Regulation of exocytosis is fundamental to many biological processes such as hormone secretion and epithelial cell polarization. More than thirty proteins involved in membrane trafficking have been identified in budding yeast, most of which are evolutionarily conserved in human. The exocyst, an octamenc protein complex, plays an essential role in late stages of post-Golgi secretion. However, the molecular mechanism of exocyst function in polarized exocytosis remains unclear. For the first part of my research, I found that Exo84p, a member of the exocyst complex, directly interacts with Sro7p/Sro77p, the yeast homologues of a tumor suppressor Lgl, which plays critical role in epithelial cell polarization. Evidence from biochemical and genetic studies suggested that although Sro7p/Sro77p are broadly distributed in the cell, their localized interaction with the exocyst and kinetic activation are important for SNARE assembly and polarized exocytosis. In the second part of my research, I investigated the regulation of exocytosis mediated by Exo84p and Sro7p/Sro77p. Yeast cells have at least 2 different types of exocytic vesicles that are typified by their cargos, Bgl2p and invertase. We found that, unlike other exocyst mutants that block both the Bgl2p vesicles and the invertase vesicles, exo84-202 preferentially exhibited Bgl2p vesicle accumulation. Further disruption of Exo84p-Sro7p/Sro77p interaction aggravates the Bgl2p secretion defect but had no effect on invertase vesicles secretion. In addition, secretion defects in cells with disruption of Exo84p-Sro7p/Sro77p interaction are pronounced at early stages of the cell cycle. Furthermore, we found a similar vesicle-specific and cell cycle-specific secretory defect in sro7 sro77 null cells and in exo70 sro7Δ double mutants. This study suggests that the interaction of Exo84p with Sro7p/Sro77p is important for Bgl2p vesicle secretion at the early stages of the cell cycle, which is critical for polarized cell growth.