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 alpha chain) and a 75 kd protein (the H chain, p75 or beta 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 degrees C was about one-third of that at 37 degrees 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 degrees C but not at 37 degrees C. We studied the kinetics of the high-affinity complex formation at 37 degrees C after the preincubation of ATL-2 cells with the 2RB antibody and IL-2 at 0 degrees C. We found that the amount of IL-2.L complex formed initially at 0 degrees C was almost equivalent to the amount of high-affinity complex formed by subsequent incubation at 37 degrees C. These results suggest that the IL-2.L 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.