Tubulin

Title: Tubulin
Additional Names: Colchicine-binding protein
Literature References: The subunit protein of microtubules, which are large protein assemblies that play an important role in eukaryotic cell form determination and dynamics. Microtubules have the general structure of long hollow cylinders within which 13 protofilaments of tubulin are arranged in a parallel manner to the cylinder axis. The axial arrangement of the protofilaments with respect to each other results in the appearance of a helical structure, and the in vitro microtubule assembly process generally follows the laws of helical protein polymerization, cf. Thermodynamics of the Polymerization of Proteins, F. Oosawa, S. Asakura, Eds. (Academic Press, New York, 1975). Tubulin is an asymmetric dimer consisting of two nearly identical molecules, a-tubulin and b-tubulin, each having mol wts of about 55,000. The two molecules can be separated due to differences in electrophoretic mobilities. Isoln from mammalian brain using colchicine binding: R. C. Weisenberg et al., Biochemistry 7, 4466 (1968). Discovery of conditions for microtubule assembly in vitro: R. C. Weisenberg, Science 177, 1104 (1972); G. G. Borisy, J. B. Olmstead, ibid. 1196. Prepn of large quantities of brain tubulin through successive assembly-disassembly cycles: M. L. Shelanski et al., Proc. Natl. Acad. Sci. USA 70, 765 (1973). Purification of tubulin from rat pancreas: J. F. Launay et al., Biochem. Biophys. Res. Commun. 111, 253 (1983). Structure of two human a-tubulin genes: C. Wilde et al., Proc. Natl. Acad. Sci. USA 79, 96 (1982). Structure and arrangement of protofilaments in microtubules and tubulin sheets: B. F. McEwen, Diss. Abstr. B 43, 942 (1982). Series of articles on prepn, isoln, and purification of tubulin from various sources: Methods Enzymol. 85, Pt. B, 376-417 (1982). Reviews: J. A. Snyder, J. R. McIntosh, Annu. Rev. Biochem. 45, 699-720 (1976); S. N. Timasheff, L. M. Grisham, ibid. 49, 565-591 (1980); M. F. Carlier, Mol. Cell. Biochem. 47, 97-113 (1982).
Properties: Purified calf brain tubulin retains many of its in vivo biochemical characteristics, such as the ability to self-assemble into microtubules and the response of the assembly reaction to inhibitory effects of cold temperature, Ca2+, and anti-microtubule agents, e.g. vinblastine and colchicine, q.q.v. uv max (PG buffer): 278 nm (e 1.33 ml mg-1 cm-1).
Absorption maximum: uv max (PG buffer): 278 nm (e 1.33 ml mg-1 cm-1)
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Tubulin is one of several members of a small family of globular proteins. The tubulin superfamily includes five distinct families, the alpha-, beta-, gamma-, delta-, and epsilon-tubulins and a sixth family (zeta-tubulin) which is present only in kinetoplastid protozoa.[1] The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of α- and β-tubulin. These subunits are slightly acidic with an isoelectric point between 5.2 and 5.8.[2]

Tubulin was long thought to be specific to eukaryotes. Recently, however, the prokaryotic cell division protein FtsZ was shown to be related to tubulin.[3]

To form microtubules, the dimers of α- and β-tubulin bind to GTP and assemble onto the (+) ends of microtubules while in the GTP-bound state.[4] The β-tubulin subunit is exposed on the plus end of the microtubule while the α-tubulin subunit is exposed on the minus end. After the dimer is incorporated into the microtubule, the molecule of GTP bound to the β-tubulin subunit eventually hydrolyzes into GDP through inter-dimer contacts along the microtubule protofilament.[5] Whether the β-tubulin member of the tubulin dimer is bound to GTP or GDP influences the stability of the dimer in the microtubule. Dimers bound to GTP tend to assemble into microtubules, while dimers bound to GDP tend to fall apart; thus, this GTP cycle is essential for the dynamic instability of the microtubule.