Differential Phosphorylation and Turnover of Nuclear Acidic Proteins during the Cell Cycle of Synchronized HeLa Cells

The phosphorylation of non-histone nuclear proteins has been studied in HeLa S-3 cells during synchronous growth. The uptake of [32P]orthophosphate into individual protein bands separated on sodium dodecyl sulfate polyacrylamide gels varies at different stages in the cell cycle while the banding patterns themselves are remarkably constant. Rates of phosphate uptake into most major phosphoprotein species are increased during the early G1 and early S phases and are minimal during the late G2 to M period. The turnover of previously incorporated phosphoryl groups during a cold chase following a 23-hour exposure to [32P]orthophosphate shows that some proteins lose their phosphoryl residues much more rapidly than do others. The halflives vary from 5 to 12 hours, with an over-all half-life of 6.7 hours. In contrast, the 14C specific activity of the proteins falls to 50% of the original activity in about 25 hours. The retention of [32P]phosphate is a more sensitive index of differential phosphoryl group turnover in different nuclear acidic proteins than is the uptake of [32P]phosphate in short term labeling experiments. If the nuclear phosphoproteins play a role in differential transcription during the cell cycle, changes in phosphorylation would appear to be more significant than changes in relative concentrations of individual protein species.