud1–GTP by its guanine-nucleotide exchange factor, Bud5. Multiple rounds of this cycle may result in a critical concentration of Bem1 and Cdc42–GTP at the presumptive bud site, which stimulates polarization of the actin cytoskeleton toward that point (see text for details).