Rings are not microtubule assembly intermediates: An analysis of the lag phase in GTP-dependent self-assembly of bovine brain tubulin

The assembly of whole microtubule protein, derived from bovine brain extracts by multiple cycles of temperature-induced assembly and disassembly, exhibits a lag phase in which there is a characteristic, transient drop in the turbidity. This drop is inconsistent with the direct participation of ring-like structures in microtubule assembly, and warming microtubule protein in the presence of GDP unmasks the kinetics of this dissociation phenomenon. These and other data suggest that rings are enthalpy-stabilized and are unlikely intermediates in the entropy-driven self-assembly of microtubules.