DIFFERENTIAL TYROSINE PHOSPHORYLATION OF THE IFNAR CHAIN OF THE TYPE-I INTERFERON RECEPTOR AND OF AN ASSOCIATED SURFACE PROTEIN IN RESPONSE TO IFN-ALPHA AND IFN-BETA

The human interferon alpha-receptor (IFNAR gene product) is a transmembranal protein of 557 amino acids with an intracytoplasmic domain of 100 amino acids containing four tyrosines. Antibodies to a C-terminal peptide (residues 521-536) were developed which efficiently immunoprecipitate the 105 kDa IFNAR protein from detergent extracts of human cells, We show that the IFNAR protein becomes tyrosine phosphorylated within 5 min after treatment of human myeloma U266 cells with IFN-alpha 2, IFN-alpha 8 or IFN-beta. The IFNAR chain interacts with both IFN-alpha 2 and IFN-beta, as demonstrated by cross-linking, Among elements involved in signal transduction by type I IFNs, the tyrosine kinase Tyk2 but not Jak1, and the ISGF3 transcription factor subunit Stat2 (p113) but not Stat1 (p91), are found associated with the IFNAR protein. After IFN-P treatment for 5 min, a tyrosine-phosphorylated protein of similar to 95 kDa (beta-PTyr) is found bound to IFNAR, but can be dissociated by denaturation. The beta-PTyr protein is present on the cell surface, like IFNAR, as shown by extracellular biotin tagging. The ratio of beta-PTyr to IFNAR tyrosine phosphorylation is much higher with IFN-P than with IFN-alpha 2 or 8, Both are IFN dependent and abrogated by a monoclonal antibody which blocks IFNAR action. The beta-PTyr component may represent an important difference in the action of IFN-beta as compared with IFN-alpha in their shared receptor system.