Stepwise formation of the high-affinity complex of the interleukin 2 receptor

The interleukin 2 receptor (IL-2R) is composed of at least two polypeptides-a 55 kd protein (the L chain, p55, or α chain) and a 75 kd protein (the H chain, p75 or β chain). The high-affinity binding of IL-2 results in the formation of a ternary complex consisting of IL-2 and the L and H chains. We found that the rate of high-affinity complex formation at 0°C was about one-third of that at 37°C. The reduction of high-affinity complex formation rate at lower temperatures correlates with the reduction of lateral diffusion within the membrane at lower temperatures. An anti-H chain antibody (2RB) inhibited the formation of the high-affinity complex at 0°C but not at 37°C. We studied the kinetics of the high-affinity complex formation at 37°C after the preincubation of ATL-2 cells with the 2RB antibody and IL-2 at 0°C. We found that the amount of IL-2 complex formed initially at 0°C was almost equivalent to the amount of high-affinity complex formed by subsequent incubation at 37°C. These results suggest that the IL-2̇L complex might be converted to the high-affinity IL-2R complex. Similar experiments using YTC3 cells, which express a smaller number of L chains, showed slower rates of high-affinity complex formation, in agreement with the affinity conversion/stepwise binding model.