N-ACETYLAMINO ACIDS AND PROTEIN SYNTHESIS

The ordinarily free amino groups of N-terminal residues in polypeptide chains are blocked in some instances by acetyl groups, e.g., in melanocyte-stimulating hormone (a-MSH),I and in the following proteins: horse heart cytochrome c,2 hemoglobin F1,3 egg albumin,4 histones,5 turnip yellow mosaic virus (TYMV),6 and tobacco mosaic virus (TMV) proteins.7 The question arises whether the introduction of these acetyl groups occurs at an early, intermediary, or late stage of protein synthesis. Some time ago we described the conversion of phenylalanine and leucine into their N-acetyl derivatives by rat liver slices,8 reactions which have remained without a known metabolic function. The existence of N-acetylproteins has now led us to consider the possibility that the acetylation of amino acids serves the purpose of furnishing the N-terminal acetyl groups of N-acetylproteins. As the first step in exploring the role of acetylation in protein synthesis, we have investigated the reactivity of N-acetylamino acids in amino acid activating systems and the transfer of the products to sRNA. We now report results demonstrating that several N-acetylamino acids, notably N-acetyltyrosine, enter the early reactions of protein synthesis as measured by the formation of hydroxamates, by the promotion of the inorganic pyrophosphate-ATP exchange, and by incorporation into sRNA. The acetyl substituent is not removed from the amino acid during these transformations.