Reduced receptor binding by a human interferon-γ fragment lacking 11 carboxyl-terminal amino acids

Treatment of recombinant human interferon-γ (IFN-γ) with either 1) the arginine-specific proteases clostripain or submaxillaris protease of 2) the broadly specific enzyme pronase produced a stable fragment with m.w. of 15,000. Structural analysis showed that the cleavage occurred between residues 120 and 130 and thus produced a fragment lacking only 11 carboxyl-terminal amino acids. The fragmented and untreated molecules showed identical amino-terminal amino acid sequences and were equally reactive with either polyclonal or monoclonal anti-IFN-γ. IFN-γ lacking carboxyl-terminal amino acids displayed a 1000- to 2000-fold reduction in its capacity to bind to cellular IFN-γ receptors at 4°C. Functionally the fragment showed a 50-fold reduction in its ability to induce antiviral activity in fibroblasts and a 10-fold reduction in its ability to induce Fc receptors on the human histiocytic lymphoma cell line U937. These results thus suggest that the carboxyl terminus of human IFN-γ contributes significantly to the formation of the receptor-binding site of the molecule.