Structure-Function Study of the Extracellular Domain of the Human Type I Interferon Receptor (IFNAR)-1 Subunit

Abstract

Despite accumulating information about the different effector molecules and signaling cascades that are invoked on interferon-α (IFN-α) binding to the type 1 IFN receptor, little is known about the specifics of the binding interactions between the ligand and the receptor complex. The IFN-α/β receptor (IFNAR)-2 subunit of the IFN receptor is considered the primary binding chain of the receptor, yet it is clear that both receptor subunits, IFNAR-1 and IFNAR-2, cooperate in the high-affinity binding of IFN to the receptor complex. Earlier results from our laboratory suggested that an association of IFNAR-1 with membrane Gal alpha 1-4Gal-containing glycolipids facilitates receptor-mediated signaling. The data implicated amino acid residues in the SD100 domain of IFNAR-1 in the glycosphingolipid(GSL) modification of the type 1 IFN receptor. Interestingly, the human and murine counterparts of IFNAR-1 exhibit remarkable species specificity despite their considerable amino acid sequence identity. Certainly, those amino acid residues that effect GSL modification of IFNAR-1 are conserved between species, yet specific regions of IFNAR-1 that confer species specificity have not been defined. To delineate further the role of the IFNAR-1 SD100A domain in receptor function, a chimeric cDNA was assembled, in which the SD100A domain of the murine IFNAR-1 chain was replaced with the human sequence. This construct was expressed in IFNAR-1^-/- mouse embryonic fibroblasts, and stable transfectants were established. Transfectants are fully sensitive to murine IFN-α4 treatment with respect to the induction of IFN-stimulated gene factor 3 (ISGF3) and sis-inducing factor (SIF) signal transducer and activator of transcription factor (Stat) complexes, exhibiting comparable levels of Stat activation to those observed in IFNAR-1^-/- cells reconstituted with intact MuIFNAR-1. Similar results were obtained with IFN-induced antiviral and growth inhibitory responses. Viewed together, these data suggest that the SD100A domain of IFNAR-1 does not contribute to species-specific IFN binding.