Exocytose

Exocytosis, also spelled exocytose, is a cellular process in which vesicles within the cell fuse with the plasma membrane to release their contents into the extracellular space. It also adds membrane to the plasma membrane and delivers membrane proteins and lipids. Exocytosis is essential for processes such as neurotransmitter and hormone secretion, digestive enzyme release, and immune cell functions.

Vesicles are trafficked to the plasma membrane and docked by tethering factors. Fusion is driven by SNARE proteins, with v-SNAREs on the vesicle pairing with t-SNAREs on the target membrane to form a SNARE complex that brings membranes into close apposition and initiates fusion. In vertebrates, typical v-SNAREs include synaptobrevin/VAMP, while t-SNAREs include syntaxin and SNAP-25. Regulatory proteins such as Rab GTPases and SM family members (for example Munc18) modulate docking and priming, and the SNARE complex is disassembled afterward by NSF and alpha-SNAP.

There are two main modes of exocytosis. Constitutive exocytosis proceeds continuously and does not require a Ca2+ signal, supporting membrane turnover and steady delivery of cargo. Regulated exocytosis is triggered by elevations in intracellular Ca2+, occurring in neurons, endocrine cells, and other secretory cells, where secretory granules release their contents in response to specific stimuli. Ca2+ acts as a key trigger in many fast, regulated secretion pathways via Ca2+-sensing proteins such as synaptotagmin.

Exocytosis plays a foundational role in cell-to-cell communication and organismal homeostasis. Disruptions in exocytotic machinery can contribute to diseases including diabetes, neurodegenerative disorders, and immune dysfunction.