GTPase activity at ends of microtubules

Abstract

MICROTUBULES are involved in many important biological processes, including cell motility, cell division, morphogenesis and axonal transport, and it is of fundamental interest to understand the mechanism and regulation of tubulin polymerisation in order to clarify their function. Previously we presented evidence that brain tubulin, devoid of microtubule associated proteins (MAPs) after fractionation by phosphocellulose chromatography, exhibits a characteristic GTPase activity which is polymerisation-dependent and can be attributed to tubulin itself1. A detailed analysis of the steady-state GTPase activity led us to propose a model, according to which the rate of the polymerisation-dependent GTP hydrolysis should be proportional to the product of the concentration of microtubule ends and of tubulin-GTP. At the polymerisation equilibrium, the concentration of tubulin dimers is just equal to the ‘critical concentration’2, which is the minimum concentration above which tubulin polymers exist. Therefore, at the polymerisation equilibrium, the rate of GTP hydrolysis should be directly proportional to the concentration of microtubule ends. We now present evidence consistent with this prediction, from experiments in which mictrotubules were fragmented by sonication to increase the number of microtubule ends and the resulting change in the rate of GTP hydrolysis was measured.