Murine interferon-β receptor-mediated endocytosis and nuclear membrane binding

Abstract

Radioiodinated mouse interferon-β (125I-MuIFN-β) bound with high affinity (K(d) = 9.8 x 10-10 M) to plasma membrane of L929 murine fibroblasts (4-6 x 103 receptor 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°C was very rapidly internalized upon warming of the cells to 37°C (t( 1/2 ) = 1.5 min). Indirect immunoferritin labeling indicated that MuIFN-β was initially located in coated pits and subsequently internalized by receptor-mediated endocytosis. Isolated L929 cell nuclei bound 125I-MuIFN-β with a 7-fold higher affinity (K(d) = 1.4 x 10-10 M) and higher receptor density (about 104 per 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.