18.12

Exocytosis

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Exocytosis is the process where molecules produced inside a eukaryotic cell are secreted out of the cell through the fusion of membrane-bound vesicles with the plasma membrane.

For example, secretory proteins that are destined for exocytosis, first leave the endoplasmic reticulum packaged along with other vesicular components in a transport vesicle. This cargo proceeds through the cisternae of the Golgi apparatus, continuously budding and fusing until the contents reach the trans-Golgi network.

Here they 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.

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Exocytosis is a process that releases molecules outside the cell. Like other bulk transport mechanisms, exocytosis requires energy.

Exocytosis is the opposite of endocytosis, which brings molecules inside the cell. Sometimes, the released materials 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 remove waste.

There are two main types of exocytosis in eukaryotes—regulated and non-regulated (or constitutive). Regulated exocytosis requires an external signal and 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.

Both regulated and constitutive exocytosis occur in a stepwise manner.

The first step is vesicle trafficking, in which vesicles transport material to the plasma membrane. Motor proteins actively move vesicles along the cytoskeletal tracks of microtubules and filaments. The second step is vesicle tethering, in which vesicles are partially 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 pair with each other.

The fourth step, vesicle priming, occurs only in regulated exocytosis. Vesicle priming includes modifications occurring after the vesicle docks but before releasing 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 the 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.

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