Definition of tubulin

The protein that constitutes microtubules is called tubulin. Tubulin is a spherical molecule with two types: α and β-tubulin, which have similar three-dimensional structures that can be tightly bound to two Aggregates, as subunits of microtubules assembled. The alpha subunit consists of 450 amino acids, and the β subunit consists of 455 amino acids with a molecular weight of about 55kDa.

A brief introduction to tubulin

Tubulin is a family of proteins that contain multiple members. The most common members areα and β-tubulin, which are the main constituents of microtubules. Microtubules are formed by units of dimer formed byα and β-tubulin. Intracellular tubulin aggregation will affect the length of microtubules, and further affect the cell morphology. Tubulin has a molecular weight of about 55 kDa, a weakly acidic protein with an isoelectric point between 5.2 and 5.8. Tubulin and actin and myosin, which belong to the kinetic protein, have many similarities in the primary structure.

In each microtubule tubulin dimer head and tail to form an elongated protofil (protofilament), 13 of these fibrils arranged in a vertical arrangement of microtubules wall. Microtubules are usually straight, but sometimes also curved. The intracellular microtubules are coiled and bundled and can be assembled with other proteins, which can be assembled into single tubes, double tubes (cilia and flagellum), triple tubes (centrioles and matrix), spindle, , Axons, nerve tube and other structures. Tubulin dimer consists of structurally similar α and β-tubulin, both subunits can bind GTP, alpha-tubulin-bound GTP never undergoes hydrolysis or exchange, is alpha-tubulin Intrinsic components; and as GTPases, beta-tubulin hydrolysable bound GTP, bound GDP can be exchanged for GTP. Microtubules, like microfilaments, have a (+) end with a slower dissociation rate and a slower dissociation rate (-) at a faster growth rate. Microtubules play a supporting role in the cell. In addition it is the two carriers, driving proteins (Kinesin) and kinematic protein (Dynein) walking orbit. Microtubules may be associated with the movement of the protein will be issued to promote the release of the sticky spot, which is the focus of the spot and the tail and the separation of the important step in the process.

Physiological function of tubulin

Some of the structures of microtubules are relatively stable, due to the role of microtubule-binding proteins and the reasons for enzyme modification. Such as nerve cell axons, microtubules in cilia and flagella. Most microtubule fibers are in a dynamic assembly and disassembly state, which is the process necessary to achieve their function (such as a spindle). Colchicine combined with tubulin can be added to microtubules but prevents other tubulin monomers from being added, thereby destroying the spindle structure, and vinca alkaloids have similar functions. Taxol can promote the assembly of microtubules and stabilize the microtubules that have been formed. But this stability will destroy the normal function of microtubules. The above drugs can prevent cell division, can be used for cancer treatment.

The formation of tubulin

γ-tubulin and β-tubulin are the main constituents of microtubules. To form microtubules, these two types of proteins need to bind to GTP first and then to the (+) end (extension) of the microtubule; after the part of the microtubule, GTP bound to the tubulin is hydrolyzed to GDP. Although both proteins can bind to GTP, only beta-tubulin has GTPase activity; this means that only beta-tubulin can hydrolyze bound GTP into GDP, whereas alpha-tubulin does not The Whether tubulin dimer binds to GTP or GDP will affect the stability of the dimer in microtubules. The dimer that binds to GTP tends to form microtubes, while the dimer that binds to GDP tends to dissociate from the microtubules; thus such a GTP and GDP cycle constitutes a dynamic equilibrium of the microtubules.

Tubulin plays a role

γ-tubulin is critical to the formation of microtubules, which predominantly play a role in nucleation and orientation polarity of microtubules. It is found mainly in the central particles and spindle, and these places is the main site of microtubules in the nucleus. In these organelles, multiple gamma-tubulin complexes with other protein molecules, known as the gamma-tubulin ring complex (γ-TuRC); this complex can mimic the (+) , Allowing the binding of [alpha]-tubulin to the [beta]-tubulin dimer. Gamma-tubulin can also be isolated in a diameric form, and this dimer is involved in the formation of gamma-tubulin small complexes (gammaTuSC). The nucleation of γ-tubulin is currently the most clear microtubule nucleation mechanism (2012); but the use of mutations and RNAi to inhibit the function of γ-tubulin found that some specific cells can adapt to the lack of γ-Tubulin, suggesting that there are other microtubule nucleation mechanisms present.

The evolutionary distance of tubulin

Unlike the first three tubulin proteins, δ, ε, ζ and η are only present in some eukaryotes; and they are far from the evolution of the first three tubulin proteins. Among them, δ and ε-tubulin are located in the central granules of the cells. At present (2012) are inconclusive about their function and may be involved in the assembly of centrioles and matrices; δ and ε-tubulin may also be involved in the formation of spindles in mitosis.

The effect of tubulin on the brain

Newborns in the world suffering from pathological head malformations of about one ten thousandth, due to brain development defects, the patient's life is usually not long. The medical profession believes that, in addition to alcohol, by excessive radiation and pregnancy rubella and other viral infection factors, genetic defects are more likely to lead to infant brain developmental defects.

Biologist David. Case-led research team found that a gene mutation called "TUBB5" could lead to fetal malformations in experimental mice. This gene mutation can cause tubulin abnormalities, tubulin is the basic structure of intracellular microtubules, in the cell movement and division play an important role. They also found mutated "TUBB5" genes in the first small malformations.

The scientific community believes that the "TUBB5" gene is an important link to unlocking the mystery of human brain development, laying the groundwork for finding a possible cure for disease.