The phosphorylation of ribosomal protein S6 on the monoribosomes and polyribosomes of baby hamster kidney fibroblasts

Abstract

In normal animal cells in vivo, ribosomal protein S6(nomenclature from [l] ) is the onlyphosphorylated protein of the 40 S ribosomal subunit [2-51. The extent to which this protein is phosphorylated can be increased in certain situations, including rapid cellular growth [2,6] , the administration of inhibitors of protein synthesis [7,8] , and the administration of cyclic AMP [9-l l] . The diversity of these stimuli has made it difficult to formulate a consistent biological role for the phosphorylation of S6, but the peculiar multiphosphorylation of this protein [2,6,12] and its evolutionary conservation among eukaryotes [ 13,141 argue’against the phosphorylation being merely gratuitous. One early result that seemed important in this regard was the finding [ 151 that a protein, which was probably S6, was only phosphorylated on the polyribosomes of reticulocytes and sarcoma cells, the inactive monoribosomes being virtually unphosphorylated. However, because this result was not confirmed with protein S6 from the ribosomes of regenerating rat liver [ 21, it has generally been disregarded. We report here that in baby hamster kidney fibroblasts (BHK cells) the phosphorylation of S6 is normally confined to polyribosomes, but that in certain circumstances S6 can become phosphorylated on monoribosomes. Our findings make it possible to reconcile the previous conflicting reports and provide a context for the reassessment of the function of the phosphorylation of ribosomal protein S6. 2. Methods