The thermodynamics of vinca alkaloid-induced tubulin spirals formation

Vinca alkaloids are antimitotic, anticancer agents that induce tubulin to form spiral polymers at physiological protein concentrations. We used sedimentation velocity to investigate the effects of six vinca alkaloids on tubulin spiraling. Fitting to a Wyman linkage model reveals a drug dependent change of over two orders of magnitude in spiraling potential, K1K2. Thermodynamic analysis of LnK1K2 data demonstrates large and positive ΔS values, indicating that tubulin spiral formation is entropically-driven. From the curvature in van't Hoff plots of vinblastine data, we estimate ΔCp for GTP and GDP conditions to be − 439 and − 396 cal/mol K. Partitioning of ΔS into the hydrophobic effect, ΔSHE, change in rotational/translational freedom, ΔSRT and change in protein conformation, ΔSother, demonstrates that the major driving force for tubulin spiral formation is burial of hydrophobic surfaces and that protein conformational changes do not make a significant contribution. Spiraling potential is an indicator of antimitotic activity in vivo, although turbidity studies indicate that there is no correlation between spiraling potential and microtubule inhibition in vitro. Mechanisms that explain this discrepancy are discussed.