Type I interferons serve as the first line of defense against pathogen invasion. Binding of IFNs to its receptors, IFNAR1 and IFNAR2, is leading to activation of the IFN response. To determine whether structural perturbations observed during binding are propagated to the cytoplasmic domain, multiple mutations were introduced into the transmembrane helix and its surroundings. Insertion of one to five alanine residues near either the N or C terminus of the transmembrane domain (TMD) likely promotes a rotation of 100°and a translation of 1.5 Å per added residue. Surprisingly, the added alanines had little effect on the binding affinity of IFN to the cell surface receptors, STAT phosphorylation, or gene induction. Similarly, substitution of the juxtamembrane residues of the TMD with alanines, or replacement of the TMD of IFNAR1 with that of IFNAR2, did not affect IFN binding or activity. Finally, only the addition of 10 serine residues (but not 2 or 4) between the extracellular domain of IFNAR1 and the TMD had some effect on signaling. Bioinformatic analysis shows a correlation between high sequence conservation of TMDs of cytokine receptors and the ability to transmit structural signals. Sequence conservation near the TMD of IFNAR1 is low, suggesting limited functional importance for this region. Our results suggest that IFN binding to the extracellular domains of IFNAR1 and IFNAR2 promotes proximity between the intracellular domains and that differential signaling is a function of duration of activation and affinity of binding rather than specific conformational changes transmitted from the outside to the inside of the cell.Type I interferons (IFNs) orchestrate the antiviral innate immunity in vertebrates. They consist of 16 members in humans as follows: 12 different IFN␣ subtypes, as well as IFN␤, IFN, IFN⑀, and IFN. They are clustered on the short arm of chromosome 9. All type I interferons elicit their innate and adaptive immune responses after binding to the receptor and forming the IFNAR1-IFN-IFNAR2 ternary complex (1, 2), the formation of which is essential for signaling. Both IFNAR1 and IFNAR2 belong to the class II helical cytokine receptors with four fibronectin type III-like subdomains for IFNAR1 and two subdomains for IFNAR2. The extracellular domains are followed by a single helix of 21 amino acids spanning the membrane, which in turn is connected to mostly natively unstructured intracellular domains and their associated effectors (Fig. 1A) (3). Cells lacking one of the receptors lack normal IFN signaling (4, 5). Structurally different members of type I IFNs bind to the same cell surface receptor but mediate differential responses that result from differences in binding affinities, concentration of IFN, and duration of activation (5-13). Differential signaling is realized through robust (antiviral) versus tunable (antiproliferative and immunomodulatory) gene induction leading to different phenotypic outcomes (14,15). Receptor dimerization drives the activation of cytosolic associated Janus family kina...