Figure 8.

Model of the conformational changes required to bring Tub4p into a nucleation-competent orientation consistent with the interprotofilament spacing of the microtubule. (A) Two tubulin dimers (pink, α-tubulin; green, β-tubulin) making lateral contacts, and the average γ-TuSC map with the smallest observed distance between the arms are shown. The Tub4p density on the right is aligned at the base of one protofilament, but the separation of the two arms precludes the possibility of both Tub4p densities making protofilament-like contacts simultaneously. (B) A rotation of 14° about the base of the mobile arm (inset) brings the two Tub4p densities into orientations compatible with the microtubule lattice. As a result of this modeled conformation, two γ-tubulin crystal structures making microtubule-like lateral contacts fit remarkably well into the density. The crystal structures were manually fit in the density in B, and this fit was extrapolated in A.