The interleukin-2 receptor is a heterotrimer composed of interleukin-2 receptor alpha (IL2RA), beta (IL2RB) and gamma (IL2RG) subunits. Individually, IL2RA and IL2RB have low affinity for interleukin-2 (IL2); IL2RG has very low affinity. The IL2RA chain has a short cytoplasmic domain and consequently does not transmit an intracellular signal, but it binds IL-2 with high affinity and is required in vivo for detection of physiological IL-2 levels (Kd for IL-2RB/G = 10-9 M versus 10-11 M for IL-2RA/B/G, Takeshita et al. 1992). The crystal structure of the trimeric complex bound to IL2 suggests that the initiating event is the binding of IL2 to IL2R alpha (Wang et al. 2005). This captures IL2 at the cell surface and allows the recruitment of the beta and gamma subunits, which then participate in signal transduction.
IL-2R alpha chains are expressed at much greater levels than the other receptor chains, usually 10-1000-fold higher compared with IL-2R beta or gamma (~1,000 sites/cell), which are usually expressed in equal numbers (Smith & Cantrell 1985). Recent single cell analysis methods have found that as the density of IL-2R alpha chains varies 1,000-fold from 100 to 100,000 sites/cell, the equilibrium dissociation constant of IL-2 binding varies to the same extent, from 100 pM to 100 fM, with the consequence that as the density of IL-2R alpha chains increases there is a marked improvement in IL-2 binding efficiency and thus signaling (Feinerman O et al. 2010). IL-2 binding to IL-2Ralpha is rapid on and rapid off.