Electron microscopic techniques have been used to reveal two classes of subunits of tubulin in ordered arrays. Presumably the two classes correspond to the α and β polypeptide chains of tubulin that have been distinguished by chemical criteria. The two types of subunits alternate along individual protofilaments in microtubules, microtubule-precursor sheets, and extended zinc-tubulin sheets. The resolution of the two types of polypeptide chains is achieved by improved negative staining methods which produce micrographs with layer lines at 28 Å -1 and 84 Å -1 in optical or computed transforms, in addition to the layer lines at 21 Å -1 and 42 Å -1 described previously [Crepeau, R. H., McEwen, B., Dykes, G. & Edelstein, S. J. (1977) J Mol. Biol. 116, 301-315]. In microtubules or microtubule-precursor sheets, adjacent protofilaments are staggered by about 10 Å, but parallel, in the sense that the α-β vector points in the same direction for all of the protofilaments of the microtubule. However, for the sheets assembled in the presence of zinc, adjacent protofilaments are staggered by about 21 Å and oriented in an antiparallel arrangement with alternate protofilaments related by a 2-fold screw axis. The antiparallel alignment of the protofilaments in the zinc-tubulin sheets accounts for their planarity (no tubular structures are found in the presence of moderate concentrations of zinc), since the intrinsic curvature found with parallel alignment of protofilaments in the absence of zinc would be cancelled by the antiparallel arrangement.

Crepeau, R. H., McEwen, B., & Edelstein, S. J. (1978). Differences in α and β polypeptide chains of tubulin resolved by electron microscopy with image reconstruction. Proceedings of the National Academy of Sciences, 75(10), 5006–5010.