Polymerising ability of C₆ glial cell microtubule protein decays much faster than its colchicine-binding activity

Abstract

BECAUSE of its high degree of specificity¹, the binding of colchicine to tubulin has become a powerful tool for the quantification and subcellular localisation of the protein. The mechanism of the binding reaction is not yet completely understood, but several of its characteristics are well documented. One of these is the decay of binding capacity with time, which follows the kinetics of a firstorder reaction^2,3 and is accentuated at elevated temperatures⁴. Although correlation between this rate of decay and the loss of other characteristic functions of tubulin, such as its ability to polymerise into microtubules, has not been directly established by experiment, carefully con trolled measurements of the decay of colchicine binding seem to be useful for estimating the stability of the tubulin molecules per se. In contrast, the decay rate of the polymerisability of tubulin preparations should depend not only on the stability of the tubulin molecules themselves but also on that of other factors that may be essential for polymerisation^5–7. As part of our recently initiated^8,9 attempt to study the mechanisms regulating microtubule assembly in cultured cells, we report here that tubulin preparations from rat glial C₆ cells exhibit a decay rate of colchicine-binding activity that is quite different from the decay rate of polymerisability.