Structure-Function Study of the N-terminal Domain of Exocyst Subunit Sec3

Department of Physiology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 02/2010; 285(14):10424-33. DOI: 10.1074/jbc.M109.096966
Source: PubMed


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.

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Available from: Sung Haeng Lee, Dec 25, 2015
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    • "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 [21]. 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 [13] [14] [21] [22]. 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 [21]. "
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    ABSTRACT: The exocytosis is a process of fusion of secretory vesicles with plasma membrane, which plays a prominent role in many crucial cellular processes, e.g. secretion of neurotransmitters, cytokinesis or yeast budding. Prior to the SNARE-mediated fusion, the initial contact of secretory vesicle with the target membrane is mediated by an evolutionary conserved vesicle tethering protein complex, the exocyst. In all eukaryotic cells, the exocyst is composed of eight subunits - Sec5, Sec6, Sec8, Sec10, Sec15, Exo84 and two membrane-targeting landmark subunits Sec3 and Exo70, which have been described to directly interact with phosphatidylinositol (4,5)-bisphosphate (PIP2) of the plasma membrane. In this work, we utilized coarse-grained molecular dynamics simulations to elucidate structural details of the interaction of yeast Sec3p and Exo70p with lipid bilayers containing PIP2. We found that PIP2 is coordinated by the positively charged pocket of N-terminal part of Sec3p, which folds into unique Pleckstrin homology domain. Conversely, Exo70p interacts with the lipid bilayer by several binding sites distributed along the structure of this exocyst subunit. Moreover, we observed that the interaction of Exo70p with the membrane causes clustering of PIP2 in the adjacent leaflet. We further revealed that PIP2 is required for the correct positioning of small GTPase Rho1p, a direct Sec3p interactor, prior to the formation of the functional Rho1p-exocyst-membrane assembly. Our results show the critical importance of the plasma membrane pool of PIP2 for the exocyst function and suggest that specific interaction with acidic phospholipids represents an ancestral mechanism for the exocyst regulation. Copyright © 2015. Published by Elsevier B.V.
    Full-text · Article · Mar 2015 · Biochimica et Biophysica Acta
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    • "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). "
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    • "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 "
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    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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