RADIATION INACTIVATION OF HUMAN GAMMA-INTERFERON - CELLULAR ACTIVATION REQUIRES 2 DIMERS

gamma-Interferon (IFN-gamma) is a 17-kDa broad-spectrum cytokine which exerts its effects on a variety of target cells through its interacti on with the IFN-gamma receptor. Although physicochemical studies of Es cherichia coli-derived IFN-gamma, as well as its crystal structure, de monstrate that it is a homodimer in solution (M(r) 34,000), previous r adiation inactivation studies yielded a functional size for IFN-gamma of 63-73 kDa in an antiviral assay. To understand the relationship bet ween the solution form of IFN-gamma and the moiety that actually binds to the cellular receptor and activates cells, we examined irradiated nonradioactive and P-32-labeled IFN-gamma for its migration in SDS/pol yacrylamide gels (to determine its physical integrity), its binding to cells, its reactivity in an ELISA, and its antiviral activity. The fu nctional size of IFN-gamma differed in the assays, being 22 +/- 2 kDa for the physical destruction of IFN-gamma, 56 +/- 2 kDa for the cellul ar binding assay, 45-50 kDa for reactivity in the ELISA, and 72 +/- 6 kDa for antiviral activity. The results from the binding assays consti tute direct evidence that IFN-gamma binds to its cellular receptor as a dimer. However, for antiviral activity, the functional mass is equiv alent to a tetramer. This is consistent with models involving ligand-i nduced receptor dimerization, whereby two dimers acting in concert (eq uivalent to the target size of a tetramer) are required to activate ce lls in the antiviral assay.