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

Microtubule nucleation from centrosomes involves a lockwasher-shaped protein complex containing γ-tubulin, named the γ-tubulin ring complex (γTuRC). Here we investigate the mechanism by which the γTuRC nucleates microtubules, using a direct labelling method to visualize the behaviour of individual γTuRCs. A fluorescently-labelled version of the γTuRC binds to the minus ends of microtubules nucleated in vitro. Both γTuRC-mediated nucleation and binding of the γTuRC to preformed microtubules block further minus-end growth and prevent microtubule depolymerization. The γTuRC therefore acts as a minus-end-capping protein, as confirmed by electron-microscopic examination of gold-labelled γTuRCs. These data support a nucleation model for γTuRC function that involves capping of microtubules.

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