Structure‐Function analysis of human IL‐6: Identification of two distinct regions that are important for receptor binding

Abstract

Interleukin‐6 (IL‐6) is a multifunctional cytokine that plays an important role in host defense. It has been predicted that IL‐6 may fold as a 4 α‐helix bundle structure with up‐up‐down‐down topology. Despite a high degree of sequence similarity (42%) the human and mouse IL‐6 polypeptides display distinct species‐specific activities. Although human IL‐6 (hIL‐6) is active in both human and mouse cell assays, mouse IL‐6 (mIL‐6) is not active on human cells. Previously, we demonstrated that the 5 C‐terminal residues of mIL‐6 are important for activity, conformation, and stability (Ward LD et al., 1993, Protein Sci 2:1472–1481). To further probe the structure‐function relationship of this cytokine, we have constructed several human/mouse IL‐6 hybrid molecules. Restriction endonuclease sites were introduced and used to ligate the human and mouse sequences at junction points situated at Leu‐62 (Lys‐65 in mIL‐6) in the putative connecting loop AB between helices A and B, at Arg‐113 (Val‐117 in mIL‐6) at the N‐terminal end of helix C, at Lys‐150 (Asp‐152 in mIL‐6) in the connecting loop CD between helices C and D, and at Leu‐178 (Thr‐180 in mIL‐6) in helix D. Hybrid molecules consisting of various combinations of these fragments were constructed, expressed, and purified to homogeneity. The conformational integrity of the IL‐6 hybrids was assessed by far‐UV CD. Analysis of their biological activity in a human bioassay (using the HepG2 cell line), a mouse bioassay (using the 7TD1 cell line), and receptor binding properties indicates that at least 2 regions of hIL‐6, residues 178–184 in helix D and residues 63–113 in the region incorporating part of the putative connecting loop AB through to the beginning of helix C, are critical for efficient binding to the human IL‐6 receptor. For human IL‐6, it would appear that interactions between residues Ala‐180, Leu‐181, and Met‐184 and residues in the N‐terminal region may be critical for maintaining the structure of the molecule; replacement of these residues with the corresponding 3 residues in mouse IL‐6 correlated with a significant loss of α‐helical content and a 200‐fold reduction in activity in the mouse bioassay. A homology model of mIL‐6 based on the X‐ray structure of human granulocyte colony‐stimulating factor is presented.