title: Conservation of functional sites on interleukin-6 and implications for evolution of signaling complex assembly and therapeutic intervention.
abstract: A number of secreted cytokines, such as interleukin-6 (IL-6), are attractive targets for the treatment of inflammatory diseases. We have determined the solution structure of mouse IL-6 to assess the functional significance of apparent differences in the receptor interaction sites (IL-6Rα and gp130) suggested by the fairly low degree of sequence similarity with human IL-6. Structure-based sequence alignment of mouse IL-6 and human IL-6 revealed surprising differences in the conservation of the two distinct gp130 binding sites (IIa and IIIa), which suggests a primacy for site III-mediated interactions in driving initial assembly of the IL-6/IL-6Rα/gp130 ternary complex. This is further supported by a series of direct binding experiments, which clearly demonstrate a high affinity IL-6/IL-6Rα-gp130 interaction via site III but only weak binding via site II. Collectively, our findings suggest a pathway for the evolution of the hexameric, IL-6/IL-6Rα/gp130 signaling complex and strategies for therapeutic targeting. We propose that the signaling complex originally involved specific interactions between IL-6 and IL-6Rα (site I) and between the D1 domain of gp130 and IL-6/IL-6Rα (site III), with the later inclusion of interactions between the D2 and D3 domains of gp130 and IL-6/IL-6Rα (site II) through serendipity. It seems likely that IL-6 signaling benefited from the evolution of a multipurpose, nonspecific protein interaction surface on gp130, now known as the cytokine binding homology region (site II contact surface), which fortuitously contributes to stabilization of the IL-6/IL-6Rα/gp130 signaling complex.
authors.affiliation: Department of Biochemistry, University of Leicester, Henry Wellcome Building, Lancaster Road, Leicester LE1 9HN, United Kingdom.
authors.forename: Vaclav
authors.initials: V
authors.lastname: Veverka
authors.forename: Terry
authors.initials: T
authors.lastname: Baker
authors.forename: Nicholas T
authors.initials: NT
authors.lastname: Redpath
authors.forename: Bruce
authors.initials: B
authors.lastname: Carrington
authors.forename: Frederick W
authors.initials: FW
authors.lastname: Muskett
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Taylor
authors.forename: Alastair D G
authors.initials: AD
authors.lastname: Lawson
authors.forename: Alistair J
authors.initials: AJ
authors.lastname: Henry
authors.forename: Mark D
authors.initials: MD
authors.lastname: Carr
mesh_terms: D000818:Animals; D001665:Binding Sites; D050822:Cytokine Receptor gp130 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D019143:Evolution, Molecular; D006801:Humans; D007249:Inflammation / Q000235:genetics / Q000378:metabolism / Q000628:therapy; D015850:Interleukin-6 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D051379:Mice; D046912:Multiprotein Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D010449:Peptide Mapping / Q000379:methods; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D019947:Receptors, Interleukin-6 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment
chemical_list: C508600:IL6 protein, human; C508603:IL6R protein, human; C494199:IL6ST protein, human; D015850:Interleukin-6; D046912:Multiprotein Complexes; D019947:Receptors, Interleukin-6; D050822:Cytokine Receptor gp130