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In biology, an endosome is a membrane-bound compartment inside cells, roughly 300-400 nm in diameter when fully mature^[1].

Function

Many endocytotic vesicles, derived from the plasma membrane, are either transported to a pre-existing endosome and fuse with it or are acidified via proton pump to become an endosome. Some endocytosed material passes through endosomes on its way to lysosomes. Endosomes are, in part, responsible for the sorting of endocytosed material before transport to lysosomes. This allows some material to be returned to the plasma membrane.

Process

Many endocytic vesicles originate at the cell surface as clathrin-coated pits. As clathrin assembles under a patch of plasma membrane, the clathrin-coated pit soon (about a minute) pinches off from the surface and forms a clathrin-coated endocytotic vesicle. Soon after forming, the clathrin-coated vesicles release their associated clathrin and become competent to fuse with early endosomes. Extracellular materials trapped in the endocytic vesicles can be either passed into the endosomal compartment or returned to the surface.

Microscopy indicates that, in some cells, the endosomal compartments comprise a network of membranous tubes and vesicles extending all the way to the cell nucleus. The deep end of the endosomal compartment is called the late endosome compartment. It may take 5-15 minutes for materials to be transported from the cell surface through early endosome compartments and on to the late endosome. The endosomal compartment is usually acidic due to the action of a proton-pumping ATPase of the endosomal membrane. Many receptors involved in endocytosis of extracellular substances change their conformation at low pH and release their bound substance. The empty receptor proteins can then be sorted back to the cell surface.

Some materials that reach the late endosomes are degraded in lysosomes. Parts of the late endosomal compartment may become lysosomes or temporarily fuse with lysosomes in order to transfer endocytosed materials into the lysosomes. In epithelial cells, a special process called transcytosis allows some materials to enter one side of a cell and exit from the opposite side. For example, the GI tract of babies can take protective antibody proteins from breast milk, and, via transcytosis, transport the antibodies into the blood stream.

Receptor Mediation

Some materials are incorporated into the endosome by receptor-mediated endocytosis. Some extracellular molecules bind to transmembrane receptor proteins that efficiently accumulate in coated pits. One example of receptor-mediated endocytosis important in human physiology is the main mechanism by which cholesterol is taken up by cells, in particular, liver cells.

Cholesterol is carried in the blood primarily by low density lipoproteins (LDL). In many people, the LDL receptor is defective, so uptake of cholesterol-containing LDL from the blood into liver cells is reduced and LDL-cholesterol accumulates in the blood. This is thought to be the cause of damage to blood vessel walls, as elevated LDL levels result in their penetration into the wall of arteries and causes atherosclerotic vascular disease atherosclerosis. Atherosclerosis is the primary cause of heart attacks and strokes.

Short-signal peptides that target certain transmembrane proteins into clathrin-coated pits have been identified. A set of proteins called adaptins bind the signal peptides. The signal for the cholesterol receptor is the tetrapeptide Asn-Pro-Val-Tyr. This is an endothermic process

Another example of receptor-mediated endocytosis involves the actions of the cell surface transferrin receptor, which binds the iron transport protein transferrin. In the acidic endosome, the iron is released from transferrin, and then the iron-free transferrin (still bound to the transferrin receptor) returns from the early endosome to the cell surface.

Multivesicular body

Multivesicular bodies (MVBs) are specialised endosomes that transit from early/sorting endosome to late endosome / lysosome. Internalized membrane receptors and their ligands are sorted into microdomains on the limiting membrane of MVBs, which are invaginated as internal vesicles into the lumen of the MVB - Hence, they are called multivesicular bodies. Those MVBs mature into lysosomes as they fuse with pre-lysosomes and intraluminal cargos are degraded by lysosomal acid hydrolases. on the other hand, MVBs are also involved in secretion, in which intraluminal vecicles are pinched off the limiting membrane as exosomes.^[2].

References

• Alberts, Bruce; et al. (2004). Essential Cell Biology, 2nd Edition, New York, NY: Garland Science. ISBN 0-8153-3480-X. (Cell and Molecular Biology)

External links

• 3D structures of some proteins associated with endosome membrane

Wikipedia content modification information:

• This page was last modified on 9 October 2008, at 01:33.

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