Exocytosis

Exocytosis is the process by which substances produced inside a eukaryotic cell are secreted out through fusion of membrane-bound vesicles to the plasma membrane. Cargo like peptide hormones leaves the endoplasmic reticulum in transport vesicles, moves through the Golgi apparatus, and finally fuses with the plasma membrane, releasing contents into the extracellular space.

Regulated exocytosis requires an external signal and is used to release neurotransmitters and secrete hormones. Constitutive exocytosis is carried out by all cells without requiring a signal, releasing components of the extracellular matrix or incorporating proteins into the plasma membrane. Regulated exocytosis has five major steps, while constitutive exocytosis has four.

Motor proteins actively move vesicles along cytoskeletal tracks composed of microtubules and filaments during the vesicle trafficking step. This active transport mechanism ensures vesicles carrying cargo reach the plasma membrane efficiently, positioning them for subsequent tethering and docking stages of exocytosis.

Vesicle priming occurs only in regulated exocytosis after the vesicle docks but before it releases its contents. This step includes modifications that prepare vesicles for fusion with the plasma membrane, ensuring they are ready to respond to external signals and release their cargo appropriately.

In complete fusion, vesicles entirely collapse and become part of the plasma membrane, expelling their contents from the cell. In kiss-and-run fusion, the vesicle only temporarily fuses with the plasma membrane, releases its contents, and returns to the cell's interior for recycling.

Cells use exocytosis to release signaling molecules like neurotransmitters, insert proteins such as ion channels into their cell membranes, secrete proteins for use in the extracellular matrix, and release waste. Like other bulk transport mechanisms, exocytosis requires energy to move material out of the cell.

Vesicle tethering links vesicles to the plasma membrane, establishing initial contact. During docking, the vesicle membrane attaches to the plasma membrane and the two membranes begin to merge. These sequential steps position vesicles correctly and bring their membranes into close proximity before the final fusion event occurs.