STABLE CONFORMATION OF AN INTERFERON-GAMMA RECEPTOR-BINDING PEPTIDE IN AQUEOUS-SOLUTION IS REQUIRED FOR INTERFERON-GAMMA ANTAGONIST ACTIVITY

A peptide antagonist for mouse interferon-gamma (IFN-gamma) activity corresponding to the amino-terminal 39 amino acids of mouse IFN-gamma [MuIFN-gamma(1-39)] has been identified previously. In an analogous manner, we have assessed the ability of the corresponding peptide of human IFN-gamma [HuIFN-gamma(1-39)] to antagonize human IFN-gamma. HuIFN-gamma(1-39) has the ability to block the antiviral activity of human IFN-gamma in a functional assay. In receptor competition, the peptide can also block human IFN-gamma receptor binding. Surprisingly, the murine analog, MuIFN-gamma(1-39), possesses a 10-fold greater ability to block human IFN-gamma antiviral activity and receptor binding than HuIFN-gamma(1-39). Both peptides showed alpha-helical structure in water by circular dichroism, however MuIFN-gamma(1-39) possessed a greater amount of alpha-helix compared to HuIFN-gamma(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 alpha-helices, suggesting that both sequences have an equal potential for helical structure. As IFN-gamma is species specific, the observation that MuIFN-gamma(1-39) can antagonize human IFN-gamma 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-gamma 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-gamma agonists and antagonists.