Crystallization and preliminary X-ray analysis of a 1:1 complex between a designed monomeric interferon-gamma and its soluble receptor.

A variant of human interferon-gamma (IFN-gamma) has been created in which the two chains of the homodimeric cytokine were linked N- to C-terminus by an eight residue polypeptide linker. The sequence of this linker was derived from a loop in bira bifunctional protein, and was determined from a structural database search. This "single-chain" variant was used to create an IFN-gamma molecule that binds only a single copy of the alpha-chain receptor, rather than the 2 alpha-chain receptor: 1 IFN-gamma binding stoichiometry observed for the native hormone. Crystals have been grown of a 1:1 complex between this single-chain molecule and the extracellular domain of its alpha-chain receptor. These crystals diffract beyond 2.0 A, significantly better than the 2.9 A observed for the native 2:1 complex. Density calculations suggest these crystals contain two complexes in the asymmetric unit; a self-rotation function confirms this conclusion.