Mutation of a Drosophila gamma tubulin ring complex subunit encoded by discs degenerate-4 differentially disrupts centrosomal protein localization

We have cloned the Drosophila gene discs degenerate-4 (dd4) and find that i t encodes a component of the gamma -tubulin ring complex (gamma TuRC) homol ogous to Spc98 of budding yeast. This provides the first opportunity to stu dy decreased function of a member of the gamma -tubulin ring complex, other than gamma -tubulin itself, in a metazoan cell. gamma -tubulin is no longe r at the centrosomes but is dispersed throughout dd4 cells and yet bipolar metaphase spindles do form, although these have a dramatically decreased de nsity of microtubules. Centrosomin (CNN) remains in broad discrete bodies b ut only at the focused poles of such spindles, whereas Asp ((a) under bar b normaI (s) under bar pindle (p) under bar rotein) is always present at the presumptive minus ends of microtubules, whether or not they are focused. Th is is consistent with the proposed role of Asp in coordinating the nucleati on of mitotic microtubule organizing centers. The centrosome associated pro tein CP190 is partially lost from the spindle poles in dd4 cells supporting a weak interaction with gamma -tubulin, and the displaced protein accumula tes in the vicinity of chromosomes. Electron microscopy indicates not only that the poles of dd4 cells have irregular amounts of pericentriolar materi al, but also that they can have abnormal centrioles. In six dd4 cells subje cted to serial sectioning centrioles were missing from one of the two poles . This suggests that in addition to its role in nucleating cytoplasmic and spindle microtubules, the gamma TuRC is also essential to the structure of centrioles and the separation of centrosomes.