Preferential utilization of phosphorylated 40-S ribosomal subunits during initiation complex formation

HeLa cells grown to a high density in spinner culture contain little or no phosphorus associated with ribosomal protein S6. When cells are transferred to fresh medium containing 10% calf serum, S6 becomes rapidly and multiply phosphorylated. Ribosomal proteins were extracted from subpolysome and polysome fractions, displayed on two-dimensional gels, and the distribution of phosphorylated S6 was quantified. Polysomal ribosomes have a higher percentage of phosphorylated S6 than subpolysomes at all times after transfer and the difference becomes more pronounced as the extent of phosphorylation increases. This difference cannot be explained by preferential phosphorylation of polysomal ribosomes, since kinase activity is equally distributed between polysomes and subpolysomes. Likewise, preferential dephosphorylation of subpolysomal ribosomes during cell fractionation does not occur. We interpret our results to mean that phosphorylated 40-S subunits form initiation complexes more efficiently than non-phosphorylated 40-S subunits.