Induction of Polymerization of Purified Tubulin by Sulfonate Buffers

Interactions of both purified tubulin and microtubule protein (tubulin plus associated proteins) with two commonly used sulfonate buffers were examined. 1,4-Piperazineethanesulfonate (Pipes) and 4-morpholineethanesulfonate (Mes) at high concentrations induce the polymerization of purified tubulin in reactions requiring only buffer, tubulin and GTP. While both reactions were temperature-dependent, cold-reversible and inhibited by GDP, colchicine or Ca2+, there were significant differences between them. Substantially lower tubulin and buffer concentrations were required for Pipes-induced polymerization; and turbidity was much more intense in the Pipes-induced than in the Mes-induced reaction at the same protein concentration. Electron microscopy demonstrated that for the most part typical smooth-walled microtubules were formed in Mes, while aberrant forms were the predominant structures formed in Pipes. When the polymerization of microtubule protein was examined as a function of buffer concentration, biphasic patterns were observed with both Pipes and Mes: polymerization occurred at both low and high, but not intermediate, buffer concentrations. The turbidity observed at high concentrations of Pipes greatly exceeded that at low concentrations. With Mes, equivalent turbidity developed at both high and low buffer concentrations. Although associated proteins copolymerized with tubulin at low buffer concentrations, they were excluded from the polymerized material at high buffer concentrations. Pipes and Mes were compared to sodium phosphate, Tris/HCl and imidazole/HCl buffers at 0.1 M in several polymerization systems using both purified tubulin and microtubule protein. The sulfonate buffers were invariably associated with more vigorous reactions than the other buffers.