LIGAND-INDUCED ASSEMBLY AND ACTIVATION OF THE GAMMA-INTERFERON RECEPTOR IN INTACT-CELLS

Functionally active gamma interferon (IFN-gamma) receptors consist of an alpha subunit required for ligand binding uction and a beta subunit required primarily for signaling. Although the receptor alpha chain h as little is known about the specific role of the receptor beta chain in IFN-gamma signaling. Expression of the wild-type human IFN-gamma re ceptor beta chain in murine L cells that stably express the human IFN- gamma receptor alpha chain (L.hgR) produced a murine cell line (L.hgR. myc beta) that responded to human IFN-gamma. Mutagenesis of the recept or beta-chain intracellular domain revealed that only two closely spac ed, membrane-proximal sequences (P263PSIP267 and I(270)EEYL(274)) are required for function. Coprecipitation, studies showed that these sequ ences are necessary for the specific and constitutive association of t he receptor beta Chain with the JAK-2 tyrosine kinase. These experimen ts also revealed that the IFN-gamma receptor alpha and beta chains are not preassociated on the surface of unstimulated cells but rather are induced to associate in an IFN-gamma-dependent fashion. A chimeric pr otein in which the intracellular domain of the beta chain was replaced by JAK-2 complemented human IFN-gamma signaling and biologic responsi veness in L.hgR. In contrast, a c-src containing beta-chain chimera di d not. These results indicate that the sole obligate role of the IFN-g amma receptor beta chain in signaling is to recruit JAK-2 into the lig and-assembled receptor complex.