In its native form IL-5 is a heterogeneous glycoprotein of mean Mr of 45 kDa. The cDNA encodes a protein of 135 residues which includes a leader sequence of 20 residues. Thus the mature polypeptide consists of 115 residues with a calculated mass of 12 000. The active molecule is a homodimer of calculated mass 24 000, indicating that the native molecule is heavily glycosylated. Human IL-5 has N-linked carbohydrate at position Asn28, and O-linked at Thr3, and murine IL-5 is glycosylated at analogous sites, with an additional N-linked oligosaccharide at Asn55. A possible N-linked site at Asn71 is not glycosylated in either species.

The two polypeptide chains form helical bundles each containing three helices (A-C) from one chain and one from the other (D'). The dimeric protein is held together by two disulfide bridges between residues 42 and 84 of opposing chains. Mutation of one of the cysteines prevents dimerization and the resulting monomer is inactive, probably because it is unstable.

Contact points between IL-5 and its receptor chains have been investigated by mutation analysis, constructing species hybrid molecules and the mapping of neutralizing antibodies. These studies indicate that residues in the D helix, Asnl08 and GlullO and in loop 3 (between the C and D helices), Lys84 and Arg91, are important for binding to the receptor a chain. The receptor (3 chain interacts with Glul3 (of helix A). Despite the dimeric nature of IL-5 suggesting two potential receptor binding sites, only one a chain binds to IL-5, although the number of (3 chains in the complex is not known.

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