Roles of β-Tubulin Residues Ala428 and Thr429 in Microtubule Formation in Vivo

The C termini of beta-tubulin isotypes are regions of high sequence variability that bind to microtubule-associated proteins and motors and undergo various post-translational modifications such as polyglutamylation and polyglycylation. Crystallographic analyses have been unsuccessful in resolving tubulin C termini. Here, we used a stepwise approach to study the role of this region in microtubule assembly. We generated a series of truncation mutants of human beta I and beta III tubulin. Transient transfection of HeLa cells with the mutants shows that mutants with deletions of up to 22 residues from beta III and 16 from beta I can assemble normally. Interestingly, removal of the next residue (Ala(428)) results in a complete loss of microtubule formation without affecting dimer formation. C-terminal tail switching of human beta I and beta III tubulin suggests that C-terminal tails are functionally equivalent. In short, residues outside of 1-429 of human beta-tubulins make no contribution to microtubule assembly. Ala(428), in the C-terminal sequence motif N-QQYQDA(428), lies at the end of helix H12 of beta-tubulin. We hypothesize that this residue is important for maintaining helix H12 structure. Deletion of Ala(428) may lead to unwinding of helix H12, resulting in tubulin dimers incapable of assembly. Thr(429) plays a more complex role. In the beta I isotype of tubulin, Thr(429) is not at all necessary for assembly; however, in the beta III isotype, its presence strongly favors assembly. This result is consistent with a likely more complex function of beta III as well as with the observation that evolutionary conservation is total for Ala(428) and frequent for Thr(429).