A NOVEL STRUCTURAL COMPONENT OF THE DICTYOSTELIUM CENTROSOME

The microtubule-organizing center of D. discoideum is a nucleus-associ ated body (NAB) that consists of a multilayered, box-shaped core embed ded in an amorphous corona from which the microtubules emerge. The com position of the NAB is still largely unresolved. Here we have examined a high molecular mass component of the NAB which was identified by a monoclonal antibody raised against isolated nucleus/NAB complexes. Thi s antibody recognized a 350 kDa component which is immunologically rel ated to the D. discoideum heavy chain of myosin II. The 350 kDa antige n was localized only at the NAB in interphase cells, while in mitotic cells it may also be found in the vicinity of the NAB as well as in as sociation with the mitotic spindle. Immunogold labeling experiments sh owed that the protein is part of the NAB corona. This association was not destroyed by treatment with 2 M urea or 0.6 M KCl. The 350 kDa ant igen was part of the thiabendazole-induced cytoplasmic microtubule-org anizing centers. A direct role in the polymerization of tubulin could not be determined in an in vitro microtubule nucleation assay, whereas antibody electroporation of live cells appeared to interfere with the generation of a normal microtubule system in a subset of cells. Our o bservations suggest that the 350 kDa antigen is a structural component of the NAB corona which could be involved in its stabilization.