A tubulin is a spherical protein, usually found in eukaryotic cells, that comes in many forms, all of which play vital roles in the structure and function of the cell. The alpha and beta forms of these proteins are the basic building blocks of microtubules, one of the main components of a cell's cytoskeleton. Gamma tubulin, a third form of this protein family, works together with other proteins to initiate the creation of microtubules in a process called microtubule nucleation. Two more proteins from this family, the delta and epsilon varieties, may play a part in cell mitosis, although research on these proteins is not as extensive. Most forms of tubulin are known to be heterodimer proteins, meaning that they consist of two nonidentical polypeptide sequences bound together.
Before the alpha and beta forms of tubulin can go to work creating microtubules in a process known as polymerization, the gamma form must first provide a framework by combining with other proteins to form a structure called the gamma tubulin ring complex (y-TUrc). Once the y-TUrc is formed, polymerization can occur using the y-TUrc as a platform. The gamma form of the protein is usually found in a cell organelle known as the centrosome, which is a microtubule organization center(MTOC).
The process of polymerization usually takes place in an MTOC, and involves the alpha and beta protein forms binding to a nucleotide known as guanosine triphosphate (GTP). While bound to GTP, tubulin dimers arrange themselves in long polymer chains called protofilaments, which are arranged by alternating the alpha and beta protein forms. These protofilaments are then packed together into the hollow fibers that are microtubules. Microtubules are an important part of a cell's cytoskeleton, playing a vital role in the maintenance of the cell's shape and structure. They are able to shift formation to assist in the execution of cell functions like mitosis and vesicle transportation, partly due to the flexible nature of alpha and beta tubulin.
It was once thought that these proteins only appeared in eukaryotic cells. However, in the 1950s, it was discovered that prokaryotic cells, specifically bacteria, possess a protein called Filamenting temperature-sensitive mutant Z (FtsZ) that is homologous to the tubulin family of proteins. FtsZ is encoded by the ftsZ gene and is a component of the prokaryotic cytoskeleton. It plays a role in prokaryotic cell division, creating a structure called the FtsZ ring that is vital to the formation of new cell walls. The FtsZ ring is also used in some eukaryotic cells for the division of chloroplasts and some varieties of mitochondria.