Fission Yeast Sec3 Bridges the Exocyst Complex to the Actin Cytoskeleton

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.

Fission yeast NDR/LATS kinase Orb6 regulates exocytosis via phosphorylation of exocyst complex

“…Our finding that Orb6 regulates exocyst (see below) is consistent with the fact that exocyst mutants display cell-separation defects rather than cytokinesis defects 34,45,46 . This in turn implies that targeting of secretion towards the midzone during cytokinesis (e.g.…”

“…In addition, after Orb6 inhibition, all four proteins showed prolonged localization to the septation zone, consistent with cell-separation defects. Notably, the cell-separation defect caused by Orb6 inhibition mimics the consequences of inactivating exocyst in fission yeast 34,45,46 .…”

“…To test the role of Sec3 serine-201 and Sec5 serine-50 phosphorylation in exocytosis, we mutated these residues to non-phosphorylatable alanine at the endogenous sec3 and sec5 loci, to make mutants sec3-S201A and sec5-S50A. In initial experiments, neither mutant displayed any obvious abnormalities similar to exocyst mutants (e.g., no cellseparation defects; 34,45,46 ). In addition, mCherry fusions of both Sec3-S201A and Sec5-S50A localized to cell tips during interphase and to the septation zone during cell division ( Fig.…”

Rho4 interaction with exocyst and septins regulates cell separation in fission yeast

“…Like the septins, the exocyst localizes to the poles and to the division site, forming a ring that doubles later on during the septation process (Jourdain et al, 2012;Wang et al, 2002). Additionally, Rho4 co-immunoprecipitates with different subunits of the exocyst complex, and the interaction with Sec8, but not with Exo70, seems to be stronger with active Rho4, suggesting that Sec8 could be an effector of this GTPase.…”

“…In S. pombe all subunits are essential, except Exo70. In fact, exocyst mutants, such as sec8-1, sec3-2 and exo70D, display cell separation defects at high temperatures, suggesting an important role for secretion during cell separation (Bendezú et al, 2012;Jourdain et al, 2012;Wang et al, 2002).…”