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Types of exocytosis

Exocytosis is a way of transporting substances out of the cell. The vesicles located in the cytosol fuse and “fuse” with the cell membrane, releasing the substances stored in them. The first connection between the lumen of the vesicle and the extracellular space is called the fusion pore . The exact nature of the fusion pore and the biophysical mechanisms of membrane fusion are still unclear. From a purely physical point of view, when two membranes approach very closely, huge repulsive forces act. Nevertheless z. B. exocytosis of synaptic vesicles within a millisecond.

The exocytosis can be divided into 2 different types:

  1. constitutive exocytosis
  2. receptor-mediated exocytosis

Constitutive exocytosis

Through constitutive or untriggered exocytosis, membrane proteins are integrated into the cell membrane and the biomembrane , consisting of a lipid bilayer , is renewed or expanded. This process is also known as cell membrane biogenesis. On the other hand, proteins are also released into the extracellular matrix via constitutive exocytosis. This type of exocytosis is particularly important in connective and supporting tissue cells, such as B. fibroblasts and osteoblasts .

Stimulated exocytosis

The activation of the stimulated or triggered exocytosis requires a specific stimulus. This stimulus is usually a hormone that binds to a specific receptor on the cell surface, thereby triggering a signal cascade inside the cell, which then ultimately causes the exosome to fuse with the cell membrane.

This form of exocytosis plays an important role in the release of hormones into the blood and the release of digestive secretions into the pulp in the digestive system. An example of this is insulin delivery . In the insulin-producing cells ( islets of Langerhans in the pancreas ), the hormone molecules are packed in vesicles and transported to the cell surface. There the vesicles fuse (fuse) with the cell membrane and the insulin is released to the outside.

New plasma membrane components are also transported from the Golgi apparatus - their place of manufacture - to the membrane.

The basic processes are shown schematically below using Fig. 3. They take place at the molecular level in the nanometer range and have even more intermediate steps.

Fig. 3: Schematic representation of the signal transduction in the sequence of a triggered exocytosis
  1. A receptor on the cell surface is converted into an active form by a ligand (e.g. a hormone). As a result, it is able to change specific molecules floating freely in the cytosol . In this way, cAMP is created from ATP and InsP3 and DAG are created from PInsP2 . Each receptor can cleave the substances mentioned more than once, until the ligand detaches from it.
  2. InsP3 and cAMP in turn cause a change in the conformation of another transmembrane molecule. This molecule is a carrier molecule which, in its active form, enables calcium ions (Ca 2+ ) to flow into the cell. This second step in signal transduction represents an amplification of the initial signal.
  3. The DAG activates a transmembrane carrier in the membrane of the endoplasmic reticulum , in which calcium ions are stored.
  4. The Ca 2+ from these two sources now flows into the cell and causes the exosome to be brought to the cell surface, where it fuses with the cell membrane, after which the now empty exosome is transported back into the cell interior ( vesicle recycling ). The transport of the exosome itself requires numerous molecules of the cytoskeleton such as microtubule molecules, which can be viewed as a kind of rails, as well as ATP-consuming kinesin molecules, which pull the exosome along the microtubules (not shown).

At the molecular level, exocytosis is a complicated interplay between the most varied of molecules and follows precisely defined functional processes. However, it has proven successful and effective in the course of evolution.

See also

Web links

Commons : Exocytosis  - collection of images, videos and audio files

Individual evidence

  1. Alberts et al .: Molecular biology of the cell. 3. Edition.