Regulation of Protein Synthesis during Density-dependent Growth Inhibition of BHK21/13 Cells

Regulation of protein synthesis in BHK21/13 cells has been examined as a function of cell growth rate and density, both in vivo by pulse-labeling of cells with [ 3 H]valine and in an in vitro cell-free protein-synthesizing system. The specific activity of [ 3 H]valyl-tRNA in the pulse-labeled cells was determined and the true in vivo rate of [ 3 H]valine incorporation into protein was measured. Unlike the reported behavior of many other cell lines, BHK21/13 cells did not shut off their protein synthesis at all while their growth rate declined 10-fold or more during density-dependent inhibition of growth or during arrest of growth by serum deprivation. In vitro assays of the cells' protein synthetic activity with both synthetic and natural messenger RNA confirmed that all of the components needed for protein synthesis were equally active in rapidly growing and in density-inhibited or serum-deprived cells. Hence, in these cells the protein synthesis rate and the cell division rate are not coupled, and the amount of protein in the cell is not regulated at the level of synthesis, but rather at the level of protein degradation or export.