-tubulin Complexes that Differ in Their Ability to Nucleate Microtubules

g -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 g -tubulin small complex ( g TuSC; z 280,000 D) and Drosophila g TuRC ( z 2,200,000 D). In addition to g -tubulin, the g TuSC contains Dgrip84 and Dgrip91, two proteins homologous to the Spc97/98p protein family. The g TuSC is a structural subunit of the g TuRC, a larger complex containing about six additional polypeptides. Like the g TuRC isolated from Xenopus egg extracts (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578–583), the Drosophila g 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 z 13 radially arranged structural repeats. The g TuSC also nucleates microtubules, but much less efficiently than the g TuRC, suggesting that assembly into a larger complex enhances nucleating activity. Analysis of the nucleotide content of the g TuSC reveals that g -tubulin binds preferentially to GDP over GTP, rendering g -tubulin an unusual member of the tu-