Immunostructural evidence for the template mechanism of microtubule nucleation

Two opposing models have been proposed to explain how the gamma-tubulin ring complex (gamma TuRC) induces microtubule nucleation. In the 'protofilament' model, the gamma TuRC induces nucleation as a partially or completely straightened protofilament that is incorporated longitudinally into the wall of the nascent microtubule, whereas the 'template' model proposes that the gamma TuRC acts as a helical template that constitutes the base of the newly-formed polymer. Here we appraise these two models, using high-resolution structural and immunolocalization methods. We show that components of the gamma TuRC localize to a narrow zone at the extreme minus end of the microtubule and that these ends terminate in a pointed cap. Together, these results strongly favour the template model of microtubule nucleation.