Molecular Machinery Regulating Exocytosis

Exocytosis is the major intracellular route for the delivery of proteins and lipids to the plasma membrane and the means by which vesicular contents are released into the extracellular space. The anterograde trafficking of vesicles to the plasma membrane is vital for membrane expansion during cell division; cell growth and migration; the delivery of specialised molecules to establish cell polarity; cell-to-cell communication; neurotransmission and the secretion of response factors such as hormones, cytokines and antimicrobial peptides. There are two major trafficking routes in eukaryotic cells, which are referred to as constitutive and regulated (Ory & Gasman, 2011). Constitutive exocytosis involves the steady state delivery of secretory carrier vesicles from the endoplasmic reticulum via the Golgi apparatus to the plasma membrane (Lacy & Stow, 2011). Regulated or granule-mediated exocytosis involves a specific trigger, usually a burst of intracellular calcium following an extrinsic stimulus. This system is utilized for secretion in neuronal cells and other specialist secretory cells, such as neuroendocrine, endocrine and exocrine cells (Burgoyne & Morgan, 2003; Jolly & Sattentau, 2007; Lacy & Stow, 2011). Regulated exocytosis enables a rapid response from a subpopulation of vesicles already primed and competent for fusion (Manjithaya & Subramani, 2011; Nickel & Seedorf, 2008; Nickel, 2010). Regulated exocytosis is also used for polarised traffic of vesicular membrane and cargo to specific spatial landmarks and this is particularly important during times of dramatic change in cell morphology, such as cell division, cell motility, phagocytosis and axonal outgrowth.