Phosphorylation of Ribosomal Proteins by Substrate-Specific Protein Kinases from Rabbit Reticulocytes

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Abstract
Three protein kinase activities have been identified in ribosome-free supernatant from rabbit reticulocytes by DEAE-chromatography. Two of the protein kinase activities have similar substrate specificities, but differ in respect to dependency on cAMP for activation. These kinases preferentially phosphorylate histone, and also phosphorylate identical ribosomal proteins. One protein band in the 40S ribosomal subunit and six protein bands in the 60S subunit are phosphorylated, as demonstrated by Na dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The third kinase activity preferentially phosphorylates casein, instead of histone, and can use both [gamma-(32)P]ATP and [gamma-(32)P]GTP as phosphate donors. This protein kinase activity phosphorylates one protein band in the 40S subunit different from that phosphorylated by the other two kinases and four bands in the 60S subunit, only one of which is coincident with the proteins phosphorylated by the histone-specific activities.