AbstractsBiology & Animal Science

Polarized exocytosis is a fundamental cellular process which mediates the delivery of intracellular lipids and proteins to the plasma membrane and secreted to the extracellular compartment. In budding yeast Saccharomyces cerevisiae, the Rho family small GTPases are important regulators of exocytosis. However, the precise mechanism by which they regulate exocytosis is not well understood. Earlier work from our laboratory suggested that Rho3 and Cdc42 have direct roles in exocytosis during different stages of the cell cycle. Here we present evidence that the functional specificity of Rho3 and Cdc42 is determined by two elements at the N terminus of Rho3.This region contains a cysteine residue that dictates the localization of Rho3, and a pair of basic residues (arginine and lysine) that are required for interacting with the exocyst component Exo70. We show that Exo70 has the biochemical and genetic properties expected of a direct effector for both Rho3 and Cdc42. Surprisingly we find that C-terminal prenylation of these GTPases both promotes the interaction and influences the sites of binding within Exo70. We identified gain-of-function mutants in EXO70 that potently suppress mutants in RHO3 and CDC42 defective for exocytic function. Taken together, these data suggests that Exo70 is the common downstream effector for both Rho3 and Cdc42. The localization of the Rho3 and Cdc42 and their interaction with the Exo70 component of the exocyst are the key determinants for their functional specificity in regulating exocytosis.