Phosphorylation of ribosomal proteins in rat cerebral cortex in vitro

Incubation of cerebral cortical tissue from immature rats in the presence of [32P]orthophosphate resulted in similar rates of incorporation of radioactivity into the proteins of free and membrane-bound ribosomes. Incorporation of label into ribosomal proteins of both species continued actively for at least 3 hours. Since recovery of membrane-bound ribosomes from rat cerebral cortex is quite low, further analyses of the radioactive phosphoproteins were restricted to the free ribosome population. A significant fraction of the radioactivity which was precipitated with trichloroacetic acid was not removed by heating in trichloroacetic acid at 90 degrees or extracted with organic solvents and therefore was presumed to be covalently bound to protein. The radioactive phosphoryl groups present in the ribosomal proteins were mainly in ester linkages since they were readily removed by exposure to 1 N NaOH, relatively unaltered by 1N HCl, and unaffected by hydroxylamine. This conclusion was supported by the isolation of labeled o-phosphoserine and o-phosphothreonine residues from hydrolysates of ribosomal proteins. A significant fraction of the labeled phosphoproteins in the purified ribosomes appeared to be bound tightly to the ribosome structure since only 40% of the radioactivity could be removed by extraction of these ribosomes with 1 M KCl. Phosphorylation of proteins of cerebral monoribosomes was more rapid than the same process in polyribosomes from the same source. Eight radioactive phosphoprotein bands could be detected by electrophoresis of proteins obtained from unfractionated cerebral ribosomes on unidimensional polyacrylamide gels containing sodium dodecyl sulfate. The protein nature of these materials was confirmed by pronase digestion. Proteins of subribosomal particles isolated from the total free ribosomal population were labeled differentially. When dissociation was carried out in the presence of EDTA, the small subunit contained four radioactive phosphoprotein bands, whereas the large subunit contained five. Three of the radioactive phosphoprotein components of the small subunit were removed when dissociation of cerebral ribosomes which were previously washed with high salt media was carried out in the presence of puromycin and high salt. However, only the largest labeled phosphoprotein band of the large subunit was removed by this procedure. This component exhibited the same electrophoretic mobility as one of the radioactive phosphoprotein bands which was removed from the small subunit by high salt treatment..