Comparison of the structure of the murine interleukin 2 (IL 2) receptor on cytotoxic and helper T cell lines by chemical cross-linking of 125I-labeled IL 2

The structure of the murine interleukin 2 receptor (IL 2R), on a cytotoxic (CTLL) and a helper (HT2) cell line, has been studied by a combination of chemical cross-linking with 125I-labeled IL 2 and immunoprecipitation with an anti-receptor monoclonal antibody (7D4). In CTLL cells both methods detected the major 57-kDa IL 2-binding protein and in addition the cross-linking studies revealed the presence of a 70-75-kDa protein associated with the high-affinity receptor. In the HT2 cell line, however, immunoprecipitation studies revealed three additional proteins of 18, 22 and 37 kDa to the expected 50-kDa receptor protein. Again cross-linking studies demonstrated the presence of a 70-75-kDa protein, which was not immunoprecipitable with the 7D4 antibody. The low molecular polypeptides in HT2 cell were associated with the low-affinity receptor and represented most likely breakdown products of the 50-kDa protein. Whereas the 18- and 22-kDa proteins were involved in ligand binding, the 37-kDa fragment carried the epitope recognized by the 7D4 antibody. Comparative studies with two IL 2R antibodies, PC61 and 7D4, revealed that only PC61 inhibited the formation of the IL 2 alpha/beta chain complex, although both antibodies reportedly prevent the biological response to IL 2. It is speculated that the 37-kDa fragment, which reacts with the 7D4 antibody, might be involved in IL 2 signal transduction. Finally there was no evidence for the existence of a high molecular weight component of the IL 2R, previously described as gamma chain. In summary, the two-chain structure of the IL 2R has been confirmed for both murine cell lines with some heterogeneity of the alpha chain. The possibility was raised that a 37-kDa fragment of the alpha chain plays a role of in signal transduction.