Structure analysis of the IL-5 ligand-receptor complex reveals a wrench-like architecture for IL-5Rα.

Interleukin-5 (IL-5) is the key mediator for the function of eosinophil granulocytes, whose deregulation is characteristic of hypereosinophilic diseases and presumably contributes to allergic asthma. IL-5 signaling involves two transmembrane receptors, IL-5Rα and the common ß chain, which upon formation of the ternary complex activate the JAK/STAT signaling cascade. To investigate the mechanism underlying ligand-receptor recognition, we determined the structure of IL-5 bound to the extracellular domain of IL-5Rα. IL-5 makes contact with all three fibronectin III-like domains of IL-5Rα, with the receptor architecture resembling a wrench. Mutagenesis data provide evidence that this wrench-like architecture is likely preformed. The structure demonstrates that for steric reasons, homodimeric IL-5 can bind only one receptor molecule, even though two equivalent receptor-binding sites exist. In regard to strong efforts being made to develop IL-5 antagonists for treating asthma and hypereosinophilic diseases, the advances in molecular understanding provided by this structure are of greatest value.

Type I receptor binding of bone morphogenetic protein 6 is dependent on N-glycosylation of the ligand.

Bone morphogenetic proteins (BMPs), together with transforming growth factor (TGF)-beta and activins/inhibins, constitute the TGF-beta superfamily of ligands. This superfamily is formed by more than 30 structurally related secreted proteins. The crystal structure of human BMP-6 was determined to a resolution of 2.1 A; the overall structure is similar to that of other TGF-beta superfamily ligands, e.g. BMP-7. The asymmetric unit contains the full dimeric BMP-6, indicating possible asymmetry between the two monomeric subunits. Indeed, the conformation of several loops differs between both monomers. In particular, the prehelix loop, which plays a crucial role in the type I receptor interactions of BMP-2, adopts two rather different conformations in BMP-6, indicating possible dynamic flexibility of the prehelix loop in its unbound conformation. Flexibility of this loop segment has been discussed as an important feature required for promiscuous binding of different type I receptors to BMPs. Further studies investigating the interaction of BMP-6 with different ectodomains of type I receptors revealed that N-glycosylation at Asn73 of BMP-6 in the wrist epitope is crucial for recognition by the activin receptor type I. In the absence of the carbohydrate moiety, activin receptor type I-mediated signaling of BMP-6 is totally diminished. Thus, flexibility within the binding epitope of BMP-6 and an unusual recognition motif, i.e. an N-glycosylation motif, possibly play an important role in type I receptor specificity of BMP-6.