Characterization of two related Drosophila γ-tubulin complexes that differ in their ability to nucleate microtubdes

gamma-tubulin exists in two related complexes in Drosophila embryo extracts (Moritz, M., Y. Zheng, B.M. Alberts, and K. Oegema. 1998. J. Cell Biol. 142:1-12). Here, we report the purification and characterization of both complexes that we name gamma-tubulin small complex (gamma TuSC; similar to 280,000 D) and Drosophila gamma TuRC (similar to 2,200,000 D). In addition to gamma-tubulin, the gamma TuSC contains Dgrip84 and Dgrip91, two proteins homologous to the Spc97/98p protein family. The gamma TuSC is a structural subunit of the gamma TuRC, a larger complex containing about six additional polypeptides. Like the gamma TuRC isolated from Xenopus egg extracts (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578-583), the Drosophila gamma TuRC can nucleate microtubules in vitro and has an open ring structure with a diameter of 25 nm. Cryo-electron microscopy reveals a modular structure with similar to 13 radially arranged structural repeats. The gamma TuSC also nucleates microtubules, but much less efficiently than the gamma TuRC, suggesting that assembly into a larger complex enhances nucleating activity. Analysis of the nucleotide content of the gamma TuSC reveals that gamma-tubulin binds preferentially to GDP over GTP, rendering gamma-tubulin an unusual member of the tubulin superfamily.