Kinetics of colchicine binding to purified beta-tubulin isotypes from bovine brain.
Tubulin, the constituent protein of microtubules, is an alpha beta heterodimer; both alpha and beta exist in several isotypic forms whose functional significance is not precisely known. The antimitotic alkaloid colchicine binds to mammalian brain tubulin in a biphasic manner under pseudo-first-order conditions in the presence of a large excess of colchicine (Garland, D. L. (1978) Biochemistry 17, 4266-4272). We have studied the kinetics of colchicine binding to purified beta-tubulin isotypes and find that each of the purified beta-tubulin isotypes binds colchicine in a monophasic manner. The apparent on-rate constants for the binding of colchicine to alpha beta II-, alpha beta III-, and alpha beta IV-tubulin dimers are respectively 132 +/- 5, 30 +/- 2, and 236 +/- 7 M-1 s-1. When the isotypes are mixed, the kinetics become biphasic. Scatchard analysis revealed that the isotypes differ significantly in their affinity constants (Ka) for binding colchicine. The affinity constants are 0.24 x 10(6), 0.12 x 10(6), and 3.31 x 10(6) M-1, respectively, for alpha beta II-, alpha beta III-, and alpha beta IV-tubulin dimers. Our results are in agreement with the hypothesis that the beta-subunit of tubulin plays a major role in the interaction of colchicine with tubulin. Our binding data raise the possibility that the tubulin isotypes might play important regulatory roles by interacting differently with other non-tubulin proteins in vivo, which in turn, may regulate microtubule-based functions in living cells.
Banerjee
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A
Department of Biochemistry, University of Texas Health Science Center, San Antonio 78284-7760.
Luduena
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RF
D000818:Animals; D000911:Antibodies, Monoclonal; D001921:Brain / Q000378:metabolism*; D002417:Cattle; D002846:Chromatography, Affinity; D003078:Colchicine / Q000378:metabolism*; D007700:Kinetics; D014404:Tubulin / Q000276:immunology / Q000378:metabolism*
D000911:Antibodies, Monoclonal; D014404:Tubulin; D003078:Colchicine