CDK5RAP2 stimulates microtubule nucleation by the γ-tubulin ring complex

CDK5RAP2 is a human microcephaly protein that contains a gamma-tubulin complex (gamma-TuC)-binding domain conserved in Drosophila melanogaster centrosomin and Schizosaccharomyces pombe Mto1p and Pcp1p, which are gamma-TuC-tethering proteins. In this study, we show that this domain within CDK5RAP2 associates with the gamma-tubulin ring complex (gamma-TuRC) to stimulate its microtubule-nucleating activity and is therefore referred to as the gamma-TuRC-mediated nucleation activator (gamma-TuNA). gamma-TuNA but not its gamma-TuC-binding-deficient mutant stimulates microtubule nucleation by purified gamma-TuRC in vitro and induces extensive, gamma-TuRC-dependent nucleation of microtubules in a microtubule regrowth assay. gamma-TuRC bound to gamma-TuNA contains NME7, FAM128A/B, and actin in addition to gamma-tubulin and GCP26. RNA interference-mediated depletion of CDK5RAP2 impairs both centrosomal and acentrosomal microtubule nucleation, although gamma-TuRC assembly is unaffected. Collectively, these results suggest that the gamma-TuNA found in CDK5RAP2 has regulatory functions in gamma-TuRC-mediated microtubule nucleation.