The role of multiply phosphorylated S6 in ribosome degradation

Rat liver ribosomes, prepared 1–24 h after intraperitoneal cortisol injection, contain multiple phosphorylated S6 consisting of four distinct derivatives in addition to the original non-phosphorylated S6. 25 h following the hormone injection the extent of S6 phosphorylation, as judged by its electrophoretic pattern in two-dimensional gels, resembles that of untreated rats. Ribosomal subunits with gradually increased degree of S6 phosphorylation, isolated at different time intervals after cortisol injection, exhibit polyphenylalanine polymerization levels inversely proportional to the extent of S6 phosphorylation. In addition, they show an elevated misincorporation of leucine in a poly(U)-programmed in vitro system. The lower amount of polyphenylalanine synthesized by multiple phosphorylated ribosomes in vitro is likely due to an enhanced susceptibility of nascent polypeptide chains synthesized in the in vitro system to proteinases present in the pH 5 and S-100 fractions. Liver polysomes derived from cortisol-treated animals lose their highly phosphorylated derivatives when exposed to S-100 enzymes. The loss can be prevented by concomitant action of proteinase and RNAase inhibitors (phenylmethylsulfonyl fluoride and heparin) but not by an inhibitor of phosphatase (sodium fluoride). In the absence of RNAase and proteinase inhibitors only degradation of old 40 S subunits can be demonstrated. 25 h after the cortisol treatment degradation of liver ribosomes occurs simultaneously with S6 dephosphorylation and is preceded by polysomal breakdown.