Biological and receptor-binding activities of human interleukin-2 mutated at residues 20Asp, 125Cys or 127Ser.

We have used site-directed mutagenesis to analyse structure-function relationships of the human Interleukin-2 molecule. The mutations introduced targetted residue 20Asp, within the N-terminal A helix, as well as residues 125Cys and 127Ser in the C-terminal D helix. The results presented here demonstrate that destabilizing the C-terminus alpha helix through introduction of Pro residues in either positions 125 or 127 reduced the biological activity of IL-2 by a factor of 10 that was correlated with a decreased ability to bind the receptor. A number of substitutions in position 20 have an even more drastic effect on biological activity and receptor binding. However, specific substitutions such as 20Asn and 20Leu displayed a differential effect on human or mouse IL-2 receptors. Furthermore, 20Leu IL-2 was found to behave as a partial antagonist of natural IL-2 when tested on murine cells.