The c-mos protein is required for the activation of M-phase promoting factor (MPF) during oocyte maturation and for MPF stabilization in unfertilized eggs. To address the question whether v-Mos plays an important role during cell cycle progression in transformed somatic cells, we have utilized a temperature-sensitive (ts) mutant of v-Mos. We examined NRK-6m2 cells chronically transformed by the ts mutant of Moloney murine sarcoma virus (MuSV ts110). NRK-6m2 cells transformed by MuSV ts 110 fail to express the viral mRNA for the heat-labile P85gag-mos protein at the restrictive temperature (39-degrees-C). To serve as controls for this study, two new transformation revertant cell lines, 6m3 and 6m4, were generated. 6m3 and 6m4 cells were found to express P85gag-mos maintained at both permissive (28-33-degrees-C) and restrictive temperatures. The growth rate of 6m2 cells at 37-degrees-C was significantly slower than either 6m3 or 6m4 cells at these same temperatures. To determine whether 6m2 cells maintained at 39-degrees-C were arrested at a specific phase in the cell cycle, flow cytometry analysis using double labeling was performed to quantitate cells in G0/G1, S- and G2/M-phases. The analyses indicated that 6m2 cells shifted to 39-degrees-C are arrested predominantly in the G1-phase of the cell-cycle. Of importance however, blocking v-mos expression also delayed progression through the G2-phase of the cell cycle.

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