exocytosis

Exocytosis is a cellular process by which vesicles derived from the Golgi apparatus or endosomes fuse with the plasma membrane to release their contents into the extracellular space and, in doing so, add membrane material to the cell surface. It is a key mechanism for secretion and for regulating the composition of the plasma membrane.

Most exocytosis occurs via two broad pathways: constitutive exocytosis, which proceeds continuously and supplies membrane components and secreted proteins, and regulated exocytosis, which is triggered by specific stimuli such as a rise in intracellular calcium. In regulated exocytosis, vesicles are trafficked to the membrane, tethered and docked, then primed for fusion. Fusion is driven by SNARE proteins: v-SNAREs on the vesicle and t-SNAREs on the target membrane form a SNARE complex that brings the lipid bilayers together. Accessory proteins, including Rab GTPases, tethering factors, and calcium-sensing proteins such as synaptotagmin, coordinate docking, priming, and the fusion event. Calcium influx commonly serves as the central trigger for fusion in many secretory cells.

The outcome of exocytosis is the release of cargo—neurotransmitters, hormones, enzymes, or extracellular matrix components—and sometimes the incorporation of vesicle membrane into the plasma membrane, which can alter surface area and receptor distribution. Examples include neurotransmitter release at synapses, insulin secretion by pancreatic beta cells, and mucus or enzyme secretion by epithelial cells.

Exocytosis is tightly regulated and involves cytoskeletal transport and membrane trafficking machinery to ensure cargo specificity and timing. Defects in exocytic pathways have been linked to various diseases, including diabetes and certain neurodegenerative conditions.