The mammalian interphase centrosome: two independent units maintained together by the dynamics of the microtubule cytoskeleton.

In mammalian cells the centrosome or diplosome is defined by the two parental centrioles observed in electron microscopy and by the pericentriolar material immunostained with several antibodies directed against various centrosomal proteins (gamma-tubulin, pericentrin, centrin and centractin). Partial destabilization of the microtubule cytoskeleton by microtubule-disassembling substances induced a splitting and a slow migration of the two diplosome units to opposite nuclear sides during most of the interphase in several mammalian cell lines. These units relocated close together following drug removal, while microtubule stabilization by nM taxol concentrations inhibited this process. Cytochalasin slowed down diplosome splitting but did not affect its relocation after colcemid washing. These results account for the apparently opposite effects induced by microtubule poisons on centriole separation. Moreover, they provide new information concerning the centrosome cycle and stability. First, the centrosome is formed by two units, distinguished only by the number of attached stable microtubules, but not by pericentrin, gamma-tubulin, centrin and centractin and their potency to nucleate microtubules. Second, the centrosomal units are independent during most of the interphase. Third, according to the cell type, these centrosomal units are localized in close proximity because they are either linked or maintained close together by the normal dynamics of the microtubule cytoskeleton. Finally, the relocalization of the centrosomal units with their centrioles in cells possessing one or two centrosomes suggests that their relative position results from the overall tensional forces involving at least partially the microtubule arrays nucleated by each of these entities.