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In cell biology, a vesicle is a relatively small intracellular, membrane-enclosed sac that stores or transports substances. The vesicle is separated from the cytosol by at least one lipid bilayer. If there is only one lipid bilayer, they are called unilamellar vesicles; otherwise they are called multilamellar. Vesicles store, transport, or digest cellular products and waste. This biomembrane enclosing the vesicle is similar to that of the plasma membrane. Because it is separated from the cytosol, the intravesicular environment can be made to be different from the cytosolic environment. Vesicles can fuse with the plasma membrane, releasing their contents outside the cell. Vesicles are a basic tool of the cell for organizing metabolism, transport, enzyme storage, as well as being chemical reaction chambers. Many vesicles are made in the Golgi apparatus, but also in the endoplasmic reticulum, or are made from parts of the plasma membrane.
Some types of vesicles
Fatty acid vesicles- aid in the long term storage of the cell
Vesicle formation and transport
Some vesicles are made when part of the membrane pinches off the endoplasmic reticulum or the Golgi complex. Others are made when an object outside of the cell is surrounded by the cell membrane.
Capturing cargo molecules
The assembly of a vesicle requires numerous coats to surround and bind to the proteins being transported. One family of coats are called adaptins. These bind to the coat vesicle (see below). They also trap various transmembrane receptor proteins, called cargo receptors, which in turn trap the cargo molecules.
The vesicle coat serves to sculpt the curvature of a donor membrane, and to select specific proteins as cargo. It selects cargo proteins by binding to sorting signals. In this way the vesicle coat clusters selected membrane cargo proteins into nascent vesicle buds.
There are three types of vesicle coats: clathrin, COPI and COPII. Clathrin coats are found on vesicles trafficking between the Golgi and plasma membrane, the Golgi and endosomes, and the plasma membrane and endosomes. COPI coated vesicles are responsible for retrograde transport from the Golgi to the ER, while COPII coated vesicles are responsible for anterograde transport from the ER to the Golgi.
Surface markers called SNAREs identify the vesicle's cargo, and complementary SNAREs on the target membrane act to cause fusion of the vesicle and target membrane. Such v-SNARES are hypothesised to exist on the vesicle membrane, while the complementary ones on the target membrane are known as t-SNAREs.
Regulatory Rab proteins are thought to inspect the joining of the SNAREs. Rab protein is a regulatory GTP-binding protein, and controls the binding of these complementary SNAREs for a long enough time for the Rab protein to hydrolyse its bound GTP and lock the vesicle onto the membrane.
Fusion requires the two membranes to be brought within 1.5 nm of each other. For this to occur water must be displaced from the surface of the vesicle membrane. This is energetically unfavourable, and evidence suggests that the process requires ATP, GTP and acetyl-coA, fusion is also linked to budding, which is why the term budding and fusing arises.
Vesicles in receptor downregulation
Membrane proteins serving as receptors are sometimes tagged for downregulation by the attachment of ubiquitin. After arriving an endosome via the pathway described above, vesicles begin to form inside the endosome, taking with them the membrane proteins meant for degregation; When the endosome either matures to become a lysosome or is united with one, the vesicles are completely degregaded. Without this mechanism, only the extracellular part of the membrane proteins would reach the lumen of the lysosome, and only this part would be degraded.
It is because of these vesicles that the endosome is sometimes known as a multivesicular body. The pathway to their formation is not completely understood; unlike the other vesicles described above, the outer surface of the vesicles is not in contact with the cytosol.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Vesicle_(biology)". A list of authors is available in Wikipedia.|