Preparation of a monoclonal antibody specific for the class IV isotype of β-tubulin. Purification and assembly of αβ(II), αβ(III), and αβ(IV) tubulin dimers from bovine brain

Tubulin, the 100-kDa subunit protein of microtubules, is a heterodimer of two 50-kDa subunits, α and β. Both α and β subunits exist as numerous isotypic forms. There are four isotypes of β-tubulin in bovine brain tubulin preparations; their designations and relative abundances in these preparations are as follows: β(I), 3%; β(II), 58%; β(III), 25%; and β(IV), 13%. We have previously reported the preparation of monoclonal antibodies specific for β(II) and β(III) (Banerjee, A., Roach, M. C., Wall, K. A., Lopata, M. A., Cleveland, D. W., and Luduena, R. F. (1988) J. Biol. Chem. 263, 3029-3034; Banerjee, A., Roach, M. C., Trcka, P., and Luduena, R. F. (1990) J. Biol. Chem. 265, 1794-1799). We here report the preparation of a monoclonal antibody specific for β(IV). By using this antibody together with those specific for β(II) and β(III), we have prepared isotypically pure tubulin dimers with the composition αβ(II), αβ(III), and αβ(IV). We have found that, in the presence of microtubule-associated proteins, all three dimers assemble into microtubules considerably faster and to a greater extent than does unfractionated tubulin. More assembly was noted with αβ(II) and αβ(III) than with αβ(IV). When assembly is measured in the presence of taxol (10 μM), little difference is seen among the isotypically purified dimers or between them and unfractionated tubulin. These results indicate that the assembly properties of a tubulin preparation are influenced by its isotypic composition and raise the possibility that the structural differences among tubulin isotypes may have functional significance.