Functional implications of large backbone amplitude motions of the glycoprotein 130‐binding epitope of interleukin‐6

Human interleukin ( IL )‐6 plays a pivotal role in the immune response, hematopoiesis, the acute‐phase response, and inflammation. IL ‐6 has three distinct receptor epitopes, termed sites I, II , and III , that facilitate the formation of a signaling complex. IL ‐6 signals via a homodimer of glycoprotein 130 (gp130) after initially forming a heterodimer with the nonsignaling α‐receptor [ IL ‐6 α‐receptor ( IL ‐6R)] via site I. Here, we present the backbone dynamics of apo‐ IL ‐6 as determined by analysis of NMR relaxation data with the extended model‐free formalism of Lipari and Szabo. To alleviate significant resonance overlap in the HSQC ‐type spectra, cell‐free protein synthesis was used to selectively 15 N‐label residues, thereby ensuring a complete set of residue‐specific dynamics. The calculated order parameters [square of the generalized model‐free order parameter ( S 2 )] showed significant conformational heterogeneity among clusters of residues in IL ‐6. In particular, the N‐terminal region of the long AB ‐loop, which corresponds spatially to one of the gp130 receptor binding epitopes (i.e. site III ), experiences substantial fluctuations along the conformation of the main chain ( S 2  = 0.3–0.8) that are not observed at the other two epitopes or in other cytokines. Thus, we postulate that dynamic properties of the AB ‐loop are responsible for inhibiting the interaction of IL ‐6 with gp130 in the absence of the IL ‐6R, and that binding of IL ‐6R at site I shifts the dynamic equilibrium to favor interaction with gp130 at site  III . In addition, molecular dynamics simulations corroborated the NMR ‐derived dynamics, and showed that the BC ‐loop adopts different substates that possibly play a role in facilitating receptor assembly.