TI  - Design, characterization, and structure of a biologically active
      single-chain mutant of human <prot>IFN-gamma</prot>.
PG  - 169-79
AB  - A mutant form of  human <prot>interferon-gamma</prot> (<prot><prot>IFN-gamma</prot> SC1</prot>)  that binds one
       <prot><prot><prot>IFN-gamma</prot> receptor</prot> alpha chain</prot> (<prot><prot>IFN-gamma</prot> R alpha</prot>)  has been designed and
      characterized. <prot><prot>IFN-gamma</prot> SC1</prot> was derived by linking the two peptide chains
      of the   <prot>IFN-gamma</prot>   dimer by a seven-residue linker and changing His111 in
      the first chain to an aspartic acid residue. Isothermal titration
      calorimetry shows that  <prot><prot>IFN-gamma</prot> SC1</prot> forms a 1:1 complex with its
      high-affinity receptor ( <prot><prot>IFN-gamma</prot> R alpha</prot> ) with an affinity of 27(+/- 9)
      nM. The crystal structure of <prot><prot>IFN-gamma</prot> SC1</prot> has been determined at 2.9 A
      resolution from crystals grown in 1.4 M citrate solutions at pH 7.6.
      Comparison of the wild-type receptor-binding domain and the
      Asp111-containing domain of <prot><prot>IFN-gamma</prot> SC1</prot> show that they are structurally
      equivalent but have very different electrostatic surface potentials. As a
      result, surface charge rather than structural changes is likely
      responsible for the inability of the His111--&gt;Asp domain of to bind
      <prot>IFN-gamma R alpha</prot>. The AB loops of <prot>IFN-gamma SC1</prot> adopt conformations
      similar to the ordered loops of <prot>IFN-gamma</prot> observed in the crystal
      structure of the <prot>IFN-gamma</prot> / <prot><prot>IFN-gamma</prot> R alpha</prot>  complex. Thus, <prot><prot>IFN-gamma</prot> R
      alpha</prot> binding does not result in a large conformational change in the AB
      loop as previously suggested. The structure also reveals the final six
      C-terminal amino acid residues of <prot><prot>IFN-gamma</prot> SC1</prot> (residues 253-258) that
      have not been observed in any other reported <prot>IFN-gamma</prot> structures. Despite
      binding to only one <prot><prot>IFN-gamma</prot> R alpha</prot>, <prot>IFN-gamma SC1</prot> is biologically
      active in cell proliferation, MHC class I induction, and anti-viral
      assays. This suggests that one domain of  <prot>IFN-gamma</prot>  is sufficient to
      recruit  <prot><prot>IFN-gamma</prot> R alpha</prot>  and  <prot>IFN-gamma</prot> R beta  into a complex competent
      for eliciting biological activity. The current data are consistent with
      the main role of the   <prot>IFN-gamma</prot>   dimer being to decrease the dissociation
      constant of <prot>IFN-gamma</prot> for its cellular receptors.
AD  - Center for Macromolecular Crystallography, University of Alabama,
