Human β-Tubulin Isotypes Can Regulate Microtubule Protofilament Number and Stability

Cell biological studies have shown that protofilament number, a fundamental feature of microtubules, can correlate with the expression of different tubulin isotypes. However, it is not known if tubulin isotypes directly control this basic microtubule property. Here, we report high-resolution cryo-EM reconstructions (3.5-3.65 angstrom) of purified human alpha 1B/beta 3 and alpha 1B/beta 2B microtubules and find that the beta-tubulin isotype can determine protofilament number. Comparisons of atomic models of 13- and 14-protofilament microtubules reveal how tubulin subunit plasticity, manifested in accordion-like distributed structural changes, can accommodate distinct lattice organizations. Furthermore, compared to alpha 1B/beta 3 microtubules, alpha 1B/beta 2B filaments are more stable to passive disassembly and against depolymerization by MCAK or chTOG, microtubule-associated proteins with distinct mechanisms of action. Mixing tubulin isotypes in different proportions results in microtubules with protofilament numbers and stabilities intermediate to those of isotypically pure filaments. Together, our findings indicate that microtubule protofilament number and stability can be controlled through beta-tubulin isotype composition.