The Human Interferon Receptor: NMR-Based Modeling, Mapping of the IFN-α2 Binding Site, and Observed Ligand-Induced Tightening,

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Abstract
The human interferon receptor (IFNAR) mediates the antiviral and antiproliferative activities of type I interferons (IFNs). This receptor is comprised of subunits IFNAR1 and IFNAR2, the latter exhibiting nanomolar affinity for IFNs. Here the extracellular domain of IFNAR2 (IFNAR2-EC), a soluble 25 kDa IFN-binding polypeptide, and its complex with IFN-α2 were studied using multidimensional NMR. IFNAR2-EC is comprised of two fibronectin-III (FN-III) domains connected by a helical hinge region. The deduced global fold was utilized to improve the alignment of IFNAR2-EC against structurally related receptors and to model its structure. A striking feature of IFNAR2-EC is the limited and localized deviations in chemical shifts exhibited upon ligand binding, observed for only 15% of its backbone 1H and 15N nuclei. Analysis of these deviations maps the IFN-α2 binding site upon IFNAR2-EC to a contiguous surface on the N-terminal domain, including the S3−S4 loop (residues 44−53), the S5−S6 loop and S6 β-strand (residues 74−82), and the S7 β-strand and the hinge region (residues 95−105). The C-terminal domain contributes only marginally to ligand binding, and no change in the hypothesized interdomain interface is observed. The proposed binding domain encompasses all residues implicated by mutagenesis studies in IFN binding, and suggests adjacent residues cooperate in forming the binding surface. D2O-exchange experiments indicate that binding of IFN-α2 induces tightening of the N-terminal domain of IFNAR2-EC. This increase in receptor rigidity may play an important role in initiating the intracellular stage of the IFN signaling cascade.