Cryo-EM of -tubulin isotype-containing microtubules revealed a contracted structure of 4A/2A microtubules

Microtubules are hollow alpha/beta-tubulin heterodimeric polymers that play critical roles in cells. In vertebrates, both alpha- and beta-tubulins have multiple isotypes encoded by different genes, which are intrinsic factors in regulating microtubule functions. However, the structures of microtubules composed of different tubulin isotypes, especially alpha-tubulin isotypes, remain largely unknown. Here, we purify recombinant tubulin heterodimers composed of different mouse a-tubulin isotypes, including alpha 1A, alpha 1C and alpha 4A, with the beta-tubulin isotype beta 2A. We further assemble and determine the cryo-electron microscopy (cryo-EM) structures of alpha 1A/beta 2A, alpha 1C/beta 2A, and alpha 4A/beta 2A microtubules. Our structural analysis demonstrates that alpha 4A/beta 2A microtubules exhibit longitudinal contraction between tubulin interdimers compared with alpha 1A/beta 2A and alpha 1C/beta 2A microtubules. Collectively, our findings reveal that alpha-tubulin isotype composition can tune microtubule structures, and also provide evidence for the "tubulin code" hypothesis.