Murine interferon-β Receptor-Mediated Endocytosis and Nuclear Membrane Binding

Radioiodinated mouse interferon-β (125I-MuIFN-β ) bound with high affinity (Kd= 9.8 × 10-10M) to plasma membrane of L929murine fibroblasts (4-6 × 103receptor sites per cell). The binding was saturable and inhibited by a 100-fold excess of unlabeled MuIFN-β but not by excess mouse IFN-γ (MuIFN-γ ). MuIFN-β bound at 4 degrees C was very rapidly internalized upon warming of the cells to 37 degrees C (t1/2= 1.5 min). Indirect immunoferritin labeling indicated that MuIFN-β was initially located in coated pits and subsequently internalized by receptor-mediated endocytosis. Isolated L929cell nuclei bound125I-MuIFN-β with a 7-fold higher affinity (Kd= 1.4 × 10-10M) and higher receptor density (about 104per nucleus) than that for the plasma membrane. Binding to the nuclear membrane was inhibited by a 100-fold excess of unlabeled MuIFN-β but not by excess MuIFN-γ . Trypsin treatment of nuclei decreased IFN binding by 80%, suggesting that the putative nuclear receptors are protein. Specific binding of MuIFN-β to nuclei was also shown by fluorescence and electron microscopy. We propose that the very rapid internalization of MuIFN-β by receptor-mediated endocytosis is important in the cellular processing of IFN and that its high-affinity binding to the nuclear membrane suggests the nucleus as an intracellular site of IFN action.