Evidence for two extracellular domains in the human interleukin-2 receptor: localization of IL-2 binding

The human interleukin-2 (IL-2) receptor was quantitatively cleaved into two large disulfide-bonded fragments by either trypsin or endoproteinase lys-C (endo lys-C). The smaller fragment contains both N-linked oligosaccharides found in the intact receptor and is derived from the amino terminus of the molecule. The larger proteolytic fragment was metabolically labeled with 32PO4 and represents the carboxy terminus. The predicted cleavage sites of both enzymes lie in the region of the molecule encoded by exon 3. This pattern of limited proteolysis provides biochemical evidence that the extracellular region of the receptor is organized into two domains. This supports a structural model of the receptor in which the regions of internal homology encoded by exons 2 and 4 form independent disulfide-bonded domains connected by a hydrophilic segment. To determine the role of these domains in IL-2 binding, [125I]IL-2 was chemically cross-linked to the proteolytically cleaved receptor on the cell surface. The 125I-labeled complex obtained displayed N-linked oligosaccharides and had an Mr consistent with one molecule of IL-2 cross-linked to the smaller proteolytic fragment of the receptor. Thus, the amino-terminal domain of the IL-2 receptor appears to form an integral part of the IL-2 binding site.