Changes in the synthesis of actin and other cell proteins after stimulation of serum-arrested cells.

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Abstract
The synthesis of both cytoplasmic and nuclear proteins was studied as quiescent, serum-deprived Swiss mouse 3T3 cells are stimulated to transit the cell cycle. In serum-arrested cells a 200,000 dalton cytoplasmic protein and a 51,000 dalton nuclear protein were preferentially synthesized. In serum-stimulated cells the 1st major protein whose synthesis increased had a molecular mass of 42,000 daltons. This protein also showed the greatest change in synthesis during the transit from Go to S phase. Its synthesis rose to a maximum 4-6 h after stinulation and then declined as cells entered S phase. The protein was present in both nuclear and cytoplasmic extracts. It was identified as actin on the basis of its mobility on sodium dodecyl sulfate and isoelectric focusing polyacrylamide gels. Other proteins synthesized preferentially by stimulated cells had molecular masses of 57,000 daltons (cytoplasmic), 33,000 daltons (cytoplasmic and nuclear) and 15,000 daltons (nuclear). The synthesis of the 57,000 and 33,000 dalton proteins increased gradually after stimulation and remained high during S phase. The 15,000 dalton proteins began to be synthesized as cells entered S phase. The preferential synthesis of these proteins provides biochemical markers for the transition from quiescence to proliferation.