HUMAN INTERFERON-GAMMA POTENTLY INDUCES THE SYNTHESIS OF A 55-KDA PROTEIN (GAMMA-2) HIGHLY HOMOLOGOUS TO RABBIT PEPTIDE-CHAIN RELEASE FACTOR AND BOVINE TRYPTOPHANYL-TRANSFER RNA-SYNTHETASE

An interferon-gamma (IFN-gamma)-inducible protein, gamma-2, was identified by two-dimensional gel electrophoresis of transformed human amnion (AMA) cell proteins. cDNA clones coding for this protein have been isolated and characterized as encoding a polypeptide with a predicted molecular weight of 53,165 and a pI of 6.16. Both values are in good agreement with those observed in two-dimensional gel electrophoresis. The gamma-2 protein is found to be highly induced by IFN-gamma, whereas no induction was seen after addition of IFN-alpha to AMA cells. A gamma-2-specific 2.7-kilobase mRNA was likewise seen to accumulate selectively in response to IFN-gamma in these cells. Comparison of the predicted amino acid sequence of gamma-2 to proteins in GenBank data bases revealed that gamma-2 is highly homologous to rabbit peptide chain release factor [Lee, C. C., Craigen, W. J., Muzny, D. M., Harlow, E. & Caskey, C. T. (1990) Proc. Natl. Acad. Sci. USA 87, 3508-3512] and bovine tryptophanyl-tRNA synthetase [M. Garret, V. Trezeguet, B. Pajot, J. C. Gandar, M. Merle, M. Guegiev, J. P. Benedetto, C. Sarger, J. Alteriot, J. La Bouessec, J. Labouesse, and J. Bonnet (1990), GenBank accession no. X52113]. Amino acid sequence similarities of 94% and 97%, respectively, are found, which in general would indicate that gamma-2 represents the human equivalent to either of these two mammalian genes. Based on these sequence similarities, the current data raise the possibility that tryptophanyl-tRNA charging and peptide chain release are carried out by the same enzyme. The gamma-2 protein is shown to possess tryptophan-dependent aminoacyl-tRNA synthetase activity and thus constitutes an enzymatic activity involved in the biological activity of IFN-gamma.