CELL-CYCLE DEPENDENT GENE EXPRESSIONS AND ACTIVITIES OF PROTEIN PHOSPHATASE-PP1 AND PHOSPHATASE-PP2A IN MOUSE NIH3T3 FIBROBLASTS

We determined the mRNA levels and the activities in nuclear and non-nuclear fractions of protein phosphatase type 1 (PP1) and type 2A (PP2A) through the cell cycle in synchronized mouse NIH3T3 fibroblasts. The mRNA level for PP1α was gradually elevated in late G1 phase, began to decrease in M phase, and reached the control level with entering into the next G1 phase. The mRNA level for PP2A was rapidly increased in early G1 phase, kept at the high level, and decreased after S phase. In nuclear fractions of cells, spontaneous activities of both PP1 and PP2A were gradually increased until M phase and rapidly decreased with entering the next G1 phase, while in non-nuclear fraction such dramatic alterations were not observed. Potential activities of PP1 in both fractions revealed by Co2+-trypsin treatment showed an oscillation patterns similar to those of the spontaneous activities. These results strongly suggest that cell cycle dependent gene expressions and activities of PP1 and PP2A play roles in DNA synthesis and mitosis during the cell cycle. © 1992 Academic Press, Inc.