Figure 8.

A model for the involvement of Rho3 in two steps of exocytosis. GTP-bound Rho3 appears to have three distinct effector pathways. Previous work (Matsui and Toh-e, 1992b; Imai et al., 1996) and results presented here show a role for Rho3 in regulating the polarized distribution of the actin cytoskeleton. Here we demonstrate a role for Rho3 in exocytosis that is independent of its effect on the actin cytoskeleton. In particular, genetic and electron microscopy data suggest that Rho3 acts both in the delivery of Golgi-derived vesicles from the mother to the bud (Step 1) and in the docking and fusion of these vesicles with the plasma membrane (Step 2). The data presented here provide strong support for the model suggested by the two-hybrid interactions identified by Robinson et al. (1999). Therefore our analysis of the rho3-V51 effector domain mutant supports the model that the vesicle delivery function of Rho3 (Step 1) is mediated by Myo2 and that the vesicle docking and fusion function of Rho3 (Step 2) is mediated by Exo70.