Creation of a biologically active interleukin-5 monomer

INTERLEUKIN-5 (IL-5) specifically induces the differentiation of eosinophils, which are important in host defence and the patho-genesis of allergies and asthma 1,2 . Structurally, IL-5 is a unique member of the short-chain helical-bundle subfamily of cytokines whose canonical motif contains four helices (A–D) arranged in an up–up–down–down topology 3,4 . In contrast to other subfamily members, which fold unimolecularly into a single helical bundle 5–8 , IL-5 forms a pair of helical bundles by the interdigita-tion of two identical monomers that contribute a D helix to the other's A–C helices 3 . We predicted that the lack of bioactivity by an IL-5 monomer 9 was due to a short loop between helices C and D which physically prevents unimolecular folding of helix D into a functionally obligate structural motif. Here we report that, by lengthening this loop, we have engineered an insertional mutant of IL-5 that was expressed as a monomer with biological activity similar to that of native IL-5. These studies demonstrate that all of the structural features necessary for IL-5 to function are contained within a single helical bundle.