The two-chain structure of high-affinity IL-2 receptors.

Binding studies with radiolabeled interleukin 2 (IL-2) have suggested that T cells possess two classes of IL-2 binding site with different affinities but a shared epitope named Tac. The gene for a 55kDa Tac-positive protein has been cloned but on transfection induced only the expression of low-affinity IL-2 binding sites. The structure of the receptor has therefore been perplexing. Here Kendall Smith discusses recent studies which disclose the existence of a new Tac-negative 75kDa IL-2 binding protein and he suggests that a high affinity IL-2 receptor consists of an α(p75) chain non-covalently linked to a β(p55) chain.