Exocytosis

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Exocytosis is a vesicular pathway that involves finalised and secretory vesicles leaving the Golgi apparatus. It is the functional opposite of endocytosis. Exocytosis is an outward pathway, beginning from the endoplasmic reticulum (ER). Once proteins have been modified and sorted in the ER and Golgi apparatus, they are stored in membrane-bound vesicles, which are then transported to the plasma membrane or organelles ( such as lysosomes/endosomes) depending upon function. This entire pathway is the outward pathway.

Vesicles exit from the trans face of the Golgi apparatus, or the "late" face, and head towards the plasma membrane via one of a further two secretory pathways: constitutive or regulatory[1].

In the constitutive phase, this pathway occurs in all cell types. The constitutive pathway directs vesicles either towards endosomes/lysosomes or immediately directs them towards the plasma membrane. It involves modified lipids and proteins in vesicles which head towards and fuse with the plasma membrane, after "budding off" from the trans face of the Golgi apparatus. The vesicle reaches a specific location on the membrane via the use of docking SNARE proteins. These proteins on the vesicle and membrane bind together to form a SNARE complex so the vesicle is in the right location on the membrane and also to catalyse the reaction for fusion to occur[2]. Once successful fusion is accomplished the contents of the vesicle are secreted into the extracellular environment, the lipids and proteins in these vesicles become components for the membrane whilst other proteins are released into the extracellular environment. Contents destined for lysosomes are usually contained in specific departing vesicles, for example in which a compound M6P (mannose-6-phosphate) allows fusion with the endosomes, whereas other vesicles do not have this compound attached[3].

Regulated pathways are more common in secretory cells such as white blood cells and epithelial cells; cells that must rapidly produce products such as hormones and specialised enzymes[4]. Vesicles containing secretory proteins are only able to fuse with the membrane under controlled conditions. These vesicles do not fuse with the plasma membrane once they arrive there, unlike the constitutive pathway; they are carried by microtubules on the cells' cytoskeleton to near the surface of the membrane. The vesicles only fuse with the membrane once they receive a signal from a receptor after a messenger (e.g. a hormone) has successfully bonded to the receptor. This complex activates intracellular signals that cause the vesicles to fuse with the membrane and release their contents into the environment. A good example is in nerve cells in the pre-synaptic membrane, when an action potential reaches the pre-synaptic membrane and causes a flow of calcium ions into the cell, these ions then allow the vesicles to fuse with the membrane and release their contents; in the case with neurons it is usually neurotransmitters such as acetylcholine and dopamine released[5].

References

  1. Alberts, B et al (2008). Molecular biology of the cell. 5th edition. US Garland Science. Page 800
  2. Pfeffer SR (2007); “Unsolved Mysteries in Membrane Traffic” Annual Review of Biochemistry 76:629-645
  3. Alberts, B et al (2008). Molecular biology of the cell. 5th edition. US Garland Science. Page 800
  4. Alberts, B et al (2008). Molecular biology of the cell. 5th edition. US Garland Science. Page 800
  5. Alberts, B et al. (2008). Molecular biology of the cell. 5th edition. US Garland Science. Page 804
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