Amino acid substitutions in loop BC and helix C affect antigenic properties of helix D in hybrid IFN-alpha21a/alpha2c molecules

Binding Characteristics of IFN-alpha Subvariants to IFNAR2-EC and Influence of the 6-Histidine Tag

The expression, purification, detection, and assay of recombinant proteins have been made more convenient and rapid by the use of small affinity tags. To facilitate the purification of interferon-alpha2c (IFN-alpha2c) by metal chelate affinity chromatography, N-terminal 6-histidine tag was introduced via genetic manipulation. Two preparations of IFN material were purified; one contained IFN-alpha2c with the 6-histidine tag, and the other contained IFN-alpha2c without the 6-histidine tag. The antigenic properties of the human IFN-alpha2c subvariant with and without the 6-histidine tag, as well as the effects of the N-terminal 6-histidine tag on IFN-alpha2c interaction with the extracellular domain of human IFN-alpha receptor chain 2 (IFNAR2-EC) were examined. For the purposes of this study, IFNs were characterized by Western blots with anti-IFN monoclonal antibodies (mAb) and bioassays. Immunoblot analyses showed differences between IFN-alpha2c-6-histidine tag and IFN-alpha2a, b, c in their interaction with IFNAR2-EC. We also observed differences between IFN-alpha2c-6-histidine tag and IFN-alpha2a, b, c in bioactivities. This study is the first report that shows that an N-terminal 6-histidine tag on IFN-alpha2c can affect its interaction with receptor and cause a different bioactivity.

Identification of Residues of the IFNAR1 Chain of the Type I Human Interferon Receptor Critical for Ligand Binding and Biological Activity

The antiviral and antiproliferative activities of human type I interferons (IFNs) are mediated by two transmembrane receptor subunits, IFNAR1 and IFNAR2. To elucidate the role of IFNAR1 in IFN binding and the establishment of biological activity, specific residues of IFNAR1 were mutated. Residues (62)FSSLKLNVY(70) of the S5-S6 loop of the N-terminal subdomain of IFNAR1 and tryptophan-129 of the second subdomain of IFNAR1 were shown to be crucial for IFN-alpha binding and signaling and establishment of biological activity. Mutagenesis of peptide (278)LRV in the third subdomain shows that these residues are critical for IFN-alpha-induced biological activity but not for ligand binding. These data, together with the sequence homology of IFNAR1 with cytokine receptors of known structure and the recently resolved NMR structure of IFNAR2, led to the establishment of a three-dimensional model of the human IFN-alpha/IFNAR1/IFNAR2 complex. This model predicts that following binding of IFN to IFNAR1 and IFNAR2 the receptor complex assumes a "closed form", in which the N-terminal domain of IFNAR1 acts as a lid, resulting in the activation of intracellular kinases. Differences in the primary sequence of individual IFN-alpha subtypes and resulting differences in binding affinity, duration of ligand/receptor association, or both would explain differences in intracellular signal intensities and biological activity observed for individual IFN-alpha subtypes.