New structural and functional aspects of the type I interferon-receptor interaction revealed by comprehensive mutational analysis of the binding interface

Type I interferons bind to two cell surface receptors, ifnar1 and ifnar2, a s the first step in the activation of several signal transduction pathways that elicit an antiviral state and an anti-proliferative response. Here, we quantitatively mapped the complete binding region of ifnar2 on interferon (IFN)alpha2 by 35 individual mutations to alanine and isosteric residues. O f the six "hot-spot" residues identified (Leu-30, Arg-33, Arg-144, Ala-145, Met-148, and Arg-149), four are located on the E-helix, which is located a t the center of the binding site flanked by residues on the A-helix and the AD-loop. The contribution of residues of the D-helix, which have been prev iously implicated in binding, proved to be marginal for the interaction wit h the extracellular domain of ifnar2. Interestingly, the ifnar2 binding sit e overlaps the largest continuous hydrophobic patch on IFN alpha2. Thus, hy drophobic interactions seem to play a significant role stabilizing this int eraction, with the charged residues contributing toward the rapid associati on of the complex. Relating the anti-viral and anti-proliferative activity of the various interferon mutants with their affinity toward ifnar2 results in linear function over the whole range of affinities investigated, sugges ting that ifnar2 binding is the rate-determining step in cellular activatio n. Dose-time analysis of the anti-viral response revealed that shortening t he incubation time of low-level activation cannot be compensated by higher IFN doses. Considering the strict dependence of the cellular response on af finity, these results suggest that for maintaining transcription of IFN-res ponsive genes over a longer time period, low but continuous signaling throu gh the IFN receptor is essential.