Exozytose

Exozytose, or exocytosis, is the cellular process by which vesicles fuse with the plasma membrane to release their contents outside the cell and to incorporate vesicle membrane into the cell surface. This mechanism is essential for secretion, membrane remodeling, and intercellular signaling.

The process typically begins with vesicle trafficking from the Golgi apparatus or endosomal compartments toward the plasma membrane. Vesicles dock at specific sites, become primed for fusion, and then merge with the membrane to form a fusion pore through which cargo is released into the extracellular space. In most cells, exocytosis is tightly regulated by cellular signals.

Two main modes of exocytosis exist. Constitutive exocytosis involves a continuous, unregulated delivery of lipids, proteins, and soluble cargo necessary for membrane maintenance and general secretion. Regulated exocytosis is Ca2+-dependent and occurs in specialized cells such as neurons and endocrine or exocrine cells, where vesicle fusion is triggered by elevations in intracellular calcium.

Molecular machinery controlling exocytosis includes SNARE proteins that drive membrane fusion. v-SNAREs on vesicles (such as synaptobrevin/VAMP) pair with t-SNAREs on the target membrane (syntaxin and SNAP-25). The SNARE complex is disassembled after fusion by NSF and α-SNAP. Rab GTPases regulate vesicle docking, while proteins like synaptotagmin serve as Ca2+ sensors in many regulated secretory pathways.

Functions of exocytosis include neurotransmitter and hormone secretion, delivery of enzymes, membrane protein integration, and immune cell degranulation. Clinically, toxins that target SNAREs (for example, botulinum and tetanus toxins) disrupt exocytosis, highlighting its critical role in health and disease.