Cellular biology of microtubules

The meaning of microtubules

Microtubules are a polar cytoskeleton. Microtubules are composed of α, β two types of tubulin subunits formed tubulin dimer, composed of tubulin dimer long tubular organelles structure. Microtubules are composed of tubulin heterodimers as the basic members, and helical coiled to form the walls of the microtubules.

The structure of microtubules

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 alpha and beta-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, Kinesin and 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.

The presence of microtubules

Microtubules can be present in all mammalian cells, except for erythrocytes, all of which are composed of about 55kD of alpha and beta tubulin. They are normally present in the form of αβ dimer (110ku) and are aggregated in the form of a head-to-tail connection to form a protofilament, consisting of 13 such fibrils forming a hollow microtubule.

Microtubule systems are widely found in eukaryotic cells. In the form of 9+0 in the flagellaries of the animals and some protozoa and the microtubule tissue center of the eukaryotic organism. In the flagellum of the flagellum, it is in the form of 9+2, and 9+0 in the flagellum matrix.

Overview of microtubule tissue

Microtubule organizing center is the region of microtubule assembly, centromere, film-forming body, central body, the matrix has a microtubule tissue center function. All microtubule tissue center has γ microglobulin, the globulin content is very low, can be polymerized into a circular complex, like a template to participate in the nucleation of tubulin, α and β globulin to help micro Tube fiber.

Centrosome is located in the center of the cell, surrounded by some amorphous or fibrous, high electron density of the material, called the outer centromeric material (PCM, pericentriolar material). Central diameter of 0.2mm, 0.4mm long, composed of 9 groups of microtubules, not directly involved in the nucleation of tubulin, with the role of convening PCM.

The tubulin is nucleated with the cyclic gamma globulin complex as a template, and the (-) pole is assembled and then begins to grow. Therefore, the microtubules (-) around the central body point to the central body, and the (+) level is far from the center body.

In 1972, R. Weisenberg demonstrated that purified microtubules spontaneously assembled into microtubules in the presence of GTP, Mg and Ca removal in a slightly acidic environment (pH = 6.9) at a suitable temperature. But this microtubule has only 11 fibrils, probably because there is no template for the formation of γ microglobulin.

Microtubule beta-globulin-bound GTP hydrolysis is not a necessary step for microtubule assembly, but the combination of GTP tubulin dimer can be added to microtubule fibers in the rapidly growing fibers at both ends of the microglobulin-bound GTP too late to hydrolysis, the formation of “hat”, so that microtubule fiber is more stable. Once the subunit microtubules associated with GDP are exposed, they begin to assemble.

The function of microtubules

The role of scaffolds: microtubules in cells like concrete in the same reinforcement, from the support role.

Intracellular transport: microtubules from the intracellular material transport of the role of the track, destruction of microtubules will inhibit the transport of substances within the cell.

Kinesin was discovered in 1985 and is a tetramer consisting of two light chains and two heavy chains, with two spherical heads (with ATPase activity), a helical rod and two fan-like tails. By combining and hydrolyzing ATP, the neck undergoes a conformational change that causes the two heads to alternate with the microtubules, thereby transporting the “cargo” (transport or organelle) of the “tail” to the other local. It is estimated that more than 50 species of kinesin-like protein or KRB (kinesin-related protein) in mammals are more than 50 species. Most KLP can transport vesicles toward microtubules (+), and some are like Ncd protein A centromere-related protein) tends to the (-) pole of the microtubule.

Dynein was discovered in 1963 and named after the flagellum and cilia movement. Dynein has a large molecular weight (close to 1.5Md) consisting of two identical heavy chains and a wide variety of light chains and binding proteins (the kinetic protein of the flagellum microtubule has three heavy chains). Its role mainly in the following areas: in the cell division to promote the separation of chromosomes, driving flagellum movement, toward the micro tube (-) pole transport vesicles.

Forming a spindle: A spindle is a dynamic structure of microtubules that acts to pull the chromosome to a splitting pole in a splitting cell.

Cilia and flagellum movement: flagellum and cilia movement is to rely on the dynein hydrolysis of ATP, so that adjacent two microtubules slide each other.

Plant cell wall formation: plant cell wall is divided into three layers, namely intercellular layer, primary wall, secondary wall. Among them, the fibers in the primary wall are in the form of a network, and the secondary walls are in parallel. The fibrils are closely related to the microtubule arrangement in plant cells.

Recognition of cancer cells: The function and function of the microtubule tissue system within the cancer cells are quite different from those before cancer. Thus, in the medical system can be based on the function and shape to determine whether the patient suffering from cancer.