XGRIP109 - A GAMMA-TUBULIN-ASSOCIATED PROTEIN WITH AN ESSENTIAL ROLE IN GAMMA-TUBULIN RING COMPLEX (GAMMA-TURC) ASSEMBLY AND CENTROSOME FUNCTION

Previous studies indicate that gamma tubulin ring complex (gamma TuRC) can nucleate microtubule assembly and may be important in centrosome formation. gamma TuRC contains approximately eight subunits, which we refer to as Xenopus gamma ring proteins (Xgrips), in addition to gamma tubulin. We found that one gamma TuRC subunit, Xgrip109, is a highly conserved protein, with homologues present in yeast, rice, flies, zebr afish, mice, and humans. The yeast Xgrip109 homologue, Spc98, is a spi ndle-pole body component that interacts with gamma tubulin. In vertebr ates, Xgrip109 identifies two families of related proteins. Xgrip109 a nd Spc98 have more homology to one family than the other. We show that Xgrip109 is a centrosomal protein that directly interacts with gamma tubulin. We have developed a complementation assay for centrosome form ation using demembranated Xenopus sperm and Xenopus egg extract. Using this assay, we show that Xgrip109 is necessary for the reassembly of salt-disrupted gamma TuRC and for the recruitment of gamma tubulin to the centrosome. Xgrip109, therefore, is essential for the formation of a functional centrosome.