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 parent al centrioles observed in electron microscopy and by the pericentriolar mat erial immunostained with several antibodies directed against various centro somal proteins (gamma-tubulin, pericentrin, centrin and centractin). Partia l destabilization of the microtubule cytoskeleton by microtubule-disassembl ing substances induced a splitting and a slow migration of the two diplosom e units to opposite nuclear sides during most of the interphase in several mammalian cell lines. These units relocated close together following drug r emoval, while microtubule stabilization by nM taxol concentrations inhibite d this process. Cytochalasin slowed down diplosome splitting but did not af fect its relocation after colcemid washing. These results account for the a pparently opposite effects induced by microtubule poisons on centriole sepa ration. Moreover, they provide new information concerning the centrosome cy cle and stability. First, the centrosome is formed by two units, distinguis hed only by the number of attached stable microtubules, but not by pericent rin, gamma-tubulin, centrin and centractin and their potency to nucleate mi crotubules. Second, the centrosomal units are independent during most of th e interphase. Third, according to the cell type, these centrosomal units ar e localized in close proximity because they are either linked or maintained close together by the normal dynamics of the microtubule cytoskeleton. Fin ally, the relocalization of the centrosomal units with their centrioles in cells possessing one or two centrosomes suggests that their relative positi on results from the overall tensional forces involving at least partially t he microtubule arrays nucleated by each of these entities.