Roles of β-tubulin residues Ala428 and ThrA429 in microtubule formation in vivo

Abstract

The C termini of Ab-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 βI and βIII tubulin. Transient transfection of HeLa cells with the mutants shows that mutants with deletions of up to 22 residues from βIII and 16 from βI can assemble normally. Interestingly, removal of the next residue (Ala428) results in a complete loss of microtubule formation without affecting dimer formation. C-terminal tail switching of human βI and βIII tubulin suggests that C-terminal tails are functionally equivalent. In short, residues outside of 1-429 of human β-tubulins make no contribution to microtubule assembly. Ala428, in the C-terminal sequence motif N-QQYQDA428, lies at the end of helix H12 of β-tubulin. We hypothesize that this residue is important for maintaining helix H12 structure. Deletion of Ala428 may lead to unwinding of helix H12, resulting in tubulin dimers incapable of assembly. Thr429 plays a more complex role. In the βI isotype of tubulin, Thr429 is not at all necessary for assembly; however, in the βIII isotype, its presence strongly favors assembly. This result is consistent with a likely more complex function of βIII as well as with the observation that evolutionary conservation is total for Ala428 and frequent for Thr429. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.