Transcriptional and translational control of cytoplasmic proteins after serum stimulation of quiescent Swiss 3T3 cells.

The synthesis of cytoplasmic proteins from quiescent and serum-stimulated Swiss 3T3 cells was compared by two-dimensional polyacrylamide gel electrophoresis. Four new proteins of Mrs 26,000, 28,000, 45,000, and 47,000 designated N26, N28, N45, and N47, which were not detectable in quiescent cells, appeared 60 min after addition of serum. During the same period, the amount of [35S]methionine incorporated into 10 proteins present in quiescent cells, ranging in Mr from 23,000 to 98,000 and designated Q23-98, increased up to 6-fold, whereas the amount incorporated into three other proteins decreased by a factor of approximately 2. Of the new proteins, N26 was no longer detectable, and the amount of [35S]methionine incorporated into N47 was significantly reduced by 150 min. During this same time, a fifth new protein, N56, appeared, and there was a large increase in the amount of radioactivity incorporated into another protein, Q121. The increases in nine of the proteins were either strongly or completely inhibited by actinomycin D, arguing that the expression of these proteins was under transcriptional control. In contrast, the increases in seven other proteins were unaffected by actinomycin D, suggesting that their expression was under translational control. These proteins will serve as useful markers for determining how cells progress through early lag phase.