5.16: Exocytosis

Exocytosis is used to release material from cells. Like other bulk transport mechanisms, exocytosis requires energy.

While endocytosis takes particles into the cell, exocytosis removes them. Sometimes, the released material are signaling molecules. For example, neurons typically use exocytosis to release neurotransmitters. Cells also use exocytosis to insert proteins, such as ion channels, into their cell membranes, secrete proteins for use in the extracellular matrix, or release waste.

There are two main types of exocytosis in eukaryotes: regulated and non-regulated (or constitutive). Regulated exocytosis, which requires an external signal, is used to release neurotransmitters and secrete hormones. Unlike regulated exocytosis, constitutive exocytosis is carried out by all cells. Cells use constitutive exocytosis to release components of the extracellular matrix or incorporate proteins into the plasma membrane.

There are five major steps in regulated exocytosis and four in constitutive exocytosis.

The first step is vesicle trafficking, in which vesicles transport material to the plasma membrane. Motor proteins actively move vesicles along cytoskeletal tracks of microtubules and filaments. The second step is vesicle tethering, in which vesicles are linked to the plasma membrane. In the third step, vesicle docking, the vesicle membrane attaches to the plasma membrane, and the two membranes begin to merge.

The fourth step, vesicle priming, occurs only in regulated exocytosis. Vesicle priming includes modifications occurring after the vesicle docks but before it releases its contents. Priming prepares vesicles for fusion with the plasma membrane.

The fifth step is vesicle fusion. Vesicle fusion can be complete or kiss-and-run. In complete fusion, vesicles entirely collapse and become part of the plasma membrane, expelling their contents from the cell in the process. In kiss-and-run fusion, the vesicle is recycled: It only temporarily fuses with the plasma membrane, releases its contents, and returns to the cell’s interior.

Exocytosis is the process in which substances produced inside a eukaryotic cell are secreted out through the fusion of membrane-bound vesicles to the plasma membrane.

For example, initially, signaling molecules, like a peptide hormone, leave the endoplasmic reticulum packaged along with other membrane components in a transport vesicle. This cargo proceeds through the cisternae of the golgi appratus, continuously budding and fusing until the contents reach the trans-golgi network.

Here they aggregate, bud off into vesicles once again, and finally head towards their target, the plasma membrane, where the membranes of the vesicle and cell fuse, releasing the signal into the extracellular space.