Changes in ribosome function by cAMP-dependent and cAMP-independent phosphorylation of ribosomal protein S6

Alterations in ribosomal function were examined following phosphorylation of 40 S ribosomal subunits by the cAMP-dependent protein kinase and two cAMP-independent protein kinases, protease-activated kinases I and II. The cAMP-dependent protein kinase incorporated 2.0 mol of phosphate/mol of 40 S ribosomal subunits; ribosomal protein S6 was the sole phosphate acceptor. Phosphorylation of 40 S ribosomal subunits by the cAMP-dependent protein kinase inhibited the binding of AUG by 41% and poly(A,U,G) by 25% when compared with nonphosphorylated 40 S ribosomal subunits. In addition, phosphorylation of 40 S ribosomal subunits by the cAMP-dependent protein kinase inhibited translation of poly(A,U,G) by 30% in a reconstituted protein-synthesizing system. Protease-activated kinase II incorporated an average of 2.5 mol of phosphate/mol of 40 S ribosomal subunits which was distributed in equimolar amounts in derivatives of S6 containing one to four phosphates. Phosphorylation of 40 S ribosomal subunits by protease-activated kinase II increased the binding of AUG and poly(A,U,G) by 26 and 42%, respectively. Poly(A,U,G)-directed translation was stimulated 15% over that observed with nonphosphorylated ribosomes and 45% over that observed with ribosomes phosphorylated by the cAMP-dependent protein kinase. Protease-activated kinase I incorporated 1.0 mol of phosphate/mol of 40 S ribosomal subunits into ribosomal protein S10. Phosphorylation of 40 S ribosomal subunits by protease-activated kinase I did not alter the binding of AUG or poly(A,U,G). The effects of phosphorylation of 40 S ribosomal subunits by protease-activated kinase I on protein synthesis could not be examined due to the rapid release of phosphate from S10 in the reconstituted translation system.