Characterization of the human type I interferon receptor by ligand blotting

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Abstract
The human type I interferon (IFN) receptor was characterized by ligand blotting. In this method, plasmalemma proteins or detergent-lysed whole-cell extracts from human Burkitt lymphoma cell lines were separated on polyacrylamide gels and subsequently transferred onto nitrocellulose sheets. Probing the blots with 3 × 10−10 M 125I-labeled recombinant IFN-αA (125I-rIFN-αA) revealed an IFN-α-binding protein with an apparent molecular mass of 95 kDa (p95). Performing the electrophoretic run under reducing conditions completely abrogated the signal on the blot, indicating that the type I IFN receptor contains a'disulfide bond essential for IFN binding. Optimal binding of 125I-rIFN-αA occurred at pH 9. The specificity of the binding reaction was established by simultaneously adding an excess of unlabeled IFN during incubation of the blots with 125I-rIFN-αA. The addition of either unlabeled IFN-α or IFN-β, but not IFN-γ, abolished the binding of 125I-rIFN-αA to p95. 125I-rIFN-γ at 1.25 × 10−11 M bound to two proteins distinct from p95 with apparent molecular mass of 92 and 87 kDa, respectively. Saturability of 125I-rIFN-αA binding was demonstrated by probing a constant amount of membrane proteins with increasing amounts of 125I-rIFN-αA. Scatchard analysis of the binding data yielded an apparent Kd of 5.4 × 10−10 M for the immobilized type I IFN receptor. The expression of p95 on IFN-α-resistant and -sensitive cells was indistinguishable. We conclude that p95 is the IFN-α/β receptor and that two proteins (p92 and p87) can specifically bind IFN-γ. These results indicate that ligand blotting is a versatile method for characterization of unmodified IFN receptors and IFN-receptor interaction and could also provide a new investigational approach for other cytokine receptor systems.