Murine interferon-beta receptor-mediated endocytosis and nuclear membrane binding.

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Abstract
Radioiodinated mouse interferon-.beta. (125I-MuIFN-.beta.) bound with high affinity (Kd = 9.8 .times. 10-10 M) to plasma membrane of L929 murine fibroblasts (4-6 .times. 103 receptor sites/cell). The binding was saturable and inhibited by a 100-fold excess of unlabeled MuIFN-.beta. but not by excess mouse IFN-.gamma. (MuIFN-.gamma.). MuIFN-.beta. bound at 4.degree. C was very rapidly internalized upon warming of the cells to 37.degree. C (t1/2 = 1.5 min). Indirect immunoferritin labeling indicated that MuIFN-.beta. was initially located in coated pits and subsequently internalized by receptor-mediated endocytosis. Isolated L929 cell nuclei bound 125I-MuIFN-.beta. with a 7-fold higher affinity (Kd = 1.4 .times. 10-10 M) and higher receptor density (about 104/nucleus) than that for the plasma membrane. Binding to the nuclear membrane was inhibited by a 100-fold excess of unlabeled MuIFN-.beta. but not by excess MuIFN-.gamma.. Trypsin treatment of nuclei decreased IFN binding by 80%, suggesting that the putative nuclear receptors are protein. Specific binding of MuIFN-.beta. to nuclei was also shown by fluorescence and EM. The very rapid internalization of MuIFN-.beta. 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.