Abstract The overall goal of this proposal is to understand the mechanism by which Rho GTPases, Rab GTPases, tethering agents, and SNAREs contribute to direct polarized trafficking and growth at the cell surface. Previous work from our laboratories and others has implicated members of the Rho/Cdc42, exocyst and Sro7/Tomosyn protein families as highly conserved factors that have important roles in both polarity and membrane trafficking to the cell surface in systems as diverse as yeast and neurons. In this proposal, we will make use of new biochemical, genetic, and structural tools we have developed during the previous funding period to examine the molecular mechanisms by with Exocyst and Sro7 proteins work together as Rab effectors and vesicle tethering agents. Importantly we make use of newly identified gain-of-function alleles within the exocyst and Sro7 to understand the distinct biochemical and structural changes that occur to these tethering agents as they respond to regulatory and spatial cues.