GCP-WD is a γ-tubulin targeting factor required for centrosomal and chromatin-mediated microtubule nucleation

The gamma-tubulin ring complex (gamma TuRC) is a large multi-protein complex that is required for microtubule nucleation from the centrosome. Here, we show that the GCP-WD protein ( originally named NEDD1) is the orthologue of the Drosophila Dgrip71WD protein, and is a subunit of the human gamma TuRC. GCP- WD has the properties of an attachment factor for the gamma TuRC: depletion or inhibition of GCP-WD results in loss of the gamma TuRC from the centrosome, abolishing centrosomal microtubule nucleation, although the gamma TuRC is intact and able to bind to microtubules. GCP-WD depletion also blocks mitotic chromatin-mediated microtubule nucleation, resulting in failure of spindle assembly. Mitotic phosphorylation of GCP-WD is required for association of gamma-tubulin with the spindle, separately from association with the centrosome. Our results indicate that GCP-WD broadly mediates targeting of the gamma TuRC to sites of microtubule nucleation and to the mitotic spindle, which is essential for spindle formation.