Exocitosis

Exocytosis is the process where molecules produced inside a eukaryotic cell are secreted out through fusion of membrane-bound vesicles with the plasma membrane. Secretory proteins first leave the endoplasmic reticulum packaged in transport vesicles, proceed through the Golgi apparatus, and bud into vesicles at the trans-Golgi network. These vesicles then travel to the plasma membrane where fusion of secretory vesicles with the plasma membrane releases the cargo into the extracellular space.

Regulated exocytosis requires an external signal and is used to release neurotransmitters and secrete hormones, occurring only in response to specific cellular signals. Constitutive exocytosis is carried out by all cells continuously and does not require external signals. Cells use constitutive exocytosis to release components of the extracellular matrix or incorporate proteins into the plasma membrane without waiting for activation.

Exocytosis occurs in five stepwise stages: vesicle trafficking, where motor proteins move vesicles along cytoskeletal tracks; vesicle tethering, where vesicles partially link to the plasma membrane; vesicle docking, where membranes begin to pair; vesicle priming, which occurs only in regulated exocytosis to prepare vesicles for fusion; and vesicle fusion, where contents are released into the extracellular space.

In complete fusion, vesicles entirely collapse and become part of the plasma membrane, expelling all contents from the cell permanently. In kiss-and-run fusion, the vesicle temporarily fuses with the plasma membrane, releases its contents, and returns to the cell's interior for recycling. Kiss-and-run fusion allows cells to reuse vesicles rather than permanently incorporating them into the membrane.

Exocytosis serves multiple functions: neurons use it to release neurotransmitters for synaptic communication, cells insert proteins such as ion channels into their membranes, and cells secrete proteins for use in the extracellular matrix. Exocytosis also removes cellular waste products. Like other bulk transport mechanisms, exocytosis requires energy to move and fuse vesicles with the plasma membrane.

Vesicle priming occurs after the vesicle docks but before releasing its contents, preparing vesicles for fusion with the plasma membrane. This step is unique to regulated exocytosis and ensures vesicles are ready to respond quickly to external signals. Priming modifications enable rapid neurotransmitter release and hormone secretion when cells receive activation signals.

Motor proteins actively move vesicles along cytoskeletal tracks composed of microtubules and filaments during the vesicle trafficking step. This active transport requires energy and directs vesicles toward the plasma membrane. Motor protein activity ensures vesicles reach their target destination efficiently, enabling timely delivery of cargo for secretion or membrane insertion.