Localization of a receptor binding site on the IL-2 molecule

The present study was performed to localize in the IL-2 molecule the active site responsible for interaction with the IL-2 receptor. To predict the receptor binding site on the IL-2 molecule, a computer programme based on the hypothesis that the active site will contain parts of the protein molecule having a high tendency to form a bend was utilized. The tendency to form a bend was evaluated by assessing the probability of beta-turn occurrence; the highest probability was found in the tetrapeptide Asn-Pro-Lys-Leu, occupying the positions 33-36 of the IL-2 molecule. Accordingly, the hexadecapeptide H-Cys-Nle-Gly-Ile-Asn-Asn-Tyr-Lys-Asn-Pro-Lys-Leu-Thr-Arg-Met-Leu-NH2 that spans over the predicted tetrapeptide Asn-Pro-Lys-Leu and comprises the region 27-40 from the IL-2 amino acid sequence was synthesized. This synthetic (I-16) peptide was found to selectively inhibit the IL-2-dependent uptake of 3H-TDR by CTLL cells, apparently by competing with IL-2 for the IL-2 receptor. The synthetic I-16 hexadecapeptide was conjugated to carrier (BSA) protein and used for immunization of rabbits. Resulting I-16 antibodies were capable of binding specifically to the I-16 hexadecapeptide in indirect ELISA test; they reacted substantially with IL-2-producing but not with IL-2-non-producing Jurkat cells in indirect cell membrane immunofluorescence, and inhibited activation of killer spleen cells with human recombinant IL-2 as detected by 51Cr microcytotoxicity assay. Taken together, these results suggest that at least one of the receptor contact sites of the IL-2 is localized within the N-terminal part of the molecule in the region defined by amino acids 27-40 and coded for by the exon 1 and 2.