Characterization of 20S component in tubulin from mammalian brain

Tubulin from porcine brain, purified by at least two cycles of assembly and disassembly, was characerized at different pH values by sedimentation velocity analysis and turbidimetric measurements. At pH 6.4 the depolymerized material was composed of two major species sedimenting with so20,w values of 6 and 36 and a minor one of 20S. By raising the pH, the amount of the 20S component increased and that of the 36S decreased, whereas that of the 6S component was unaltered. At pH 7.6 the mixture contained 20S and 6S components but hardly any 36S. The 20S species can be separated from the 6S ones by gel filtration on agarose A-15m at pH 7.6. On electron microscopic examination this preparation contains far fewer double rings compared to the material at pH 6.4, but single rings could often be seen. Sodium dodecyl sulfate gel electrophoresis of the 20S and 36S components showed that they consist almost entirely of tubulin and some higher and lower molecular weight fractions. Turbidity measurements showed that the minimal protein concentration necessary for polymerization increases with increasing pH. The turbidity plateau reached at a given pH can be raised by decreasing the pH. From these results it is suggested that the 20S component is an intermediate of the 36S species. The results further indicate the existence of a pH-dependent equilibrium between the 20S species and the 36Soligomers.