Stable Conformation of an Interferon-γ Receptor Binding Peptide in Aqueous Solution Is Required for Interferon-γ Antagonist Activity

Abstract

A peptide antagonist for mouse interferon-γ (IFN-γ) activity corresponding to the amino-terminal 39 amino acids of mouse IFN-γ [MuIFN-γ(1–39)] has been identified previously. In an analogous manner, we have assessed the ability of the corresponding peptide of human IFN-γ [HuIFN-γ(1–39)] to antagonize human IFN-γ. HuIFN-γ(1–39) has the ability to block the antiviral activity of human IFN-γ in a functional assay. In receptor competition, the peptide can also block human IFN-γ receptor binding. Surprisingly, the murine analog, MuIFN-γ(1–39), possesses a 10-fold greater ability to block human IFN-γ antiviral activity and receptor binding than HuIFN-γ(1–39). Both peptides showed α-helical structure in water by circular dichroism, however MuIFN-γ(1–39) possessed a greater amount of α-helix compared to HuIFN-γ(1–39), suggesting a requirement for a stable secondary structure for optimal antagonist activity. In trifluoroethanol, a helix-stabilizing agent, both peptides showed relatively equal α-helices, suggesting that both sequences have an equal potential for helical structure. As IFN-γ is species specific, the observation that MuIFN-γ(1–39) can antagonize human IFN-γ raises questions about the role of this region in species specificity. These studies provide insight into the structural requirements, both primary and secondary, for IFN-γ receptor binding. In the future, this information, along with more detailed three-dimensional structural analyses of the peptides, could prove useful in the rational design of IFN-γ agonists and antagonists.