The exocyst is an octameric protein complex implicated in tethering post-Golgi secretory vesicles at the plasma membrane in preparation for fusion. However, it is not clear how the exocyst is targeted to and physically associates with specific domains of the plasma membrane and how its functions are regulated at those regions. We demonstrate that the N terminus of the exocyst component Sec3 directly interacts with phosphatidylinositol 4,5-bisphosphate. In addition, we have identified key residues in Sec3 that are critical for its binding to the guanosine triphosphate–bound form of Cdc42. Genetic analyses indicate that the dual interactions of Sec3 with phospholipids and Cdc42 control its function in yeast cells. Disrupting these interactions not only blocks exocytosis and affects exocyst polarization but also leads to defects in cell morphogenesis. We propose that the interactions of Sec3 with phospholipids and Cdc42 play important roles in exocytosis and polarized cell growth.

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The exocyst is an evolutionarily conserved octameric complex involved in polarized exocytosis from yeast to humans. The Sec3 subunit of the exocyst acts as a spatial landmark for exocytosis through its ability to bind phospholipids and small GTPases. The structure of the N-terminal domain of Sec3 (Sec3N) was determined ab initio and defines a new subclass of pleckstrin homology (PH) domains along with a new family of proteins carrying this domain. Respectively, N-and C-terminal to the PH domain Sec3N presents an additional ␣-helix and two ␤-strands that mediate dimerization through domain swapping. The structure identifies residues responsible for phospholipid binding, which when mutated in cells impair the localization of exocyst components at the plasma membrane and lead to defects in exocytosis. Through its ability to bind the small GTPase Cdc42 and phospholipids, the PH domain of Sec3 functions as a coincidence detector at the plasma membrane.The exocyst is an evolutionarily conserved octameric protein complex composed of subunits Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84. This complex was first identified by genetic and biochemical methods in the budding yeast Saccharomyces cerevisiae (1, 2). A homologous complex was subsequently discovered in mammalian cells (3). The exocyst mediates initial tethering of post-Golgi secretory vesicles to the plasma membrane, a step that precedes SNARE 7 -driven membrane fusion (4, 5). The exocyst is regulated by numerous cellular factors, and in particular small GTPases, which are primarily responsible for the spatiotemporal control of exocytosis (6).Recent studies have provided insights into the molecular architecture and function of tethering proteins. Crystal structures of nearly full-length Exo70 (7-9) and large fragments of Sec6 (10), Sec15 (11), and Exo84 (7) have been determined. Despite the lack of sequence similarity, these structures all reveal a similar fold, consisting of elongated tandem repeats of helical bundles, which are predicted to pack against one another during assembly of the exocyst complex (4). The recently determined structure of the yeast Dsl1p complex implicated in Golgi-to-endoplasmic reticulum transport provided the first glance into an assembled tethering complex consisting of helical bundles similar to those of exocyst subunits (12). The structure suggested a similar architecture, and possibly a common origin, among multisubunit tethering complexes. Structures have also been determined of the RalA-binding domains of the mammalian exocyst subunits Sec5 (13) and Exo84 (14), which display immunoglobulin-like and pleckstrin homology (PH) folds, respectively. However, these two domains are missing in the yeast complex and are not considered part of the conserved core of the exocyst (4).Studies in yeast suggest that subunit Sec3 plays a pivotal role in exocyst function and vesicle tethering. Sec3 localizes, together with Exo70, to the growing end of the daughter cell (known as the "bud tip"). Although the localization of other exocyst c...

The establishment and maintenance of cell polarity is important to a wide range of biological processes ranging from chemotaxis to embryogenesis. An essential feature of cell polarity is the asymmetric organization of proteins and lipids in the plasma membrane. In this article, we discuss how polarity regulators such as small GTP-binding proteins and phospholipids spatially and kinetically control vesicular trafficking and membrane organization. Conversely, we discuss how membrane trafficking contributes to cell polarization through delivery of polarity determinants and regulators to the plasma membrane.

The small GTPase Cdc42p is a master regulator of cell polarity. We analyzed Cdc42p localization using yeast mutants and found that endo-exocytic trafficking and septin-based diffusion barrier synergistically control Cdc42p polarization during asymmetric cell growth.