In this study we demonstrate that 125I-labelled interleukin (IL) 1 alpha binds specifically to its receptor on the surface of EL4 6.1 cells and is subsequently endocytosed and translocated from the cell membrane to the nucleus, where it progressively accumulates. Two-dimensional polyacrylamide-gel electrophoresis revealed that the internalized 125I-IL1 alpha associated with the nucleus was intact, with negligible breakdown products present. Specific and saturable binding of 125I-IL1 alpha was demonstrated on purified nuclei isolated from these cells. Binding of the radiolabelled ligand showed similar kinetics to that of the plasma-membrane receptor, and was inhibited by both unlabelled IL1 alpha and IL1 beta. Equilibrium binding studies on isolated nuclei revealed a single high-affinity binding site, with a Kd of 17 +/- 2 pM, and 79 +/- 12 binding sites per nucleus. These studies demonstrate that receptor-mediated endocytosis of IL1 results in its accumulation in the nucleus, and this mechanism may play an important role in mediating some of the actions of IL1.

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