The Role of Xgrip210 in γ-Tubulin Ring Complex Assembly and Centrosome Recruitment

The γ-tubulin ring complex (γTuRC), purified from the cytoplasm of vertebrate and invertebrate cells, is a microtubule nucleator in vitro. Structural studies have shown that γTuRC is a structure shaped like a lock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the γTuRC. Consequently, the cap structure of the γTuRC is distal to the base of the microtubules, giving the end of the microtubule the shape of a pointed cap. Here, we report the cloning and characterization of a new subunit of Xenopus γTuRC, Xgrip210. We show that Xgrip210 is a conserved centrosomal protein that is essential for the formation of γTuRC. Using immunogold labeling, we found that Xgrip210 is localized to the ends of microtubules nucleated by the γTuRC and that its localization is more distal, toward the tip of the γTuRC-cap structure, than that of γ-tubulin. Immunodepletion of Xgrip210 blocks not only the assembly of the γTuRC, but also the recruitment of γ-tubulin and its interacting protein, Xgrip109, to the centrosome. These results suggest that Xgrip210 is a component of the γTuRC cap structure that is required for the assembly of the γTuRC.