Loss of mitotic centrosomal microtubule initiation capacity at the metaphase-anaphase transition.

The microtubule initiation capacity of the mitotic centrosome was studied in PtK1 cells by using the highly reversible microtubule inhibitor nocodazole. Cells blocked with nocodazole at any stage prior to onset of anaphase completed mitosis by reforming the spindle following release from the drug. Cells treated with nocodazole immediately upon onset of sister chromatid separation and then released did not complete mitosis, but instead progressed directly to an interphase state. This anaphase-linked transition in response to treatment was clearly evident as a change in centrosomal microtubule initiation capacity coincident with commencement of sister chromatid separation. Cells blocked in very late metaphase and then released were found to have retained the enhanced centrosomal microtubule initiation capacity characteristic of early mitosis. Cells blocked after the beginning of anaphase and then released, however, displayed dramatically reduced centrosomal microtubule initiation capacity. Mitotic cells blocked with colcemid or nocodazole and lysed into microtubule protein containing buffers also exhibited stage-specific differences in their ability to initiate microtubule in the centrosomal region. Cells blocked prior to anaphase onset and lysed into microtubule protein nucleated a number of microtubules typical of that found in a metaphase aster; anaphase cells nucleated substantially fewer microtubules.