Enzymatic Modification of Proteins

Twenty-five highly purified protein preparations were tested for their ability to accept leucine or phenylalanine in the reaction catalyzed by leucyl, phenylalanyl-tRNA-protein transferase. Three were identified as true acceptors on the basis of their capacity to accept nearly molar equivalents of amino acid. These three were modified bovine serum albumin to which arginine had been enzymatically added at the NH2-terminal position, αs1-casein, and β-casein. The latter are also known to possess arginine as their NH2-terminal residue. Acceptor specificity was similar for the transfer of leucine and phenylalanine. Although phenylalanine could be almost quantitatively transferred from phenylalanyl-tRNA to protein, no transfer was observed from the 3'-pentanucleotide fragment of E. coli tRNAphe, suggesting that an internal portion of the tRNA molecule functions in the reaction. Phenylalanine but not valine was transferred from E. coli tRNAval, and no transfer occurred from N-acetylphenylalanyl-tRNA, indicating a specificity for the aminoacyl residue of aminoacyl-tRNA. However, p-fluorophenylalanyl-tRNA was found to replace phenylalanyl-tRNA as substrate. The results suggest that an NH2-terminal arginine residue of the protein acceptor as well as both the aminoacyl and tRNA moieties of aminoacyl-tRNA participate in the reaction catalyzed by leucyl,phenylalanyl-tRNA-protein transferase.