Dictyostelium γ-tubulin: molecular characterization and ultrastructural localization

The centrosome of Dictyostelium discoideum is a nucleusassociated body consisting of an electron-dense, threelayered core surrounded by an amorphous matrix, the corona. To elucidate the molecular and supramolecular architecture of this unique microtubule-organizing center, we have isolated and sequenced the gene encoding γ-tubulin and have studied its localization in the Dictyostelium centrosome using immunofluorescence and postembedding immunoelectron microscopy. D. discoideum possesses a single copy of a γ-tubulin gene that is related to, but more divergent from, other γ-tubulins. The low-abundance gene product is localized to the centrosome in an intriguing pattern: it is highly concentrated in the corona in regularly spaced clusters whose distribution correlates with the patterning of dense nodules that are a prominent feature of the corona. These observations lend support to the notion that the corona is the functional homologue of the pericentriolar matrix of ‘higher’ eukaryotic centrosomes, and that nodules are the functional equivalent of γ-tubulin ring complexes that serve as nucleation sites for microtubules in animal centrosomes.