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Abstract

The two-dimensional projected structure of tubulin hoops is reconstructed at a resolution of 1.6 nm from electron images of negatively stained specimens taken at a low electron dose. Like most tubulin aggregates, the hoops consist of protofilaments. A unique feature of hoops is the grouping of protofilaments into triplets. Each triplet is made up of one lone protofilament, separated from its neighbors by 5 nm or more, and a close pair of protofilaments, spaced less than 4 nm. The protofilaments have identical polarities. The shapes of tubulin subunits are different in each protofilament of a triplet. The lateral asymmetry of triplets is a strong indicator of polarity, while the polarity of individual subunits is weak. There is a small difference between alpha- and beta-tubulin monomers. The arrangement of the hetcrodimers is similar to that of flagellar B tubules. The shapes of subunits in some protofilaments are similar to those rported for normal or zinc-induced sheets (Crepeau et al., 1978; Baker and Amos, 1978), but others are distinctly different. The appearance of the hoop wall depends strongly on whether its outside faces toward or away from the microscope grid. This affects both the contrast between triplet protofilaments and their subunit structure. It can be explained by considering the protofilament triplets as small microtubule walls with a cylindrical curvature reminiscent of microtubules. The junction between two triplets may be analogous to other junctions between microtubule walls found in the same preparations (Mandelkow and Mandelkow, 1979). The implications of the hoop structure and protofilament tripling for tubulin assembly are discussed.