Cryo-EM structures of human α1B/βI+βIVb microtubules shed light on isoform specific assembly

Microtubules are dynamic polymers assembled from {beta}-tubulin dimers. Mammals have multiple and {beta}-tubulin isoforms. Despite a high degree of conservation, microtubules assembled from different tubulin isoforms have unique dynamic properties. How isoform sequence variation affects polymerization interfaces and nucleotide dependent conformational changes in microtubules is still not well understood. Here we report 2.9[A] resolution cryo-EM structures of human 1B/{beta}I+{beta}IVb microtubules in the GDP state, and a GTP-like state, bound to GMPCPP. Our structures show that, similar to microtubules assembled from other mammalian isoforms, transition from the GTP to the GDP states in 1B/{beta}I+{beta}IVb microtubules results in strengthening of the longitudinal interface and an overall compaction of the axial dimer repeat distance in the lattice. Interestingly, we find that -tail residues link longitudinally adjacent tubulin dimers through interactions with two conserved arginine residues in {beta}-tubulin that are mutated in human disease. Comparative analysis of tubulin isoforms shows minimal isoform-specific effects at the longitudinal interface or the -tubulin lateral interface, but a high concentration of sequence variability in the second shell of residues away from the {beta}-tubulin lateral interface which can modulate polymerization interfaces and thus impact microtubule dynamics.