The centrosome is not only important for the regulation of cell division, but also forms the main microtubule-organising centre in the cell. The γ-tubulin ring complex (γ-TuRC) is a key component of the centrosome and is required for the nucleation of microtubules. The γ-TuRC is an assembly of several γ-tubulin small complexes (γ-TuSCs) and several other proteins, including GCP4, GCP5, GCP6 and NEDD1, whose functions have not been identified so far. Here, Ingrid Hoffmann and co-workers (p. ) describe a role for GCP6 in γ-TuRC assembly, mitotic spindle formation and centriole duplication downstream of the serine/threonine kinase PLK4. Cells depleted of GCP6 by siRNA form monopolar mitotic spindles and are unable to recruit γ-tubulin to the centrosome. Immunoelectron microscopy reveals that GCP6 localises to both the PCM at the proximal end of the centriole and the distal part of the centriolar lumen, thereby mirroring the localisation pattern of γ-tubulin. Furthermore, GCP6 has an important role in centriole duplication, as depletion of the protein results in a decrease in centriole number. This role depends on PLK4-mediated serine phosphorylation of GCP6. Disruption of the phosphorylation event does not abolish recruitment of GCP6 to the γ-TuRC. However, it does result in defects in centriole duplication, which suggests that GCP is an essential component of PLK4-mediated centriole biogenesis.
