Evidence for control of protein synthesis in HeLa cells via the elongation rate

The effects of fresh medium and serum on protein synthesis in suspension-cultured HeLa cells after growth to high cell density (> 5 x 10(5) cells/ml) were studied. Cells which were resuspended in fresh medium plus serum and grown for 24 hours (control) were compared with cells grown for 2 hours after resuspension (stimulated). The spectrum of proteins being synthesized by control and stimulated cells does not appear to be grossly different; that is, the weight and number average molecular weights of newly synthesized whole-cell protein are about the same in both cultures. Also, no significant differences were observed in the number of ribosomes per polysome or in the fraction of total ribosomes in polysomes. However, the transit times (combined elongation and termination times) were found to differ significantly; the average transit time for control cells was 2.24 minutes, while the average transit time for stimulated cells was 1.26 minutes. (An appendex evaluating the methodology involved in measuring the transit time is included.) In aggreement with the difference in transit time, the absolute rate of protein synthesis in stimulated cells was approximately 1.8 times the rate measured in control cells. These data are taken as evidence that under certain conditions, the rate of elongation and/or termination of polypeptide chains limits the overall rate of translation, and that cells can respond to growth conditions by changing the elongation and/or termination rate of protein synthesis.