Fig 6.

(a) Space-filling model of the proposed hexameric receptor complex, consisting of two molecules each of IL-6, IL-6R, and gp130 chains (shown in purple and blue, light and dark green, orange and red, respectively). IL-6 is labeled with the binding sites I, II, and III. (b) A modular representation of the hexameric complex (D₁–D₆ are labeled 1–6) colored as in a, but with the additional three fibronectin domains (D₄–D₆) of gp130 that have not been modeled placed to indicate how the association of the two D_5′ modules from both gp130 molecules, under the “tunnel” of the IL-6R dimer, can activate signaling by dimerization of the cytoplasmic domains (white circles). The membrane proximal extracellular region of IL-6R (49 residues) is shown as an extended chain connecting IL-6R to the membrane.