Mouse interleukin-2 structure-function studies: substitutions in the first α-helix can specifically inactivate p70 receptor binding and mutations in the fifth α-helix can specifically inactivate p55 receptor binding

Abstract

The function of two α-helical regions of mouse interleukin-2 were analyzed by saturation substitution analysis. The functional parts of the first α-helix (A) was defined as residues 31-39 by the observation that proline substitutions within this region inactivate the protein. Four residues within α-helix A, Leu31, Asp34, Leu35 and Leu38, were found to be crucial for biological activity. Structural modeling suggested that these four residues are clustered on one face of α-helix A. Residues 31 and 35 had to remain hydrophobic for the molecule to be functional. At residue 38 there was a preference for hydrophobic side chain residues, while at residue 34 some small side chain residues as well as acidic or amide side chain residues were functionally acceptable. Inactivating changes at residue 34 had no effect upon the ability of the protein to interact with the p55 receptor. Disruption of the fifth α-helix (E), which had little effect upon biological activity, resulted in an inability of the protein to interact with the p55 receptor. Mutagenesis of the α-helix E region demonstrated that α-helicity and the nature of the side chain residues in this region were unimportant for biological activity. The region immediately proximal to α-helix E was important only for the single intramolecular disulfide linkage.