Structure of the γ-tubulin ring complex:: a template for microtubule nucleation

The gamma-tubulin ring complex (gamma TuRC) is a protein complex of relative molecular mass -2.2x10(6) that nucleates microtubules at the centrosome. Here we use electron-microscopic tomography and metal shadowing to examine the structure of isolated Drosophila gamma TuRCs and the ends of microtubules nucleated by gamma TuRCs and by centrosomes. We show that the gamma TuRC is a lockwasher-like structure made up of repeating subunits, topped asymmetrically with a cap. A similar capped ring is also visible at one end of microtubules grown from isolated gamma TuRCs and from centrosomes. Antibodies against gamma-tubulin label microtubule ends, but not walls, in centrosomes. These data are consistent with a template-mediated mechanism for microtubule nucleation by the gamma TuRC.