Tubulin dimers

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

FIGURE 6.47 The structure of a typical microtubule, showing the arrangement of the a- and /3-monomers of the tubulin dimer. 

Because all tubulin dimers in a microtubule are oriented similarly, microtubules are polar structures. The end of the microtubule at which growth occurs is the plus end, and the other is the minus end. Microtubules in vitro carry out a GTP-dependent process called treadmiUing, in which tubulin dimers are added to the plus end at about the same rate at which dimers are removed from the minus end (Figure 17.3). 

Colchicine has a long history of successful use and was the treatment of choice for many years. It is used infrequently today because of its low therapeutic index. Colchicine is thought to exert its anti-inflammatory effects by interfering with the function of mitotic spindles in neutrophils by binding of tubulin dimers this inhibits phagocytic activity. 

Recently, another class of MAPs has been identified that lead to depolymerization of MT, stathmin and katanin [11], Stathmin destabilizes MT either by reducing available tubulin dimer or by altering the frequency of catastrophe at MT plus ends [18]. In contrast, katanin is an ATP-ase that severs MT [19]. In neurons, one important role of katanin is to release the MT from the MTOC and allow transport of the assembled MT into axons and dendrites. 

Colchicine (6) is used in the treatment of a broad range of diseases including acute gout and Mediterranean fever [28] and induces depolymerization of tubulin. This compound (6) distorts the tubulin/microtubule equilibrium by binding to the tubulin dimer and halting mitosis in the metaphase. The reason this approach is such a successful target in cancer therapy is that... 

Microtubules have a key role in mitosis and cell-proliferation. They are dynamic assemblies of heterodimers of a and f3 tubullin. In the cell-reproduction cascade tubulin polymerizes fast and subsequently depolymerizes. Tubulin dimers are unusual guanyl nucleotide binding (G) proteins, which bind GTP reversibly at a site in the (3-tubulin. GTP irreversibly hydrolyzes to GDP during polymerization. 

Cholera toxin This protein catalyzes the covalent insertion of an ADP-ribosyl moiety derived from NAD into an unidentified site of tubulin, based on SDS—gel electrophoresis and peptide fragmentation patterns (Hawkins and Browning, 1982). Because only about 0.1% of the tubulin dimers appear to become modified, the association with microtubule control remains tenuous... 

Earlier studies indicated that vinblastine binding to tubulin dimers involves interaction of two molecules of vinblastine with each dimeric pro-... 

Treatment of cells with vinblastine or vincristine can result in the formation of paracrystals, complexes containing the alkaloid molecules and tubulin dimers in a 1 1 ratio. Paracrystal formation in neuronal tissue of a freshwater snail has been proposed as a model for the neurotoxic effects of Catharanthus alkaloids and derivatives 44). Vincristine is approximately 10-fold more active than vinblastine as an inducer of paracrystal formation when snail neuronal tissue is treated with high concentrations (150 lM) of the alkaloids. 

This enzyme [EC 6.3.2.25] catalyzes the ATP-dependent religation of L-tyrosine to the C-terminal L-glutamate residue in the detyrosinated a/S-tubulin dimer, yielding ADP, orthophosphate, and the religated tubulin dimer. L-Phenylalanme and 3,4-dehydroxy-L-phenylalanine are also substrates. This is the only known C-terminal peptide bond-forming reaction involving a protein substrate. See also Arsenolysis... 

Paclitaxel (Taxol) is a highly complex, organic compound isolated from the bark of the Pacific yew tree. It binds to tubulin dimers and microtubulin filaments, promoting the assembly of filaments and preventing their depolymerization. This increase in the stability of microfilaments results in disruption of mitosis and cytotoxicity and disrupts other normal microtubular functions, such as axonal transport in nerve fibers. 

Each tubulin dimer binds one molecule of GTP strongly in the a subunit and a second molecule of GTP or GDP more loosely in the P subunit. In this respect, tubulin resembles actin, whose subunits are about the same size. However, there is little sequence similarity. Labile microtubules of cytoplasm can be formed or disassembled very rapidly. GTP is essential for the fast growth of these microtubules and is hydrolyzed to GDP in the process.320 However, nonhydro-lyzable analogs of GTP, such as the one containing the linkage P-CH2-P between the terminal and central phosphorus atoms of the GTP, also support polymerization.321 Since microtubules have a distinct polarity, the two ends have different tubulin surfaces exposed, and polymerization and depolymerization can occur at different rates at the two ends. As a consequence, microtubules often grow at one end and disassemble... 

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.

In interphase, microtubules are stabilized by several kinds of proteins that are found all along microtubules and are called MAPs. They tend to have repeating domains, which allow each MAP molecule to associate with more than one tubulin dimer. This produces a doubly effective method of controlling assembly, in that the conformations of several tubulin dimers may be individually stabilized and the stabilized subunits are also cross-linked. The binding of these structural MAPs is in turn controlled by kinases and phosphatases (Cassimeris and Spittle, 2001). During mitosis they are phosphorylated and detach from tubulin, whose assembly and disassembly comes under the control of proteins that operate more at the ends of microtubules. Differentiated cells, such as neurons, do not divide. However, as microtubules and MAPs are slowly transported along axons (Baas and Buster, 2004), the MAPs maybe phosphorylated in particular places, at times when structural plasticity is required for making synapses or other contacts. 

N-terminal domain binds only to microtubules, whereas the C-terminal domain binds to both microtubules and soluble tubulin dimers (Kim et al., 2003). The second domain might, therefore, be active only at a microtubule end. 

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