The orderly progression of eukaryotic cells from interphase to mitosis requires the close coordination of various nuclear and cytoplasmic events. Studies from our laboratory and others on animal cells indicate that two activities, one present mainly in mitotic cells and the other exclusively in G1-phase cells, play a pivotal role in the regulation of initiation and completion of mitosis, respectively. The purpose of this study was to investigate whether these activities are expressed in the slime mold Physarum polycephalum in which all the nuclei traverse the cell cycle in natural synchrony. Extracts were prepared from plasmodia in various phases of the cell cycle and tested for their ability to induce germinal vesicle breakdown and chromosome condensation after microinjection into Xenopus laevis oocytes. We found that extract of cells at 10-20 min before metaphase consistently induced germinal vesicle breakdown in oocytes. Preliminary characterization, including purification on a DNA-cellulose affinity column, indicated that the mitotic factors from Physarum were functionally very similar to HeLa mitotic factors. We also identified a number of mitosis-specific antigens in extracts from Physarum plasmodia, similar to those of HeLa cells, using the mitosis-specific monoclonal antibodies MPM-2 and MPM-7. Interestingly, we also observed an activity in Physarum at 45 min after metaphase (i.e., in early S phase since it has no G1) that is usually present in HeLa cells only during the G1 phase of the cell cycle. These are the first studies to show that maturation-promoting factor activity is present in Physarum during mitosis and is replaced by the G1 factor (or anti-maturation-promoting factor) activity in a postmitotic stage. A comparative study of these factors in this slime mold and in mammalian cells would be extremely valuable in further understanding their function in the regulation of eukaryotic cell cycle and their evolutionary relationship to one another.

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