Randomization of the receptor alpha chain recruitment epitope reveals a functional interleukin-5 with charge depletion in the CD loop

We report the functional phage display of single chain human interleukin-5 (scIL-5) and its use for receptor-binding epitope randomization. Enzyme-lin ked immunosorbent assays and optical biosensor analyses verified expression of scIL-5 on the phage surface and binding of scIL-5 phage to interleukin- 5 receptor Lu chain. Furthermore, an asymmetrically disabled but functional scIL-8 mutant, (wt/A5)scIL-5, was displayed on phage, (wt/A5)scIL-5 was co nstructed from an N-terminal half containing the original five charged resi dues ((EERRR92)-E-88) in the CD loop, including the Glu(89) and Arg(91) bel ieved key in the alpha chain recognition site, combined with a C-terminal h alf containing a disabled CD loop sequence ((88)AAAAA(92)) missing the key recognition residues. This asymmetric variant was used as a starting point to generate an scIL-5 library in which the intact 88-92 N-terminal CD loop was randomized. From this epitope library, a receptor-binding variant of IL -5 was detected, (SLRGG/A5)scIL-5, in which the only charged residue in the CD loop is an Arg at position 90, Characterization of this variant express ed as a soluble protein in E. coli shows that the IL-5 pharmacophore for re ceptor alpha chain binding can function with a single positive charge in th e CD loop. Charge-depleted CD loop mimetics of IL-5 suggest the importance of charge distribution in functional IL-5 receptor recruitment.