How does IL-2 transduce its signals?
Analogous to other cytokines/growth factors, IL-2 activates a number of signaling pathways. As shown in Figure 1, the IL-2 receptor is composed of three subunits, IL-2R, IL-2R, and c (Lin and Leonard, 1997; Sugamura et al., 1996; Taniguchi, 1995). On the basis of their binding affinities to IL-2, the receptor complexes are denoted as low- ( chain only, Kd=10-8 M), intermediate- ( and c, Kd=10-9 M) and high-affinity (, and c, Kd=10-11 M) receptors (Lin and Leonard, 1997). Only the intermediate- and high-affinity receptors are able to transduce IL-2 signals, and the cytoplasmic domains of both IL-2R and c are essential for this purpose. Like most type I cytokine receptors, the cytoplasmic domains of both IL-2R and c contain certain structural features, including ‘Box1/Box2’ regions for binding JAK kinases, and a number of tyrosine residues (Bazan, 1990; Leonard and O’Shea, 1998). At least in the case of IL-2R, phosphorylation of some of these tyrosine residues creates docking sites for t
