Quiescent cells but not cycling cells exhibit enhanced actin synthesis before they synthesize DNA

Major proteins synthesized by Swiss 3T3 cells at different stages of the cell cycle have been analyzed using double isotope labeling and one-dimensional SDS-polyacrylamide slab gels. The synthesis of actin was previously shown to be markedly enhanced a few hours after quiescent cells initiated growth following addition of serum. In contrast, the synthesis of actin remained at a constant rate, similar to that in quiescent cells, relative to synthesis of other proteins during the entire cell cycle. We conclude that enhanced actin synthesis is a process specific for the G0 to S transit, and may serve as a marker event during this interval. In contrast, three other proteins (90,000, 57,000, and 33,000 daltons) were synthesized throughout the cell cycle at higher rates than in G0 cells, and thus, are markers characteristic of cells traversing the cell cycle. A transient increase, such as seen for actin synthesis, by cells emerging from quiescence, may represent a process that these cells must perform before they can enter the G1 portion of the cell cycle. A transient event such as this need not be a periodic event that occurs during each cycle.