P-Tubulin

Nogales E, Wolf S G and Downing K 1998 Struoture of a,p-tubulin dimer by eleotron orystallography A/afure 391 199-202... 

Microtubules (MT) are the largest of the cytoskeletal filaments with an outer diameter of about 25 nm, a wall thickness of about 5 nm, and a central lumen measuring about 15 nm. They consist of tubulin and associated proteins. Vertebrate brain tissue is a rich source of extractable tubulin because of the large numbers of microtubules that are present in axons and dendrites. Tubulin obtained from such a natural source is a heterodimer of 100 kD composed of a-tubulin and P-tubulin. Brain a-tubulin is a globular polypeptide that contains 451 amino acid residues, whereas P-tubulin, which is somewhat shorter, is made up of 445 amino acid residues. These two molecular species of tubulin share in common 40% of their amino acid residues. 

When tubulin heterodimers are assembled into microtubules, they form linear protofilaments with the P-tubulin subunit of one tubulin molecule linking covalently with the a-subunit of the next. Direct examination by electron microscopy of tannic acid-treated specimens has shown that micrombules in neurons and the A-microtubules of cilia and flagella have 13 protofilaments arranged side to side to form a cylinder around what appears to be an empty lumen. 

Both dynein and MAP2 interact with microtubules at the same binding sites, namely, the C termini of a- and p-tubulin. Also, MAP2 inhibits the microtubule-activated ATPase of dynein and prevents microtubule gliding on dynein-coated glass coverslips. Thus, MAP2 and other fibrous MAPs may be regulators of microtubule-based motility in vivo (Paschal et al., 1989). 

Binding to P-tubulin. The anthelmintic benzimidazoles, such as thiabendazole and albendazole, are drugs that bind selectively to nematode... 

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

Although taxanes bind to p-tubulin promoting microtubule polymerization and stabilization of the spindle complex, they serve to cause a sustained mitotic block at the metaphase/anaphase boundary. This block will occur at a lower concentration than that which is required to increase the microtubule mass (10). However, it is not completely clear how this interaction with microtubules translates into cell death. Morphologic features and the characteristic DNA fragmentation patterns seen in the setting of apoptosis have been documented in tumor cells after therapy with taxanes (10). These observations are accompanied by the phosphorylation of Bcl-2, an anti-apoptotic protein, changing the cellular balance between Bax and Bcl-2 to a status that favors apoptosis (11). 

Mutations in the amino acid sequence of P-tubulin that abolish the paclitaxel binding site (19). 

RAPD marker of Lees, 1995 in Doohan etal, 1998 P-tubulin gene... 

It binds specifically to the P-tubulin subunit of microtubule and appears to antagonise the disassembly of this cytoskeletal protein i.e. enhances... 

Fig. 1.52. Model for the control of translation by tubulin. The amount of tubuhn in animal cells is determined partially by the stabrhty of P-tubuhn mRNA, whereby tubuhn itself acts as the regulating signal. Starting from the 5 cap, various stages of the translation of P-tubuhn mRNA, represented as a chain of small circles, is illustrated in the figure. As soon as the N-terminus of the growing P-chain emerges from the ribosome, the a- and P- subunits of tubulin bind to the terminal MREI sequence, upon which an endonuclease becomes activated by a presently unknown mechanism. The degradation of the P-tubulin mRNA then proceeds.

These bind to the a- and p-tubulin heterodimers, which block the GTP binding site and prevent the addition of further heterodimers so the whole micro tubular spindle cannot form. 

In a nematode that infects sheep a single tyrosine to phenylalanine mutation at position 200 in the P-tubulin subunit confers resistance.1... 

Fig. 7-3331 1 316 are thought to be packed into an imperfect helix as indicated in Fig. 7-34. The structure can also be regarded as an array of longitudinal protofilaments. Naturally formed microtubules usually have precisely 13 protofilaments and a discontinuity in the helical stacking of subunits as shown in Fig. 7-34. When grown in a laboratory the microtubules usually have 14 protofilaments317 and rarely 10 or 16 protofilaments with regular helical packing.318 Microtubules of some moths and also of male germ cells of Drosophila have 16-protofilament microtubules without a discontinuity, an architecture that is specified by the geometry of a specific P-tubulin isoform.319...

Figure 7-33 Stereoscopic ribbon diagram of the tubulin dimer with a-tubulin with bound GTP at the top and P-tubulin with bound GDP at the bottom. The p-tubulin subunit also contains a bound molecule of taxotere (see Box 7-D) which is labeled TAX. This model is based upon electron crystallography of zinc-induced tubulin sheets at 0.37-nm resolution and is thought to approximate closely the packing of the tubulin monomers in microtubules.315 The arrow at the left points toward the plus end of the microtubule. Courtesy of Kenneth H. Downing.

Brehm, K., Kronthaler, K., Jura, H. and Frosch, M. (2000) Cloning and characterization of P-tubulin genes from Echinococcus multilocularis. Molecular and Biochemical Parasitology 1 07, 297-302. 

Robinson, M.W., Hoey, E.M., Fairweather, I., Dalton, J.P., McConigle, S. and Trudgett, A. (2001) Characterisation of a P-tubulin gene from the liver fluke, Fasciola hepatica. International Journal for Parasitology 31,1 264-1 268. 

Thus, the fluorine probe approach has proved useful for the conformational analysis of paclitaxel and taxoids in connection with the determination of possible bioactive conformations.77 The previously unrecognized conformer C might be the molecular structure first recognized by the P-tubulin binding site on microtubules. 

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