Formation of a Uniquely Stable Type I Interferon Receptor Complex by Interferon β Is Dependent upon Particular Interactions between Interferon β and Its Receptor and Independent of Tyrosine Phosphorylation

Human type I interferons (IFN) require two receptor chains, IFNAR1 and IFNAR2c for high affinity (pM) binding and biological activity. Our previous studies have shown that the ligand dependent assembly of the type I IFN receptor chains is not identical for all type I IFNs. IFNβ appears unique in its ability to assemble a stable complex of receptor chains, as demonstrated by the observation that IFNAR2c co-immunoprecipitates with IFNAR1 when cells are stimulated with IFNβ but not with IFNα. The characteristics of such a receptor complex are not well defined nor is it understood if differential signaling events can be mediated by variations in receptor assembly. To further characterize the factors required for formation of such a stable receptor complex we demonstrate using IFN stimulated Daudi cells that (1) IFNAR2c co-immunoprecipitates with IFNAR1 even when tyrosine phosphorylation of receptor chains is blocked with staurosporine, and (2) IFNβ1b but not IFNα2, is present in the immunoprecipitated receptor complex. These results demonstrate that the unique IFNβ induced assembly of type I IFN receptor chains is independent of receptor tyrosine phosphorylation and the recruitment of additional proteins to the receptor by such events. Furthermore, the presence of IFNβ1b in the immunoprecipitated IFN receptor complex suggests that IFNβ interacts and binds differently to the receptor than IFNα2. These results suggest that the specific assembly of type I IFN receptor chains is ligand dependent and may represent an early event which leads to the differential biological responses observed among type I IFNs.