TI  - New structural and functional aspects of the <prot>type I interferon-receptor</prot>
      interaction revealed by comprehensive mutational analysis of the binding
      interface.
PG  - 40425-33
AB  -   Type I interferons   bind to two cell surface receptors,  <prot>ifnar1</prot>  and <prot>ifnar2</prot>,
      as the first step in the activation of several signal transduction
      pathways that elicit an anti-viral state and an anti-proliferative
      response. Here, we quantitatively mapped the complete binding region of
       <prot>ifnar2</prot>  on  <prot>interferon (IFN)alpha2</prot>  by 35 individual mutations to alanine and
      isosteric residues. Of the six &quot;hot-spot&quot; 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 at the center of the binding site flanked by
      residues on the A-helix and the AB-loop. The contribution of residues of
      the D-helix, which have been previously implicated in binding, proved to
      be marginal for the interaction with the extracellular domain of <prot>ifnar2</prot>.
      Interestingly, the <prot>ifnar2</prot> binding site overlaps the largest continuous
      hydrophobic patch on <prot>IFNalpha2</prot>. Thus, hydrophobic interactions seem to
      play a significant role stabilizing this interaction, with the charged
      residues contributing toward the rapid association of the complex.
      Relating the anti-viral and anti-proliferative activity of the various
      interferon mutants with their affinity toward <prot>ifnar2</prot> results in linear
      function over the whole range of affinities investigated, suggesting that
      <prot>ifnar2</prot> binding is the rate-determining step in cellular activation.
      Dose-time analysis of the anti-viral response revealed that shortening the
      incubation time of low-level activation cannot be compensated by higher
      IFN doses. Considering the strict dependence of the cellular response on
      affinity, these results suggest that for maintaining transcription of
      IFN-responsive genes over a longer time period, low but continuous
      signaling through the IFN receptor is essential.
AD  - Department of Biological Chemistry, Weizmann Institute of Science, 76100
