Structure of the microtubule anchoring factor NEDD1 bound to the γ-tubulin ring complex.

The γ-tubulin ring complex (γ-TuRC) is an essential multiprotein assembly, in which γ-tubulin, GCP2-6, actin, MZT1 and MZT2 form an asymmetric cone-shaped structure that provides a template for microtubule nucleation. The γ-TuRC is recruited to microtubule organizing centers (MTOCs), such as centrosomes and pre-existing mitotic spindle microtubules, via the evolutionarily-conserved attachment factor NEDD1. NEDD1 contains an N-terminal WD40 domain that binds to microtubules, and a C-terminal domain that associates with the γ-TuRC. However, the structural basis of the NEDD1-γ-TuRC interaction is not known. Here, we report cryo-electron microscopy (cryo-EM) structures of NEDD1 bound to the human γ-TuRC in the absence or presence of the activating factor CDK5RAP2, which interacts with GCP2 to induce conformational changes in the γ-TuRC and promote its microtubule nucleating function. We found that the C-terminus of NEDD1 forms a tetrameric α-helical assembly that contacts the lumen of the γ-TuRC cone, is anchored to GCP4, 5 and 6 via protein modules consisting of MZT1 & GCP3 subcomplexes, and orients its microtubule-binding WD40 domains away from the complex. We biochemically tested our structural models by identifying NEDD1 mutants unable to pull-down γ-tubulin from cultured cells. The structure of the γ-TuRC simultaneously bound to NEDD1 and CDK5RAP2 reveals that both factors can associate with the "open" conformation of the complex. Our results show that NEDD1 does not induce conformational changes in the γ-TuRC, but suggest that anchoring of γ-TuRC-capped microtubules by NEDD1 would be structurally compatible with the significant conformational changes experienced by the γ-TuRC during microtubule nucleation.