Exocitoz

Exocitoz, also known as exocytosis, is the cellular process by which vesicles derived from within the cell fuse with the plasma membrane to release their contents into the extracellular space. It is essential for secretion of proteins and lipids, intercellular signaling, and plasma membrane growth and maintenance.

The mechanism involves vesicle trafficking to the plasma membrane, where vesicles are tethered and docked at specific sites. Priming prepares the vesicle for fusion. Fusion is driven by SNARE proteins: v-SNAREs on the vesicle (such as VAMP/synaptobrevin) pair with t-SNAREs on the target membrane (syntaxin and SNAP-25) to form a SNARE complex that brings membranes together. Regulatory factors, including Rab GTPases, Munc18, Munc13, NSF, and alpha-SNAP, modulate assembly and disassembly of the complex. In many cells, a rise in intracellular calcium triggers regulated exocytosis: calcium sensors like synaptotagmin detect Ca2+ and promote rapid SNARE-mediated fusion.

Two main modes are recognized. Constitutive exocytosis proceeds continuously to deliver membrane components and secreted proteins, supporting membrane turnover and routine secretion. Regulated exocytosis is stimulated by specific signals (such as neuronal activity or hormonal signals) and results in rapid, space- and time-constrained release of cargo.

Roles and significance include neurotransmitter release at synapses, hormone secretion (for example, insulin), enzyme and mucous secretion, and the delivery of membrane proteins and cell wall components in various cell types. Exocytosis is coordinated with endocytosis and membrane recycling. Disruptions in exocytotic machinery are linked to a range of diseases, including certain neurological disorders and metabolic conditions.