A and p Tubulin

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). 

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. 

Damage to the mitotic spindle (B). The contractile proteins of the spindle apparatus must draw apart the replicated chromosomes before the cell can divide. This process is prevented by the so-called spindle poisons (see also colchicine, p. 326) that arrest mitosis at metaphase by disrupting the assembly into spindle threads of microtubuli. These consist of the proteins a-and p-tubulin. Surplus tubuli are broken down, enabling the tubulin subunits to be recycled. 

Microtubules are built from two kinds of homologous 50-kd subunits, a- and P-tubulin, which assemble in an helical array of alternating tubulin types to form the wall of a hollow cylinder (Figure 34.21). Alternatively, a microtubule can be regarded as 13 protofilaments that run parallel to its long axis. The outer diameter of a microtubule is 30 nm, much larger than that of actin (5 nm). Like actin, microtubules are polar structures. One end, termed the minus end, is anchored near the center of a cell, whereas the plus end extends toward the cell surface. 

Each type of cytoskeletal filament is a polymer of protein subunits (Table 5-4). Monomeric actin subunits assemble into mlcrofllaments dimeric subunits composed of a- and p-tubulin polymerize into microtubules. Unlike mlcrofllaments and microtubules, which are assembled from one or two proteins, intermediate filaments are assembled from a large diverse family of proteins. The most common intermediate filaments, found in the nucleus, are composed of lamins. Intermediate filaments constructed from other proteins are expressed preferentially in certain tissues for example, keratin-containing filaments in epithelial cells, desmin-contalnlng filaments In muscle cells, and vimentin containing filaments In mesenchymal cells. 

Each tubulin subunit binds two molecules of GTP. One GTP-bIndIng site, located in a-tubulin, binds GTP irreversibly and does not hydrolyze it. The second site, located on p-tubulln, binds GTP reversibly and hydrolyzes it to GDP. Thus, tubulin Is a GTPase like bacterial FtsZ protein. In the atomic structure of the tubulin subunit, the GTP bound to a-tubulln is trapped at the interface between the a- and p-tubulin monomers and is thus nonexchangeable. The second GTP lies at the surface of the p-tubulin monomer this GTP is freely exchangeable with GDP (Figure 20-3a). As discussed later, the... 

Colchicine itself and colchicine analogues predominantly bind to a high affinity site, the so-called colchicine binding site located at the intradimer interface between a- and P-tubulin, facing the lumen of the microtubule. Colchicine (1) Fig. (1) inhibits microtubule formation and disrupts microtubules as a tubulin destabilising agent. 

It is well known that various chemical agents can interfere with tubulin (which consists of a- and P-tubulins), and prevent it from combining effectively with microtubular-associated proteins to afford microtubules. The microtubules themselves are important sub-cellular structures involved in construction of the cytoskeleton, and in cell devision and cell movement. Representative chemicals which bind to tubulin are shown in Table 1. Most of these compounds are potent inhibitors of mitosis, and for this reason some of them appear to be promising chemotherapeutic agents in the treatment of certain cancers (e.g. breast and ovarian carcinomas). 

Although dicistronic and polycistronic mRNAs can be identified in steady-state RNA, two lines of evidence suggest that these molecules probably are not authentic precursors to mature mRNAs. First, in Trypanosoma brucei cells, trans-splicing at three different splice acceptor sites (the a. and P tubulin, and the actin RNA coding regions). 

MTs are made of a head-to-tail alignment of a- and P-tubulin dimers, each of which provides a single binding site for kinesins. The 8nm step size of the kinesin motor matches with the periodicity of the tubulin dimers along the protofllament (see Figure I.IB). 

Tubulin assembly into MTs is similar to actin assembly but with three major differences. The MT subunit is a heterodimer of two closely related polypeptides a- and p-tubulin the tubulin dimer hydrolyzes GTP rather than ATP following... 

Calligaris, D., Villard, C., Terras, L, Braguer, D., Verdier-Pinard, P., and Lafitte, D. (2010) MALDI in-source decay of high mass protein isoforms applications to a- and P-tubulin variants. Anal. Chem., 82, 6176-6184. 

Protofiia merits The 13 linear columns of tubulin units that can be visualized in the structure of a microtubule they result because each turn of the microtubule helix contains exactly 13 tubulin units. Each protofilament consists of alternating a and P tubulin subunits. 

Figure 3 Double labelling of a- and p-tubulin components of microtubules labelled with 5 nm and 10 nm gold conjugates in a section of a PLT embedded cell. Whilst lines of >ld marker can be seen occasionally following the profile of a microtubule (arrow) this figure illustrates the point that although the microtubules can be seen in the section, only those antigens accessible at the section surface can be labelled. Magnification x 74000.

Microtubule consists of a- and p-tubulin proteins. Tubulin polymerizes end to end with the a-subunit of one tubulin dimer joining with the p-subunit of the next. The protofilament bundles are parallel to one another. Thus, in a microtubule, there is one end with only p-subunits exposed called (+) end, while the other end has only a-subunits exposed called (—) end. That is why a microtubule shows polarity. The ( ) end is capped so elongation of the microtubule occurs from the (+) direction. 

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