Cryo-EM of α-tubulin isotype containing microtubules revealed a contracted structure of α4A/β2A microtubules

Microtubules are hollow α/β-tubulin heterodimeric polymers playing critical roles in cells. In vertebrates, both α- and β-tubulin have multiple isotypes encoded by different genes, which are intrinsic factors in regulating microtubule functions. However, structures of microtubules composed of different tubulin isotypes, especially α-tubulin isotypes, remain largely unknown. Here, we purified recombinant tubulin heterodimers composed of different mouse α-tubulin isotypes, including α1A, α1C and α4A, with β-tubulin isotype β2A. We further assembled and determined the cryo-electron microscopy (cryo-EM) structures of α1A/β2A, α1C/β2A, and α4A/β2A microtubules. Our structural analysis demonstrated that α4A/β2A microtubules exhibit a longitudinal contraction between tubulin interdimers compared with α1A/β2A and α1C/β2A microtubules. Collectively, our findings reveal that α-tubulin isotype composition could tune microtubule structures, and also provide evidence for the “tubulin code” hypothesis.