Multiple phosphorylation of ribosomal protein S6 during transition of quiescent 3T3 cells into early G1, and cellular compartmentalization of the phosphate donor.

Abstract

At 5 min after quiescent cells [Swiss mouse 3T3 fibroblasts] are induced to enter G1, there is a large increase in the amount of 32P incorporated into 40S ribosomal protein S6. Changes in the specific activities of 32Pi and [.gamma.-32P]ATP in stimulated as compared to quiescent cultures apparently do not account for this large increase. By decreased electrophoretic mobility on 2-dimensional polyacrylamide gels it was shown that this increase is due to a quantitative increase in the total amount of phosphate incorporated into S6. Pulse-chase experiments also show that the phosphate incorporated into S6 is metabolically stable during at least the first 60 min of induction and that the incorporation of 32P into S6 responds immediately to the replacement of 32Pi by Pi in the medium, in contrast to [.gamma.-32P]ATP which changes very slowly. The S6 phosphate donor must be a compartment separate from that of the total cellular ATP.