A SPECIFIC ROLE FOR THE PHOSPHORYLATION OF MAMMALIAN ACIDIC RIBOSOMAL-PROTEIN P2

The acidic ribosomal proteins P1-P2 from rat liver were overproduced f or the first time by expression of their cDNA in Escherichia coli, The y were tested for their ability to reactivate inactive P1-P2-deficient core particles derived from 60 S ribosomal subunits treated with dime thylmaleic anhydride, in poly(U)-directed poly(Phe) synthesis, The rec ombinant P1-P2 were unable to reactivate these core particles although they could bind to them, When recombinant P1-P2 had been phosphorylat ed first with casein kinase II? they mere as efficient in the reactiva tion process as P1-P2 extracted with ethanol/KCl from the 60 S subunit s, Reconstitution experiments were carried out using all possible comb inations of the two recombinant proteins phosphorylated or not. Reacti vation of the core particles required the presence of both P1 and P2 w ith the latter ill its phosphorylated form, These experiments reveal a distinct role for P1 and P2 in protein synthesis, Phosphorylated P2 p roduced a partial quenching of the intrinsic fluorescence of eukaryoti c elongation factor 2, which was not observed with the unphosphorylate d protein, This result demonstrates the existence of an interaction be tween phosphorylated P2 and eukaryotic elongation factor 2, P2 also qu enched part of the intrinsic fluorescence of P1, due to the interactio n between the two proteins.