Differential nuclear localization of the IFNGR-1 and IFNGR-2 subunits of the IFN-γ receptor complex following activation by IFN-γ

We have recently identified a nuclear localization sequence (NLS) in the C-terminus of murine type II interferon (IFN), IFN-gamma, that is responsible for the internalization and nuclear translocation of extracellularly added IFN-gamma. Because the uptake of IFN-gamma is a receptor-mediated endocytotic process, we examined in this study the fate of both the receptor subunits (IFNGR-1 and IFNGR-2) of the heterodimeric IFN-gamma receptor complex. Human IFN-gamma (HuIFN-gamma) was also found to contain a polybasic NLS in a conserved C-terminal region capable of directing its nuclear translocation. Like the ligand, the IFNGR-1 subunit of the receptor complex on WISH cells was found to be translocated to the nucleus on treatment with HuIFN-gamma. Using a combination of immunoprecipitation and immunofluorescence techniques, we found the nuclear accumulation of IFNGR-1 to be ligand dependent, and it was evident within 10-20 min after ligand stimulation. IFNGR-1 was found to colocalize, in a time-dependent and dose-dependent fashion, with the nuclear translocation of the transcription factor Stat1 alpha, which is activated by this ligand-receptor system. In addition, Stat1a was found to be complexed with IFNGR-1 over the time period of its nuclear translocation. In marked contrast, IFNGR-2 was not transported to the nucleus. The surface immunofluorescence pattern of IFNGR-2 suggested that, following ligand stimulation, the majority of IFNGR-2 remains at the cell surface, whereas IFNGR-1 is endocytosed and targeted to the cell nucleus. These findings suggest that IFNGR-1 plays an active intracellular role in signal transduction events subsequent to the binding of ligand to the dimeric receptor complex. Furthermore, these studies provide the first example of the selective endocytosis and nuclear translocation of a subunit of a multimeric receptor complex.