A new function for the gamma-tubulin ring complex as a microtubule minus-end cap

Microtubule nucleation from centrosomes involves a lockwasher-shaped protei n complex containing gamma-tubulin, named the gamma-tubulin ring complex (g amma TuRC). Here we investigate the mechanism by which the gamma TuRC nucle ates microtubules, using a direct labelling method to visualize the behavio ur of individual gamma TuRCs. A fluorescently-labelled version of the gamma TuRC binds to the minus ends of microtubules nucleated in vitro. Both gamm a TuRC-mediated nucleation and binding of the gamma TuRC to preformed micro tubules block further minus-end growth and prevent microtubule depolymeriza tion. The gamma TuRC therefore acts as a minus-end-capping protein, as conf irmed by electron-microscopic examination of gold-labelled gamma TuRCs. The se data support a nucleation model for gamma TuRC function that involves ca pping of microtubules.