Analysis of the Role of the Interleukin-2 Receptor γ Chain in Ligand Binding†

Interleukin-2 is the primary T cell growth factor secreted by activated T cells. IL-2 is an α-helical cytokine that binds to a multisubunit receptor expressed on the surface of a variety of cell types. IL-2Rα, IL-2Rβ, andIL-2Ryc receptor subunits expressed on the surface of cells may aggregate to form distinct binding sites of differing affinities. IL-2Ryc was the last receptor subunit to be identified. It has since been shown to be shared by at least five other cytokine receptors. In this study, we have probed the role of IL-2Ryc in the assembly of IL-2R complexes and in ligand binding. We demonstrate that in the absence of ligand IL-2Ryc does not possess detectable affinity for IL-2Ra, IL-2Rβ, or the pseudo-high-affinity binding site composed of preformed IL-2Rα/β. We also demonstrate that IL-2Ryc possesses an IL-2-dependent affinity for IL-2Rβ and IL-2Rα/β. We performed a detailed biosensor analysis to examine the interaction of soluble IL-2Ryc with IL-2-bound IL-2Rβ and IL-2-bound IL-2Rα/β. The kinetic and equilibrium constants for sIL-2Ryc binding to these two different liganded complexes were similar, indicating that IL-2Ra does not play a role in recruitment of IL-2Ryc. We also determined that the binding of IL-2 to the isolated IL-2Rγc was very weak (approximate K D = 0.7 mM). The experimental methodologies and principles derived from these studies can be extended to at least five other cytokines that share IL-2Ryc as a receptor subunit.