Differences in α and β polypeptide chains of tubulin resolved by electron microscopy with image reconstruction

Publisher Website
Google Scholar
Abstract
EM techniques were used to reveal 2 classes of subunits of [pig brain] tubulin in ordered arrays. Presumably the 2 classes correspond to the .alpha. and .beta. polypeptide chains of tubulin that have been distinguished by chemical criteria. The 2 types of subunits alternate along individual protofilaments in microtubules, microtubule-precursor sheets and extended Zn tubulin sheets. The resolution of the 2 types of polypeptide chains is achieved by improved negative staining methods which produce micrographs with layer lines at 28 .ANG.-1 and 84 .ANG.-1 in optical or computed transforms, in addition to the layer lines at 21 .ANG.-1 and 42 .ANG.-1 described previously [Crepeau et al (1977)]. In microtubules or microtubule-precursor sheets, adjacent protofilaments are staggered by about 10 .ANG., but are parallel, in the sense that the .alpha.-.beta. vector points in the same direction for all of the protofilaments of the microtubule. However, for the sheets assembled in the presence of Zn, adjacent protofilaments are staggered by about 21 .ANG. and oriented in an antiparallel arrangement with alternate protofilaments related by a 2-fold screw axis. The antiparallel alignment of the protofilaments in the Zn-tubulin sheets accounts for their planarity (no tubular structures are found in the presence of moderate concentrations of Zn), since the intrinsic curvature found with parallel alignment of protofilaments in the absence of Zn would be cancelled by the antiparallel arrangement.