Figure 8.
A model for the structural pathway of spontaneous nucleation. (A) In a solution of GTP tubulin, tubulin dimers and oligomers of variable sizes and curvatures are in rapid equilibrium. The color intensity represents the probability for each dimer and oligomer. The distance between tubulin dimers in an oligomer symbolically represents the strength of the interdimer bonds. Straight oligomers crucial for nucleation can be formed only from GTP tubulin, the T5 loop “out” conformation being favored. (B) While the reaction of tubulin binding to a two-stranded oligomer composed of straight protofilaments can be thermodynamically favorable (bottom), the reaction of tubulin binding to a two-stranded oligomer formed from curved protofilaments is not (top). Here, we assume that a tubulin dimer that binds to a multi-stranded oligomer gains energy equivalent to the energy gained by a tubulin dimer that binds to a MT protofilament (ΔGMT).