| Cell polarity, the asymmetric distribution of lipids and proteins in distinct cellular domains, is essential for a broad range of functions, ranging from cell migration to cell fate determination. Although both RhoGTPase signaling and vesicle trafficking pathways play central roles in cell polarity establishment, the mechanisms of their co-ordinated regulation and the crosstalk between their machineries is far from clear. Epsins, classical endocytic adaptor proteins, play additional signaling roles by mediating cell polarity establishment in the budding yeast, Saccharomyces cerevisiae; this provides an excellent model to study the intricate interplay between vesicle trafficking and signaling machineries. In this study, we demonstrate that by interacting with the Cdc42 GTPase activating protein and septin regulator, Bem3, yeast Epsin2 plays a specific role in cell division signaling. Specifically, a dominant negative approach by overexpression of the conserved E&barbelow;psin N&barbelow;-t&barbelow;erminal h&barbelow;omology domain of Ent2 (ENTH2) led to complete disruptions in Bem3 localization and septin organization, consequently leading to a delay in cell cycle progression and cytokinesis defects. We mapped three residues in ENTH2 critically important for direct interactions with Bem3 and demonstrated that the interaction was necessary for the ability of Ent2 to rescue a cell division defect mediated by a hyperactive mutant of Bem3. In turn, Bem3 occupied endosomal compartments during early stages of the cell cycle and regulated the proper targeting of secretory vesicles to active exocytic sites for efficient bud emergence and apical growth. Importantly, Bem3 deletion led to a depolarization of secretory vesicles, while its overexpression resulted in massive aggregations of secretory vesicles in large internal compartments and a concomitant defect in bud emergence and loss of cell polarity. Therefore, in addition to extending our understanding of regulatory mechanisms involved in Bem3 localization and function, our study provides mechanistic insights into the broad and growing topic describing extensive crosstalks between signaling and vesicle trafficking pathways during cell polarity establishment and maintenance. |
