Structure of the native gamma-Tubulin Ring Complex capping spindle microtubules

Microtubules (MTs) are fundamental to cellular architecture, function and organismal development. They are nucleated from microtubule organizing centres by the evolutionary conserved ?-tubulin ring complex (?TuRC). However, the molecular mechanism of nucleation remains elusive. Here, we used cryo-electron tomography (cryo-ET) to determine the structure of the native ?TuRC capping the minus end of a MT in the context of enriched budding yeast spindles. In our structure, ?TuRC presents a ring of ?-tubulin subunits to seed nucleation of exclusively 13-protofilament microtubules, and it adopts an active closed conformation to function as a perfect geometric template for MT nucleation. Our cryo-ET reconstruction also revealed that a novel coiled-coil protein staples the first row of ?/?-tubulin molecules to alternating positions along the ?-tubulin ring. This positioning of ?/?-tubulin onto ?TuRC suggests a role for the coiled-coil protein in augmenting ?TuRC-mediated microtubule nucleation. Based on our results we describe a molecular model for budding yeast ?TuRC activation and MT nucleation.