Limited tryptic digestion of recombinant human interleukin-2: structure-binding relationships with the alpha chain of the interleukin-2 receptor.

The surface topography and structural features of interleukin-2 (IL-2) in relation to its interaction with the alpha subunit of its receptor (IL-2R alpha) have been probed by limited tryptic digestion followed by detailed structural analyses. Four sensitive cleavage sites in IL-2 (Lys8, Lys9, Lys35, and Arg38) were identified as surface amino acids, suggesting that they are potential binding sites for IL-2R alpha. To examine the involvement of these residues in IL-2R alpha binding, a truncated IL-2 molecule lacking the amino-terminal residues through Arg38 was generated and it was found to be incapable of binding IL-2R alpha in a solid-phase receptor binding sequencing assay. These studies have led to the conclusion that the IL-2R alpha contact region of IL-2 includes residues Lys35 and Arg38. This finding is supported by the refined three-dimensional structure of IL-2 in which these residues are located outside of the compact bundle of four helices and thus are readily available for interaction with IL-2R alpha.