An activated S6 kinase in extracts from serum- and epidermal growth factor-stimulated Swiss 3T3 cells

Soluble extracts from serum- or epidermal growth factor-stimulated Swiss 3T3 cells show up to a 25-fold increase in their ability to phosphorylate 40 S ribosomal protein S6. The increased S6 phosphorylation is due to increased protein kinase activity in extracts of stimulated cells and not due to the inactivation of an S6 phosphatase. However, the presence of phosphatase inhibitors as well as EGTA is required during the preparation of cell extracts to obtain fully active S6 kinase(s). Epidermal growth factor has little effect at concentrations below 10(-10) M: activity increases sharply at 10(-9) M epidermal growth factor and reaches saturation at 10(-8) M (50-60% of the activity obtained by stimulating with 10% serum). Activation of the kinase activity in cell extracts is observed as early as 2 min after serum stimulation, reaches 50% between 10 and 15 min, and is maximal by 60 min of serum stimulation. Phosphorylation in vitro of ribosomal protein S6 with extracts from serum-stimulated cells followed by analysis of the tryptic phosphopeptides shows the presence of 9 of the 11 phosphopeptides induced by serum in vivo.