The binding of vinca domain agents to tubulin: structural and biochemical studies.

Vinca domain ligands are small molecules that interfere with the binding of vinblastine to tubulin and inhibit microtubule assembly. Many such compounds cause isodesmic association which results in difficulties in biochemical or structural studies of their interaction with tubulin. The complex of two tubulins with the stathmin-like domain of the RB3 protein (T(2)R) is a protofilament-like short assembly that does not assemble further. This has allowed structural studies of the binding of several vinca domain ligands by X-ray crystallography as crystals of the corresponding complexes diffract to near atomic resolution. This proved that their sites are located at the interface of two tubulin molecules arranged as in a curved protofilament. These sites overlap with that of vinblastine. Structural data are generally consistent with the results of available structure-function studies, though subtle differences exist. Binding in solution to the vinca domain displayed in T(2)R is conveniently studied by fluorescence spectroscopy or by monitoring inhibition of the T(2)R GTPase activity. In addition, inhibition of nucleotide exchange allows characterization of the binding to the vinca domain moiety displayed by the beta-subunit of an isolated tubulin molecule. T(2)R is therefore a useful tool to characterize and dissect the binding of vinca domain ligands to tubulin. In addition, these studies have provided new information on the interaction of tubulin with guanine nucleotides, namely on the mechanisms of nucleotide exchange and hydrolysis.