Microtubule nucleation at the centrosome is mediated by the gamma tubulin ring complex (gamma TuRC) (reviewed in Raynaud-Messina and Merdes, 2006; Wiese and Zheng, 2006). In humans, this large complex contains the tubulin superfamily member gamma-tubulin, five gamma complex proteins (GCP2-GPC6) and NEDD1/GCP-WD. A current model of the arrangement of subunits within the gamma-TuRC proposes that 6-7 TuSC subcomplexes are held together by the other Grip proteins (at an unknown stoichiometry), which together form the cap subunits.
In many animal cells, the recruitment of gamma-tubulin complexes to the centrosome rapidly increases (3–5 fold ) before mitosis to support the formation of new spindle microtubules (Khodjakov and Rieder 1999). NEDD1/GCP-WD plays an essential role in recruitment of these complexes to the centrosomes (Haren et al., 2006; Luders et al., 2006) and to the mitotic spindle (Luders et al., 2006). GCP-WD/NEDD1 associates directly with the gamma-TuRC. The carboxy-terminal half binds to the gamma-TuRC whereas the amino-terminal half, corresponding to the WD-repeat domain, is responsible for its attachment to the centrosome (Haren et al., 2006). Additional centrosomal proteins have also been implicated in the docking of gamma-TuRC to the centrosomes. CG-NAP/AKAP450 and kendrin are necessary for the initiation of microtubule nucleation and interact with GCP2/GCP3 and GCP2, respectively (Takahashi et al., 2002). Pericentrin plays an important role in microtubule organization in mitotic cells and anchors gamma- TuRC through domains that bind GCP2 and GCP3 (Zimmerman et al. 2004). Ninein localizes to the centriole via its C-terminus and interacts with gamma-tubulin-containing complexes via its N-terminus.
Nucleoside diphosphate kinase (NME7) is a poorly characterised member of the NME family and has been observed to exhibit no NADPK activity (Yoon et al. 2005, Liu et al. 2014). NME7 has recently been found to be a component of the γ-tubulin ring complex (γTuRC) where it regulates the microtubule-nucleating activity (the event that initiates de novo formation of microtubules) of the γTuRC. NME7 contains two putative kinase domains, A and B; domain A is involved in autophosphorylation whereas domain B is inactive. NME7 interacts with the γTuRC through both domains, with Arg-322 in domain B being critical for binding. In association with the γTuRC, NME7 localizes to centrosomes throughout the cell cycle and to mitotic spindles during mitosis (Liu et al. 2014).