CELL-CYCLE REGULATION OF P70 S6 KINASE AND P42 P44 MITOGEN-ACTIVATED PROTEIN-KINASES IN SWISS MOUSE 3T3 FIBROBLASTS/

We show here using synchronized Swiss mouse 3T3 fibroblasts that p70 S 6 kinase (p70(S6k)) and mitogen-activated protein kinases (p42(mapk)/p 44(mapk)) are not only activated at the G(0)/G(1) boundary, but also i n cells progressing from M into G(1). p70(S6k) activity increases 20-f old in G(1) cells released from G(0). Throughout G(1), S, and G(2) it decreases constantly, so that during M phase low kinase activity is me asured. The kinase is reactivated 10-fold when cells released from a n ocodazole-induced metaphase block enter G(1) of the next cell cycle. p 42(mapk)/ p44(mapk) in G(0) cells are activated transiently early in G (1) and are reactivated late in mitosis after nocodazole release. p70( S6k) activity is dependent on permanent signaling from growth factors at all stages of the cell cycle. Immunofluorescence studies showed tha t p70(S6k) and its isoform p85(S6k) become concentrated in localized s pots in the nucleus at certain stages in the cell cycle. Cell cycle-de pendent changes in p70(S6k) activity are associated with alterations i n the phosphorylation state of the protein. However, examination of th e regulation of a p70(S6k) mutant in which the four carboxyl-terminal phosphorylation sites are changed to acidic amino acids suggests that a mechanism independent of these phosphorylation sites controls the ac tivity of the enzyme during the cell cycle.