Mutated interleukin-5 monomers are biologically inactive.

Interleukin-5 contains only two cysteine residues both of which appear to be involved in the dimerisation of the molecule to form a disulphide-linked homodimer (Minamitake et al., J. Biochem. 107, 292-297, 1990). However, it remains unclear whether this linkage is necessary for the bioactivity of this cytokine. Site-directed mutagenesis was used to produce amino acid substitutions of either or both of the cysteines. The mutant proteins were all biologically inactive monomers, however when the two single mutant constructs were co-transfected, biologically active IL5 was produced. This is consistent with the dimer forming in a head-to-tail configuration.