vesicle

Scheme of a simple vesicle (liposome).

A vesicle is a bubble of liquid within a cell. More technically, a vesicle is a small, intracellular, membrane-enclosed sac that stores or transports substances within a cell. Vesicles form naturally because of the properties of lipid membranes (see micelle). 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 of vesicles without sampling their contents.

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. (Lamella means membrane).

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.

Sarfus image of lipid vesicles.

Types of vesicles

Electron micrograph of a cell containing a food vacuole (fv) and transport vacuole (tv)in a malaria parasite.

Vacuoles

Vacuoles are vesicles which contain mostly water.

• Plant cells are known for having a large central vacuole in the center of the cell that is used for osmotic control and nutrient storage.

• Food vacuoles are used in phagocytosis and other forms of endocytosis. The vesicles used in endocytosis are sometimes called endosomes.

• Contractile vacuoles are found in certain protists, especially those in Phylum Ciliophora. These vacuoles take water from the cytoplasm and excrete it from the cell to avoid bursting due to osmotic pressure.

Lysosomes

• Lysosomes are found within vesicles which contain digestive enzymes used to break down substances in the cell into smaller compounds. They are involved in cellular digestion. Food can be taken from outside the cell into food vacuoles by a process called endocytosis. These food vacuoles fuse with lysosomes which break down the components so that they can be used in the cell. This form of cellular eating is called phagocytosis.

• Lysosomes are also used to destroy defective or damaged organelles in a process called ENDOPHAGOCYTOSIS . They fuse with the membrane of the damaged organelle digesting it.

Transport vesicles

• Transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus.

• Membrane-bound and secreted proteins are made on ribosomes found in the rough endoplasmic reticulum. Most of these proteins mature in the Golgi apparatus before going to their final destination which may be to lysosomes, peroxisomes, or outside of the cell. These proteins travel within the cell inside of transport vesicles.

Secretory vesicles

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.

Types of secretory vesicles

• Synaptic vesicles are located at presynaptic terminals in neurons and store neurotransmitters. When a signal comes down an axon, the synaptic vesicles fuse with the cell membrane releasing the neurotransmitter so that it can be detected by receptor molecules on the next nerve cell.

• In animals endocrine tissues release hormones into the bloodstream. These hormones are stored within secretory vesicles. A good example is the endocrine tissue found in the Islets of Langerhans in the pancreas. This tissue contains many cell types that are defined by which hormones they produce.

• Secretory vesicles hold the enzymes that are used to make the cell walls of plants, protists, fungi, bacteria, and archea cells as well as the extracellular matrix of animal cells.

Other types of vesicles

• Gas vesicles are used by Archaea, Bacteria and planktonic microorganisms, possibly to control vertical migration by regulating the gas content and thereby buoyancy, or possibly to position the cell for maximum solar light harvesting.

• Matrix vesicles are located within the extracellular space, or matrix. Using electron microscopy but working independently, they were discovered in 1967 by H. Clarke Anderson^[2] and Ermanno Bonucci.^[3] These cell-derived vesicles are specialized to initiate biomineralization of the matrix in a variety of tissues, including bone, cartilage, and dentin. During normal calcification, a major influx of calcium and phosphate ions into the cells accompanies cellular apoptosis (genetically determined self-destruction) and matrix vesicle formation. Calcium-loading also leads to formation of phosphatidylserine:calcium:phosphate complexes in the plasma membrane mediated in part by a protein called annexins. Matrix vesicles bud from the plasma membrane at sites of interaction with the extracellular matrix. Thus, matrix vesicles convey to the extracellular matrix calcium, phosphate, lipids and the annexins which act to nucleate mineral formation. These processes are precisely coordinated to bring about, at the proper place and time, mineralization of the tissue's matrix.

• Multivesicular body, or MVB, is a membrane-bound vesicle containing a number of smaller vesicles.

Vesicle formation and transport

Overview of cellular organelles, showing vesicle (4), and related structures (rough endoplasmic reticulum (5) and Golgi apparatus (6).

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.

Vesicle coat

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.

Vesicle docking

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

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.^[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.

See also

• Endoplasmic reticulum
• Golgi apparatus
• Micelle
• Membrane nanotube
• Endocytosis
• Synaptic vesicle
• Membrane contact sites

References

Other references

• Bruce Alberts, et al. (1994); Molecular Biology of the Cell; Third Edition

External links

• Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology