G protein α subunits activate tubulin GTPase and modulate microtubule polymerization dynamics

G proteins serve many functions involving the transfer of signals from cell. surface receptors to intracellular effector molecules. Considerable evidence suggests that there is an interaction between G proteins and the cytoskeleton. In this report, G protein alpha subunits G(i1)alpha, G(B)alpha, and G(o)alpha are shown to activate the GTPase activity of tubulin, inhibit microtubule assembly, and accelerate microtubule dynamics. G(i)alpha inhibited polymerization of tubulin-GTP into microtubules by 80-90% in the absence of exogenous GTP, Addition of exogenous GTP, but not guanylylimidodiphosphate, which is resistant to hydrolysis, overcame the inhibition. Analysis of the dynamics of individual microtubules by video microscopy demonstrated that G(i1)alpha increases the catastrophe frequency, the frequency of transition from growth to shortening. Thus, G alpha may play a role in modulating microtubule dynamic instability, providing a mechanism for the modification of the cytoskeleton by extracellular signals.