THE N-TERMINUS AND C-TERMINUS OF IFN-GAMMA ARE BINDING DOMAINS FOR CLONED SOLUBLE IFN-GAMMA RECEPTOR

The mechanism of binding of murine IFN-gamma to its receptor has not been determined. We have studied this mechanism by examining the binding of overlapping synthetic peptides of IFN-gamma to cloned soluble murine IFN-gamma-R. IFN-gamma(1-39) and IFN-gamma(95-133) were able to compete with [I-125]IFN-gamma for binding to cloned soluble receptor. Peptides corresponding to the inner region of IFN-gamma-IFN-gamma(36-60), IFN-gamma(54-91), and IFN-gamma(78-107)-showed a markedly reduced ability to compete with [I-125]IFN-gamma for receptor binding relative to the N-terminal and C-terminal peptides. In direct binding studies, the binding of [I-125]IFN-gamma(1-39) to soluble receptor could only be competed by IFN-gamma(1-39) and IFN-gamma and not by any of the other peptides including IFN-gamma(95-133). This suggests that the N- and C-termini of IFN-gamma bind to different regions of the receptor. These data in conjunction with previous structure/function studies and x-ray crystallographic data have allowed us to formulate a "velcro-key" model of IFN-gamma binding to receptor that involves both the N- and C-terminal domains. The N-terminus binds in the classical "lock-and-key" manner characterized by specific ligand-receptor binding. The hydrophilic C-terminus binds to a region of the receptor distinct from the N-terminus likely through the polycationic region, which is conserved across species barriers. Binding of this type would exhibit high affinity and low specificity similar to a piece of velcro. This interaction becomes specific when the C-terminus is in the context of the whole IFN-gamma molecule and may act to increase the affinity of receptor binding and/or facilitate signal transduction.