THREE-DIMENSIONAL STRUCTURE OF HUMAN INTERFERON-[[gamma]] COMPLEXED TO ITS CELL RECEPTOR. Daniel Thiel¹, Marie le Du¹, Richard Walter¹, Allan D'Arcy², Christiane Chene², Michael Fontoulakis², Gianni Garotta², Fritz Winkler² and Steve Ealick¹. Section of Biochemistry, Molecular and Cell Biology, Cornell University, Ithaca, NY 14853 and Pharma Research, F. Hoffman-La Roche Ltd, 4002 Basel, Switzerland.

Interferon-[[gamma]] (IFN-[[gamma]]) was originally identified because of its ability to "interfere" with viral replication after infection. Since then IFN-[[gamma]] has been shown to be responsible for many types of immune responses. IFN-[[gamma]] is secreted by activated T-cells and natural killer cells and functions by binding to a specific high affinity cell receptor (R[[alpha]]) and a second low affinity receptor (R[[beta]]). We have used X-ray crystallography to determine the structure of IFN-[[gamma]] complexed to its high affinity receptor (R[[alpha]]) at 2.7 Å resolution. The complex crystallizes in space group C2 with a = 202 Å, b = 114 Å, c = 74 Å and [[beta]] = 117deg. and with one complex per asymmetric unit. The structure was determined by MAD phasing in which the eight methionines of IFN-[[gamma]] were substituted by selenomethionine. Data were measured at four different wavelengths using frozen crystals on CHESS beamline F-2. The complex consists of one IFN-[[gamma]] dimer, two ordered receptors and one disordered receptor; the IFN-[[gamma]] and ordered receptors show approximate two-fold symmetry. The IFN-[[gamma]] dimer is made up of identical subunits with six [[alpha]] helices each. The subunits are interdigitated such that four helices from one subunit form a cleft that cradles the C-terminal helix from the other subunit. Each receptor molecule contains two 100 amino acid domains joined by a hinge. Each of these domains contains seven [[beta]] strands with a fribronectin type III fold. In the crystal structure, the receptor molecules are truncated at the point where they normally attach to the cell membrane. Most of the interactions between IFN-[[gamma]] and the two ordered R[[alpha]]'s occur between an N-terminal helix, C-terminal helix and the loop connecting helices A and B of IFN-[[gamma]] and the N-terminal domain of the receptor. There is no interaction between these two R[[alpha]] chains within one complex. A third partially disordered R[[alpha]] chain was observed in the electron density maps. The C-terminal domain of the disordered receptor packs against the C-terminal domain of an ordered receptor chain. The N-terminal domain of the third receptor is almost completely disordered and is located near the IFN-[[gamma]] molecule. Several pieces of evidence suggest that the disordered R[[alpha]] chain may be mimicking an R[[beta]] chain. This observation has lead to a hypothetical model of a ternary IFN-[[gamma]]/R[[alpha]]/R[[beta]] complex.