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 alpha-helix and two beta-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.
"A couple of recent reports showed that Sec3p and Exo70p interact with phosphatidylinositol (4,5)-bisphosphate (PIP 2 ), which is present predominantly in the inner leaflet of PM . Sec3p binds PIP 2 via its N-terminal PH-like domain and basic residues located at the C-terminal part are important for Exo70p interaction with PIP 2    . The disruption of the interaction of PIP 2 with both Sec3p and Exo70p inhibits PM-association of the exocyst and results in severe growth defects . "
"Accordingly, loss of the Rho–GTPase activating protein (GAP) Lrg1 results in much thicker secondary septa (Svarovsky and Palecek 2005). Rho1 is also involved in activation and recruitment of Sec3 and the exocytic apparatus, which delivers chitin synthases for construction of both primary and secondary septa (Guo et al. 2001; Roumanie et al. 2005; Baek et al. 2010; Wu and Brennwald 2010; Yamashita et al. 2010). Overall, these findings suggest that Rho1 directly coordinates multiple distinct processes to ensure that cytokinesis and septation proceed rapidly and robustly (Figure 3). "
[Show abstract][Hide abstract] ABSTRACT: Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.
"The above results suggest that S. pombe Sec3 is a member of the exocyst complex. During the course of this study, Baek et al. 2010 published the crystal structure of the N-terminal domain of S. cerevisiae Sec3. Using this new fold as a template, the authors predicted that S. pombe SPAC17G8.12 "
[Show abstract][Hide abstract] ABSTRACT: The exocyst complex tethers post-Golgi secretory vesicles to the plasma membrane prior to docking and fusion. In this study, we identify Sec3, the missing component of the Schizosaccharomyces pombe exocyst complex (SpSec3). SpSec3 shares many properties with its orthologs, and its mutants are rescued by human Sec3/EXOC1. Although involved in exocytosis, SpSec3 does not appear to mark the site of exocyst complex assembly at the plasma membrane. It does, however, mark the sites of actin cytoskeleton recruitment and controls the organization of all three yeast actin structures: the actin cables, endocytic actin patches and actomyosin ring. Specifically, SpSec3 physically interacts with For3 and sec3 mutants have no actin cables as a result of a failure to polarize this nucleating formin. SpSec3 also interacts with actin patch components and sec3 mutants have depolarized actin patches of reduced endocytic capacity. Finally, the constriction and disassembly of the cytokinetic actomyosin ring is compromised in these sec3 mutant cells. We propose that a role of SpSec3 is to spatially couple actin machineries and their independently polarized regulators. As a consequence of its dual role in secretion and actin organization, Sec3 appears as a major co-ordinator of cell morphology in fission yeast.
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