Structure-function study of the extracellular domain of the human type I interferon receptor (IFNAR)-1 subunit

Despite accumulating information about the different effector molecules and signaling cascades that are invoked on interferon-alpha (IFN-alpha) bindin g to the type 1 IFN receptor, little is known about the specifics of the bi nding interactions between the ligand and the receptor complex, The IFN-alp ha/beta receptor (IFNAR)-2 subunit of the IFN receptor is considered the pr imary binding chain of the receptor, Set it is clear that both receptor sub units, IFNAR-1 and IFNAR-2, cooperate in the high-affinity binding of IFN t o the receptor complex. Earlier results from our laboratory suggested that an association of IFNAR-1 with membrane Gal alpha 1-4Gal-containing glycoli pids facilitates receptor-mediated signaling, The data implicated amino aci d residues in the SD100 domain of IFNAR-1 in the glycosphingolipid (GSL) mo dification of the type 1 IFN receptor. Interestingly, the human and murine counterparts of IFNAR-1 exhibit remarkable species specificity despite thei r considerable amino acid sequence identity. Certainly, those amino acid re sidues that effect GSL modification of IFNAR-1 are conserved between specie s, 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 i n receptor function, a chimeric cDNA was assembled, in which the SD100A dom ain of the murine IFNAR-1 chain was replaced with the human sequence. This construct was expressed in IFNAR-1(-/-) mouse embryonic fibroblasts, and st able transfectants were established. Transfectants are fully sensitive to m urine IFN-alpha 4 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 l evels of Stat activation to those observed in IFNAR-1-/- cells reconstitute d with intact MuIFNAR-1, Similar results were obtained with IFN-induced ant iviral and growth inhibitory responses. Viewed together, these data suggest that the SD100A domain of IFNAR-1 does not contribute to species-specific IFN binding.