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Figure 3.
Fig. 3. Stepwise energetic and structural assembly of the
functional human IL-6 hexamer signaling complex. Isothermal
titration calorimetry (ITC) was used to measure the
thermodynamic parameters for each step in the hexamer assembly
pathway. The ITC titrations are designated as follows: site I:
IL-6 and IL-6R  forming a binary
complex; site II: single-chain (sc) IL-6/IL-6R and gp130 D2D3
form a nonsignaling binary complex in the absence of the gp130
D1 domain; sites II and III: scIL-6/IL-6R and gp130 D1D2D3
form the signaling-competent hexamer; sites II and III 6 domain:
scIL-6/IL-6R and gp130
D1D2D3D4D5D6 resulting in the hexamer with the three
membrane-proximal domains of gp130. As discussed in the text,
the single-chain version of the IL-6/R complex was used
for these measurements to deconvolute the trimolecular
equilibrium (i.e., IL-6 + IL-6R + gp130) into a
bimolecular interaction event (i.e., single-chain IL-6/IL-6R
+ gp130).
Thermodynamic parameters for each titration are provided in
tabular format. Surfaces are calculated from the coordinates of
the hexamer components. The D1 domains of IL-6R are included as
schematic modules.
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