Tubulin, microtubules formed

Microtubule a hollow rod formed from the protein tubulin. Microtubules form part of the cytoskeleton of cells as well as cilia and flagella. 

Fig. 4 Tubulin microtubules and Zn-sheets. The paired series of images show these polymeric forms of tubulin from different views. Top - parallel (in line) with the protofilament axis. Middle - 45° to the pf axis. Bottom - perpendicular to the pf axis (for the microtubule, this is the luminal view)...

Fig. 24 Laulimalide stabilizes microtubules, but destabilizes zinc sheets. This multipart figure illustrates the effect of addition of laulimalide upon the polymerization of tubulin. Tubulin polymers, as sheets or microtubules, are inherently unstable and disassemble rapidly at low temperatures. Addition of paclitaxel (or epothilone, not shown) protect sheets from disassembly, a Control zinc sheets stabilized with PTX after 20 minutes in a bath of ice and water, b The effect of laulimalide under the same ice bath conditions as a. After 20 minutes, sheets exposed to laulimalide have significantly disassembled and reformed into microtubules, c Microtubules formed at room temperature from addition of laulimalide to zinc-stabilized sheets and incubated overnight. Almost all sheets have converted to microtubules (enlarged microtubules shown inset). By comparison, paclitaxel and epothilone-stabilized sheets remain intact under the same conditions, d Magnified microtubules seen in b. Figure provided by Huilin Li...

Figure 5.15. Turbidity-time curve for microtubule assembly.The diagram illustrates the turbidity-time changes that occur during microtubule assembly. Initially during the turbidity lag phase, tubulin monomers form rings of tubulin subunits that cause microtubule elongation during the growth phase.

Eukaryotic cells have an internal scaffolding called the cytoskeleton or cytomatrix that maintains their cellular morphology and enables them to migrate, undergo shape changes, and transport vesicles. Microfilaments, made of actin, intermediate filaments, which are composed of laminin and other proteins, and microtubules, formed from the protein tubulin, along with many different accessory proteins, comprise the cytoskeleton. Both the microfilaments and the microtubules can assemble and disassemble rapidly in the cell, whereas disassembly of intermediate filaments may require their destruction. Although much is known about the molecular composition of the cytoskeleton, the molecular events involved in most cell movements are still unknown. 

Site of action Paclitaxel binds reversibly to tubulin, but unlike the vinca alkaloids, it promotes polymerization and stabilization of the polymer rather than disassembly (Figure 38.15). Thus it shifts the depolymerization-polymerization to favor the formation of microtubules. The overly stable microtubules formed in the presence of paclitaxel are dysfunctional, thereby causing the death of the cell. 

Since the discovery 117) of its action on tubulin 118) (the protein which, in the form of microtubules, constitutes the mitotic spindle), taxol has been of great utility to biologists. Numerous publications, not all of which need be cited here, describe the use of taxol for the isolation of tubulin from cellular preparations in which the concentration of this protein is too low to permit its polymerization, as in the pancreas 119) or the vegetal domain 120). Taxol has permitted not only the discovery of new microtubules in the Xenopus oocyte cortex 121) but also the study of the role of microtubules in certain cellular processes owing to its lack of destructive effects, in contrast to other known spindle poisons such as colchicine or vinblastine (722). Among other problems, taxol has helped in studies of the influence of the tubulin-microtubule equilibrium on the fluidity of platelet membranes 123) and of the function of the meiotic spindle in spermatocytes 124). 

The self-assembly of tubulin to form microtubules was described initially in a classic polymerization model of nucleated helical polymerization by Maruyama and Oosawa (4). Assembly involves two phases a nucleation phase followed by an elongation phase. With purified systems in vitro, nucleation can... 

Microtubule assembly in cells differs in some ways from assembly in vitro. In cells, nucleation of microtubules requires a third type of tubulin, which is called y-tubulin, that functions in concert with other proteins in the form of a y-tubulin ring complex. In most animal cells, the y-tubuIin ring complex is located at the pericentriolar region of the microtubule organizing center (or centrosome) where it nucleates microtubule assembly at the minus ends (7). The y-tubulin does not become incorporated into the microtubule, but rather it only localizes to the minus ends. Assembly of tubulin to form microtubules during the early stages of polymerization in vitro can be considered a pseudo first-order reaction. A steady state is eventually attained in which both the soluble tubulin concentration and the microtubule polymer mass attain stable plateaus (8). The critical concentration at apparent equilibrium (actually a steady state, see below) is the concentration of soluble tubulin in apparent equilibrium with the microtubule polymers. 

The assembly of tubulin to form functional microtubules is a complex process (see References 2, 8, and 9). Two GTP binding sites are on the tubulin dimer One is on P-tubulin, which is readily exchangeable when the tubulin is in solution, and the other on a-tubulin, which is not exchangeable, located at the... 

Paclitaxel acts by enhancing microtubule assembly and stabilizing microtubules (1,2). Microtubules consist of polymers of tubulin in dynamic equilibrium with tubulin heterodimers. Their principal function is the formation of the mitotic spindle during cell division, but they are also active in many interphase functions, such as cellular motility, intracellular transport, and signal transmission. Paclitaxel inhibits the depolymerization of tubulin, and the microtubules formed in the presence of paclitaxel are extremely stable and dysfunctional. This stabilization impairs the essential assembly and disassembly required for dynamic cellular processes, and death of the cell results through disruption of the normal microtubular dynamics required for interphase processes and cell division. In tumor cells, cytotoxicity is represented by the appearance of abnormal microtubular bundles, which accumulate during G2 and mitosis, blocking the cell cycle (3). 

These are Ca -binding proteins found in the brain and the central nervous system, which belong to the same branch of the calmodulin family as the intestinal proteins, to which they are closely related. Both contain one variant and one normal " site for Ca . The SlOO protein exists in two forms, a and b. Binding of calcium results in conformational change. The SlOOb protein also binds two Zn ions, with conformational change, and with higher affinity than Ca. Zn is involved with the tubulin microtubule system in brain. SlOOb may be associated with this process. 

Figure 3 A typical microtubule assembly reaction is initiated by warming a solution of ice-cold tubulin dimers to 37°C in the presence of GTP. Tubulin dimers (adjacent white and gray circles) slowly form nucleating seeds (heptameric tubulin aggregate), which catalyze a rapid phase of microtubule elongation (growing microtubule) enroute to a steady state condition of microtubule formation and destruction. The assembly reaction is monitored by measuring the change in absorbance at 350 nm. In vitro incubation of microtubules with 2,5-HD or in vivo exposure of animals to 2,5-HD followed by tubulin purification yields pyrrolylated tubulin with altered assembly behavior. 2,5-HD-modified tubulin quickly forms numerous seeds, resulting in more rapid assembly into greater numbers of shorter microtubules compared to the control.

FIGURE 20-11 Dynamic instability model of microtubule growth and shrinkage. GTP-bound a(3-tubulin subunits (red) add preferentially to the (-t) end of a preexisting microtubule. After incorporation of a subunit, the GTP (red dot) bound to the (i-tubulin monomer is hydrolyzed to GDP Only microtubules whose (+) ends are associated with GTP-tubulin (those with a GTP cap) are stable and can serve as primers for the polymerization of additional tubulin. Microtubules with GDP-tubulin (blue) at the (+) end (those with a GDP cap) are rapidly depolymerized and may disappear within 1 minute. At high concentrations of unpolymerized GTP-tubulin, the rate of addition of tubulin is faster than the rate of hydrolysis of the GTP bound in the microtubule or the rate of dissociation of GTP-tubulin from microtubule ends thus the microtubule grows. At low concentrations of unpolymerized GTP-tubulin, the rate of addition of tubulin is decreased consequently, the rate of GTP hydrolysis exceeds the rate of addition of tubulin subunits and a GDP cap forms. Because the GDP cap is unstable, the microtubule end peels apart to release tubulin subunits. [See T Mitchison and M. Kirschner, 1984, Nature 312 237 ... 

Vinca alkaloids are extracted from the periwinkle plant. Vinca rosea, although newer ones are semi-synthetic. They bind to tubulin and prevent its polymerization into microtubules. Microtubules form the spindle during mitosis, so vinca alkaloids halt mitosis at a certain stage. 

Microtubules form some of the most prominent structures of protozoa. They are composed of copolymerized a and j tubulin subunits, and are decorated with an array... 

During mitosis, the mitotic spindle is responsible for the physical movement of chromosomes. The spindle is composed of hundreds of microtubules. During prophase, numerous microtubules form at the kinetochores and then move the chromosomes first to a central plane (metaphase) and then to the poles during anaphase. At telophase, microtubules are present at the site of cell plate formation. At interphase, microtubules are located in the cytoplasm near the cell wall. Microtubules do not move from one location to another during the cell cycle (e.g., from the cell wall at interphase to the kinetochore of the chromosome during prophase), but rather are depolymerized at one location into a common pool of protein subunits called tubulin. At their new location, the microtubules repolymerize from the tubulin pool. 

Tubulin is a protein (Mw = 110.000) present in all eucario-tic cells, where it forms microtubules. These are tubular structures of 25 nm diameter and variable length. Microtubules form together with other biopolymers e.g. actin, the cytoskeleton. 

During cell division, the duplicated chromatids that are localized in the equatorial plane have to be separated and pulled apart into the daughter cells. This process is achieved by a complex interaction of microtubules. Microtubules are elements of the cellular cytoskeleton and are pol3uners of tubulin. Tubulin dimers form protofilaments when GTP is present and several protofilaments organize themselves into microtubules. When GDP dominates, the microtubules... 

Tubulin is a dimer composed of similar but not identical polypeptides, a and )8, one of which (a) is phosphorylated. The molecular weight of tubulin is 100,000, with each of the subunits having a molecular weight of 55,000. Tubulin polymerizes, forming a polar, helical surface lattice. Microtubules are stabilized by Mg + and GTP, which also promote the polymerization. Ca " inhibits polymerization and may promote depolymerization (Weisenberg, 1972 Staprans et al., 1975). Brain tubulin is phosphorylated, possibly by cAMP-stimulated protein kinase, which is one of many microtubule-associated proteins. Colchicine blocks polymerization by binding to the growing end of a micro-... 

The influences of herbicides on cell division fall into two classes, ie, dismption of the mitotic sequence and inhibition of mitotic entry from interphase (G, S, G2). If ceU-cycle analyses indicate increases in abnormal mitotic figures, combined with decreases in one or more of the normal mitotic stages, the effect is upon mitosis. Mitotic effects usually involve the microtubules of the spindle apparatus in the form of spindle depolymerization, blocked tubulin synthesis, or inhibited microtubule polymerization (163). Alkaloids such as colchicine [64-86-8J,viahla.stiae [865-21-4] and vincristine [57-22-7] dismpt microtubule function (164). Colchicine prevents microtubule formation and promotes disassembly of those already present. Vinblastine and vincristine also bind to free tubulin molecules, precipitating crystalline tubulin in the cytoplasm. The capacities of these dmgs to interfere with mitotic spindles, blocking cell division, makes them useful in cancer treatment. 

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

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