Identification of an Interferon- Receptor Chain Sequence Required for JAK-1 Binding

We have shown previously that a four-amino acid block residing at positions 266-269 (LPKS) in the intracellular domain of the human interferon- (IFN- ) receptor α chain is critical for IFN- -dependent tyrosine kinase activation and biologic response induction. Herein we show that this sequence is required for the constitutive attachment of the tyrosine kinase JAK-1. Using a vaccinia expression system, a receptor α chain-specific monoclonal antibody coprecipitated JAK-1 from cells coexpressing JAK-1 and either ( a ) wild type IFN- receptor α chain, ( b ) a receptor α chain truncation mutant containing only the first 59 intracellular domain amino acids, or ( c ) a receptor mutant containing alanine substitutions for the functionally irrelevant residues 272-275. In contrast, JAK-1 was not coprecipitated when coexpressed with a receptor α 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 residue obligatorily required for receptor function. In addition, Pro-267 is required for JAK-1 binding. These results thus identify a site in the IFN- receptor α chain required for constitutive JAK-1 association and establish that this association is critical for IFN- signal transduction.