Molecular Dynamics Simulations Reveal Key Roles of the Interleukin-6 Alpha Receptor in the Assembly of the Human Interleukin-6 Receptor Complex

Human interleukin-6 (hIL-6) is a pleiotropic cytokine with three distinct receptor epitopes, termed sites I, II, and III, which function to assemble a signaling complex. hIL-6 signals via a glycoprotein 130 (gp130) homodimer after initially forming a heterodimer with the nonsignaling alpha-receptor (IL-6R alpha). The molecular description of the assembly of the hIL-6 signaling complex remains elusive because available structures provide descriptions of hIL-6 in its free and fully bound receptor forms, but not for intermediate steps that are crucial in the stepwise assembly of the signaling complex. In this report, molecular dynamics simulations provide atomic details describing the functional role of the initial hIL-6/IL-6R alpha complex in facilitating subsequent interactions with gp130, which have not been previously shown. IL-6R alpha binding to hIL-6 rigidifies the flexible N-terminus of the hIL-6 AB-loop through interactions with the D2 domain of IL-6R alpha. This rigidification Combined with repositioning, of residues involved in gp130 receptor recognition promotes gp130 binding at site III. Binding of gp130 receptors at sites II and III is coupled with the release of the hIL-6 N-terminal AB-loop interaction and a pivoting of IL-6R alpha around the hIL-6 helix bundle to the state of the hIL-6/IL-6R alpha/gp130 complex.