Compensatory Energetic Mechanisms Mediating the Assembly of Signaling Complexes Between Interleukin-2 and its α, β, and γc Receptors

Interleukin-2 is a key immuno-regulatory cytokine whose actions are mediated by three different cell surface receptors: the α, β and the “common γ” (γc) chains. We have undertaken a complete thermodynamic characterization of the stepwise assembly cycle for multiple possible combinations of the receptor–ligand, and receptor–receptor interactions that are necessary for formation of the high-affinity IL-2/αβγc signaling complex. We find an entropically favorable high affinity interaction between IL-2 and its α receptor, a moderately entropically favorable low affinity interaction between IL-2 and its β receptor, and no interaction between IL-2 and the shared receptor, γc. Formation of the stable intermediate trimolecular complexes of IL-2 with α and β receptors, as well as IL-2 with β and γc receptors proceeds through enthalpy–entropy compensation mechanisms. Surprisingly, we see a moderate affinity interaction between the unliganded receptor α and β chains, suggesting that a preformed αβ complex may serve as the initial interaction complex for IL-2. Reconstitution of the IL-2/Rαβγc high-affinity quaternary signaling complex shows it to be assembled through cooperative energetics to form a 1:1:1:1 assembly. Collectively, the favorable entropy of the bimolecular interactions appears to be offset by the loss in rigid body entropy of the receptor components in the higher-order complexes, but overcome by the formation of increasingly enthalpically favorable composite interfaces. This enthalpic mechanism utilized by γc contrasts with the favorable entropic mechanism utilized by gp130 for degenerate cytokine interaction. In conclusion, we find that several energetically redundant pathways exist for formation of IL-2 receptor signaling complexes, suggesting a more complex equilibrium on the cell surface than has been previously appreciated.