Mechanism of how augmin directly targets the γ-tubulin ring complex to microtubules

Microtubules (MTs) must be generated from precise locations to form the structural frameworks required for cell shape and function. MTs are nucleated by the γ-tubulin ring complex (γ-TuRC), but it remains unclear how γ-TuRC gets to the right location. Augmin has been suggested to be a γ-TuRC targeting factor and is required for MT nucleation from preexisting MTs. To determine augmin's architecture and function, we purified Xenopus laevis augmin from insect cells. We demonstrate that augmin is sufficient to target γ-TuRC to MTs by in vitro reconstitution. Augmin is composed of two functional parts. One module (tetramer-II) is necessary for MT binding, whereas the other (tetramer-III) interacts with γ-TuRC. Negative-stain electron microscopy reveals that both tetramers fit into the Y-shape of augmin, and MT branching assays reveal that both are necessary for MT nucleation. The finding that augmin can directly bridge MTs with γ-TuRC via these two tetramers adds to our mechanistic understanding of how MTs can be nucleated from preexisting MTs.