Characterization and Reconstitution of Drosophila g -Tubulin Ring Complex Subunits

The g -tubulin ring complex ( g TuRC) is important for microtubule nucleation from the centrosome. In addition to g -tubulin, the Drosophila g TuRC contains at least six subunits, three of which [ Drosophila gamma ring proteins (Dgrips) 75/d75p, 84, and 91] have been characterized previously. Dgrips84 and 91 are present in both the small g -tubulin complex ( g TuSC) and the g TuRC, while the remaining subunits are found only in the g TuRC. To study g TuRC assembly and function, we first reconstituted g TuSC using the baculovirus expression system. Using the reconstituted g TuSC, we showed for the first time that this subcomplex of the g TuRC has microtubule binding and capping activities. Next, we characterized two new g TuRC subunits, Dgrips128 and 163, and showed that they are centrosomal proteins. Sequence comparisons among all known g TuRC subunits revealed two novel sequence motifs, which we named grip motifs 1 and 2. We found that Dgrips128 and 163 can each interact with g TuSC. However, this interaction is insufficient for g TuRC assembly.