A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes (see micelle), or they may be prepared. Artificially prepared vesicles are known as liposomes. Most vesicles have specialized functions depending on what materials they contain.
Because vesicles tend to look alike, it is very difficult to tell the difference between different types.
The vesicle is separated from the cytosol by at least one phospholipid bilayer. If there is only one phospholipid bilayer, they are called unilamellar vesicles; otherwise they are called multilamellar.
Vesicles store, transport, or digest cellular products and waste. The membrane enclosing the vesicle is similar to that of the plasma membrane, and vesicles can fuse with the plasma membrane to release their contents outside of the cell. Vesicles can also fuse with other organelles within the cell.
Because it is separated from the cytosol, the inside of the vesicle can be made to be different from the cytosolic environment. For this reason, vesicles are a basic tool used by the cell for organizing cellular substances. Vesicles are involved in metabolism, transport, buoyancy control,[1] and enzyme storage. They can also act as chemical reaction chambers.
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Vacuoles are vesicles which contain mostly water.
Secretory vesicles contain materials that are to be excreted from the cell. Cells have many reasons to excrete materials. One reason is to dispose of wastes. Another reason is tied to the function of the cell. Within a larger organism, some cells are specialized to produce certain chemicals. These chemicals are stored in secretory vesicles and released when needed. Some examples include the following.
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.
The assembly of vesicles requires numerous coats to surround and bind to the proteins being transported. these bind to the coat vesicle. 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.
The clathrin coat is thought to assemble in response to regulatory G protein. A coatomer coat assembles and disassembles due to an ARF protein.
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.
Often SNAREs associated with vesicles or target membranes are instead classified as Qa, Qb, Qc or R SNAREs owing to further variation than simply v- or t-SNAREs. An array of different SNARE complexes can be seen in different tissues and subcellular compartments, with 36 isoforms currently identified in humans.
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.
Vesicle fusion can occur in one of two ways: full fusion or kiss-and-run fusion. 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.
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 degradation; When the endosome either matures to become a lysosome or is united with one, the vesicles are completely degraded. 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.[4]
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.
Phospholipid vesicles have been studied in biochemistry. For such studies, a homogeneous phospholipid vesicle suspension can be prepared by sonication,[5] injection of a phospholipid solution into the aqueous buffer solution membranes.[6] In this way aqueous vesicle solutions can be prepared of different phospholipid composition, as well as different sizes of vesicles.
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