Microtubules, an integral component of the cellular cytoskeleton, consist of cytoplasmic tubes, 25 nm in diameter that are often of extreme length (Figure1). Each cylindrical tube is comprised of 13 longitudinally arranged protofilaments comprised of α-tubulin and β-tubulin, closely related proteins of ≈50 kDa molecular mass. Assembly starts with the formation of tubulin dimers that assemble into protofilaments that associate with one another in parallel to form sheets and eventually cylinders. GTP is required for assembly. A microtubule organizing center, located around a pair of centrioles, nucleates the growth of new microtubules. A third species of tubulin, γ-tubulin, appears to play an important role in this process.

Fig1. Schematic representation of microtubules.At left and center are shown drawings of microtubules as seen in the electron microscope following fixation with tannic acid in glutaraldehyde. The unstained tubulin subunits are delineated by the dense tannic acid. Cross sections of tubules reveal a ring of 13 subunits of dimers arranged in a spiral. Changes in microtubule length are due to the addition or loss of individual tubulin subunits. Characteristic arrangements of microtubules (not shown here) are found in centrioles, basal bodies, cilia, and flagellae. (Reproduced with permission from Junqueira LC, Carneiro J, Kelley RO: Basic Histology, 7th ed. New York, NY: Appleton & Lange; 1992.)

Microtubules are necessary for mitotic spindle formation and function. The latter include chromosome segregation during cell division, intracellular movement of endocytic and exocytic vesicles, and structural components of cilia and flagella. Microtubules also maintain the structure of axons and dendrites as well as participate in the axoplasmic flow of material along neuronal processes. A variety of microtubule associated proteins [MAPs], one of which is tau, play important roles in microtubule assembly and stabilization.

Microtubules exist in a state of dynamic instability, constantly assembling and disassembling. They exhibit polarity (plus and minus ends); a feature important for growth from centrioles and directing intracellular movement. For instance, in axonal transport, the protein kinesin, with a myosin-like ATPase activity, uses hydrolysis of ATP to move vesicles down the axon toward the positive end of the microtubular formation. Flow of materials in the opposite direction, toward the negative end, is powered by cytosolic dynein, another protein with ATPase activity. Similarly, axonemal dyneins power ciliary and flagellar movement while dynamin, which uses GTP instead of ATP, is involved in endocytosis. Kinesins, dyneins, dynamin, and myosins are referred to as molecular motors.

An absence of dynein in cilia and flagella results in immotile cilia and flagella, leads to Kartagener syndrome (OMIM 244400), which can cause male sterility, situs inversus, and chronic respiratory infections. Mutations in genes affecting the synthesis of dynein have been detected in individuals with this syndrome. Pharmaceuticals such as colchicine (used for treatment of acute gouty arthritis), vinblastine(a vinca alkaloid used for treating certain types of cancer), paclitaxel (Taxol) (effective against ovarian cancer), and griseofulvin (an antifungal agent) also disrupt microtubule assembly or disassembly.

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استطلاع يكشف "الأكثر قلقا" من تأثر الوظائف بالذكاء الاصطناعي

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مواضيع ذات صلة

Plasma Contains A Complex Mixture of Proteins

Intermediate Filaments Differ From Microfilaments & Microtubules

Muscle Tissue are The Target of Several Genetic Disorders

Skeletal Muscle Contains Slow (Red) & Fast (White) Twitch Fibers

Muscle Contraction Requires Large Quantities of ATP

Cardiac Muscle Resembles Skeletal Muscle in Many Respects

Contraction is Orchestrated by the Second Messenger Ca2+

Muscle Converts Chemical Energy to Mechanical Energy

Cholesterol Side Chain Cleavage Enzyme (p450scc)

Steroidogenic Acute Regulatory Protein (StAR)

Cytochrome P450 Steroid Hydroxylases

Major Protein Components of Muscle Fibers