A larger number of L chains (Tac) enhance the association rate of interleukin 2 to the high affinity site of the interleukin 2 receptor.
The IL-2-R is composed of at least two proteins, that is, a 55-kD protein (p55, the L chain, or Tac) and a 75-kD protein (p75, the H chain, or converter). The high affinity binding of IL-2 results in the formation of the ternary complex consisting of IL-2, and the L and H chains. To distinguish the affinity conversion model and the binary complex model we have carried out kinetic studies on the IL-2 binding to the high affinity IL-2-R on T lymphocytes expressing various numbers of L chains and a relatively constant number of H chains. We found that expression of a larger number of L chains accelerated the association of IL-2 to the high affinity receptor. The results are not compatible with the binary complex model that assumes a fixed number of high affinity sites determined by the numbers of a limiting chain. Instead, the results are consistent with the prediction of the affinity conversion model that assumes association of IL-2 to the L chain as the first step of the ternary complex formation and they indicate that the possible role of excess L chains is to accelerate the formation of the ternary complex. The reaction rate constants calculated from the affinity conversion model were reasonably constant.
Saito
Y
Y
Department of Medical Chemistry, Kyoto University Faculty of Medicine, Japan.
Sabe
H
H
Suzuki
N
N
Kondo
S
S
Ogura
T
T
Shimizu
A
A
Honjo
T
T
D006801:Humans; D066298:In Vitro Techniques; D007376:Interleukin-2 / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances; D015375:Receptors, Interleukin-2 / Q000378:metabolism*; D013329:Structure-Activity Relationship; D014407:Tumor Cells, Cultured
D007376:Interleukin-2; D046911:Macromolecular Substances; D015375:Receptors, Interleukin-2