Vesicular trafficking through cortical actin during exocytosis is regulated by the Rab27a effector JFC1/Slp1 and the RhoA-GTPase-activating protein Gem-interacting protein

Cytoskeleton remodeling is important for the regulation of vesicular transport associated with exocytosis, but a direct association between granular secretory proteins and actin-remodeling molecules has not been shown, and this mechanism remains obscure. Using a proteomic approach, we identified the RhoA-GTPase–activating protein Gem-interacting protein (GMIP) as a factor that associates with the Rab27a effector JFC1 and modulates vesicular transport and exocytosis. GMIP down-regulation induced RhoA activation and actin polymerization. Importantly, GMIP-down-regulated cells showed impaired vesicular transport and exocytosis, while inhibition of the RhoA-signaling pathway induced actin depolymerization and facilitated exocytosis. We show that RhoA activity polarizes around JFC1-containing secretory granules, suggesting that it may control directionality of granule movement. Using quantitative live-cell microscopy, we show that JFC1-containing secretory organelles move in areas near the plasma membrane deprived of polymerized actin and that dynamic vesicles maintain an actin-free environment in their surroundings. Supporting a role for JFC1 in RhoA inactivation and actin remodeling during exocytosis, JFC1 knockout neutrophils showed increased RhoA activity, and azurophilic granules were unable to traverse cortical actin in cells lacking JFC1. We propose that during exocytosis, actin depolymerization commences near the secretory organelle, not the plasma membrane, and that secretory granules use a JFC1- and GMIP-dependent molecular mechanism to traverse cortical actin

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e. secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules