The role of Xgrip210 in gamma-tubulin ring complex assembly and centrosomerecruitment

The gamma -tubulin ring complex (gamma TuRC), purified from the cytoplasm o f vertebrate and invertebrate cells, is a microtubule nucleator in vitro. S tructural studies have shown that gamma TuRC is a structure shaped like a l ock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the gamma TuRC. Consequently, the cap structure of th e gamma TuRC is distal to the base of the microtubules, giving the end of t he 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 th at its localization is more distal, toward the tip of the gamma TuRC-cap st ructure, than that of gamma -tubulin. Immunodepletion of Xgrip210 blocks no t only the assembly of the gamma TuRC, but also the recruitment of gamma -t ubulin and its interacting protein, Xgrip109, to the centrosome. These resu lts suggest that Xgrip210 is a component of the gamma TuRC cap structure th at is required for the assembly of the gamma TuRC.