Ligand-induced assembly and activation of the gamma interferon receptor in intact cells

Functionally active gamma interferon (IFN-γ) receptors consist of an α subunit required for ligand binding and signal transduction and a β subunit required primarily for signaling. Although the receptor α chain has been well characterized, little is known about the specific role of the receptor β chain in IFN-γ signaling. Expression of the wild-type human IFN-γ receptor β chain in murine L cells that stably express the human IFN-γ receptor α chain (L.hgR) produced a murine cell line (L.hgR.mycβ) that responded to human IFN-γ. Mutagenesis of the receptor β-chain intracellular domain revealed that only two closely spaced, membrane-proximal sequences (P₂₆₃PSIP₂₆₇ and I₂₇₀EEYL₂₇₄) are required for function. Coprecipitation studies showed that these sequences are necessary for the specific and constitutive association of the receptor β chain with the JAK- 2 tyrosine kinase. These experiments also revealed that the IFN-γ receptor α and β chains are not preassociated on the surface of unstimulated cells but rather are induced to associate in an IFN-γ-dependent fashion. A chimeric protein in which the intracellular domain of the β chain was replaced by JAK-2 complemented human IFN-γ signaling and biologic responsiveness in L.hgR. In contrast, a c-src-containing β-chain chimera did not. These results indicate that the sole obligate role of the IFN-γ receptor β chain in signaling is to recruit JAK-2 into the ligand-assembled receptor complex.