IFNAR1 and IFNAR2 play distinct roles in initiating type I interferon-induced JAK-STAT signaling and activating STATs.

Type I interferons bind to cell surface receptors composed of the subunits IFNAR1 and IFNAR2, the intracellular domains (ICDs) of which are associated with the kinases TYK2 and JAK1, respectively. Ligand binding results in the cross-phosphorylation of TYK2 and JAK1, which then phosphorylate tyrosine residues in the ICDs of the receptor subunits and members of the STAT family of transcription factors. The phosphorylated STATs migrate to the nucleus and drive transcription. We analyzed receptor mutants in knockout cells to study the functional importance of various regions of the receptor ICDs. For IFNAR1, only the TYK2 binding site in the ICD was required for signaling. In contrast, successive truncations of the ICD of IFNAR2 proportionally decreased constitutive STAT binding, STAT phosphorylation, and target gene activation. These findings fit a model in which nonsuccessive stretches along the ICD interact with STATs. Tyrosine residues in the IFNAR1 ICD were not required for signaling, and single tyrosine mutations in the IFNAR2 ICD did not affect signal activation. However, simultaneous mutation of all the tyrosine residues in IFNAR2-ICD reduced STAT phosphorylation, STAT-mediated transcriptional activation, and antiviral activity but not constitutive STAT2 binding. We suggest that tyrosine phosphorylation on IFNAR2-ICD drives the dissociation of phosphorylated STATs, thus maintaining high signaling flux.