The role of Xgrip210 in γ-tubulin ring complex assembly and centrosome recruitment

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