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