A NEGATIVE REGULATORY REGION IN THE INTRACELLULAR DOMAIN OF THE HUMANINTERFERON-ALPHA RECEPTOR

Interferon-alpha (IFN-alpha)-mediated intracellular signaling is initi ated by ligand-induced receptor dimerization, tyrosine phosphorylation of the Tyk2 and Jak1 tyrosine kinases, and subsequent phosphorylation of the Stat1 and Stat2 proteins. The IFN-alpha receptor consists of a t least two distinct subunits. One subunit, IFNAR1, has low affinity b inding for interferon yet is required for signal transduction. We intr oduced mutations in the cytoplasmic domain of human IFNAR1 in order to identify residues involved in the mediation of biological responses. We took advantage of the species specificity of the interferon recepto rs by analyzing human IFN-alpha-induced major histocompatibility compl ex class I antigen expression in mouse L929 cells stably transfected w ith mutant human receptors. The membrane proximal 60-amino acids were insufficient to signal a biological response even though within these residues Tyk2 and Stat2 binding sites have been identified. IFN-alpha- induced receptor tyrosine phosphorylation was not critical for signali ng because mutation of Tyr residues to Phe did not prevent the biologi cal response to IFN-alpha. The deletion of a 16-amino acid region high ly homologous between species created a receptor which signals an enha nced response. Tyrosine dephosphorylation is a component of this enhan ced response as mutation of the Tyr residues within this region to Phe resulted in a receptor with increased sensitivity to IFN. The known s ignaling molecules that interact with IFNAR1 are positive regulators o f IFN-alpha function. The presence of this domain in the COOH-terminal region suggests that the receptor may interact with signaling molecul es that negatively regulate interferon responses.