Thermodynamics of tubulin polymerization into zinc sheets: assembly is not regulated by GTP hydrolysis

The thermodynamics of tubulin assembly into Zn sheets have been studied, with special emphasis on the role of bound nucleotide and of GTP hydrolysis in polymerization. In contrast to microtubules, Zn sheets could be assembled from GDP-tubulin as well as from GTP-tubulin. Accordingly, no appreciable destabilization of the Zn sheets was observed following GTP hydrolysis and P(i) release, indicating that the binding of Zn2+ to tubulin has abolished the regulatory switch role played by GTP hydrolysis in tubulin assembly. As a consequence, the critical concentration for assembly of Zn sheets did not increase with tubulin concentration, a feature characteristic of microtubule assembly. Zn sheets do not bind P(i) analogs, indicating that the gamma-phosphate binding locus of the E-site of tubulin is occluded following GTP hydrolysis in these GDP-tubulin polymers. Nonlinear van't Hoff plots were obtained for assembly of Zn sheets in the presence of either GTP or GDP, consistent with a change in heat capacity. Enthalpy, entropy, and heat capacity changes had values similar to those reported for assembly of microtubules or polymerization of tubulin-colchicine, indicating that hydrophobic tubulin-tubulin interactions are of comparable size in these different polymers.