IDENTIFICATION OF AN INTERFERON-GAMMA RECEPTOR-ALPHA CHAIN SEQUENCE REQUIRED FOR JAK-1 BINDING

We have shown previously that a four-amino acid block residing at posi tions 266-269 (LPKS) in the intracellular domain of the human interfer on-gamma (IFN-gamma) receptor alpha chain is critical for IFN-gamma-de pendent tyrosine kinase activation and biologic response induction, He rein we show that this sequence is required for the constitutive attac hment of the tyrosine kinase JAK-1. Using a vaccinia expression system , a receptor alpha chain-specific monoclonal antibody coprecipitated J AK-1 from cells coexpressing JAK-1 and either (a) wild type IFN-gamma receptor alpha chain, (b) a receptor alpha chain truncation mutant con taining only the first 59 intracellular domain amino acids, or (c) a r eceptor mutant containing alanine substitutions for the functionally i rrelevant residues 272-275, In contrast, JAK-1 was not coprecipitated when coexpressed with a receptor alpha chain mutant containing alanine substitutions for the functionally critical residues 266-269 (LPKS), Mutagenesis of the LPKS sequence revealed that Pro-267 is the only res idue obligatorily required for receptor function. In addition, Pro-267 is required for JAK-1 binding, These results thus identify a site in the IFN-gamma receptor alpha chain required for constitutive JAK-1 ass ociation and establish that this association is critical for IFN-gamma signal transduction.