The in vitro polymerization of tubulin from beef brain.

The conditions for the in vitro polymerization of beef brain tubulin have been examined using turbidity and electron microscopic examination as methods of following the reaction. The optimal conditions are 37°, pH 6.0--6.4, and an ionic strength of approximately 0.08 to 0.1 μ. The ionic strength is particularly critical, at 0.25 μ the polymerization does not occur and increasing the ionic strength of a solution contaning microtubules to this value causes the tubules to depolymerize. GTP and Mg2+ are required. After an initial lag period the rate of polymerization appears to follow first-order kinetics. Although colchicine inhibits the polymerization reaction it does not cause the tubules formed in vitro to depolymerize. Binding studies show that colchicine binds to free tubulin better than to tubules. Calcium also inhibits polymerization and causes the depolymerization of tubules.