The common ␥ chain (␥c), a subunit of the interleukin (IL)-2, IL-4, IL-7, IL-9, and IL-15 receptors, contributes to both cytokine binding and subsequent signal transduction. Using a model-based site-directed mutagenesis strategy, we have identified residues of the mouse ␥c extracellular domain that are required for normal ␥c-dependent enhancement of IL-2 and IL-7 binding. One of these sites, Tyr-103, is homologous to key ligand-interacting residues in the growth hormone and erythropoietin receptors, whereas Cys-161, Cys-210, and Gly-211 may function indirectly by maintaining the functional conformation of ␥c via formation of an intramolecular disulfide bond. These two cysteines are also required for the integrity of a putative epitope recognized by TUGm2, an antagonistic monoclonal antibody that blocks ␥c-dependent cytokine binding and bioactivity. These results are consistent with the involvement of three predicted loops in ␥c that contribute to the binding of both IL-2 and IL-7. Mutations in these loops have also been noted in the ␥c gene of patients with X-linked severe combined immunodeficiency.The common ␥ chain (␥c) 1 is a member of the type I cytokine receptor superfamily and functions as a shared subunit of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15 (reviewed in Refs. 1 and 2). In each of these receptors ␥c regulates signaling via the JAK3 tyrosine kinase associated with its cytoplasmic tail. The biological importance of ␥c is most dramatically illustrated by the fact that mutations of either ␥c or JAK3 are the primary causes of human X-linked severe combined immunodeficiency (X-SCID), characterized by a failure in T and natural killer cell development (3). Although ␥c does not detectably bind cytokines by itself, its recruitment into the IL-2, IL-4, and IL-7 receptor complexes enhances ligand binding. This is most evident in the context of the IL-2 receptor (IL-2R) where ␤␥ heterodimers bind IL-2 with 100-fold higher affinity than IL-2R␤ (4), and the affinity of ␣␤␥ trimers is 10-fold increased over ␣␤ complexes (5). A similar but more modest 3-5-fold increase in affinity has also been observed in the IL-4R (6) and IL-7R (7).Cytokines binding to type I cytokine receptors are typically folded as bundles of four ␣-helices. The receptor subunits, including ␥c, are characterized by extracellular homology regions composed of two fibronectin type III domains that are each composed of seven ␤-strands. The first domain contains four highly conserved cysteine residues linked by disulfide bonds whereas the second domain contains a highly conserved WSXWS motif (8). The prototypical members of the type I cytokine receptor superfamily are the growth hormone and erythropoietin receptors (EPORs) for which the crystal structure has been determined for the extracellular portion of the receptors together with their ligands (9, 10). The structure of the growth hormone receptor (GHR) complex (9) has been used as a template for homology modeling of a number of receptors within the family. Subsequent use of such m...

The common γ-chain (γc), a subunit of the IL-2R, is essential for high affinity ligand binding and signal transduction due to Jak3 association to γc. Another consequence of IL-2/IL-2R interaction is rapid receptor-mediated endocytosis of the receptor-ligand complex. In the present study, we establish that this rapid endocytosis of IL-2 in a T cell tumor line is dependent upon the cytoplasmic tail of γc. Deletion mutants of the cytoplasmic tail mapped this activity to 9 aa of γc, 45–54 aa distal to the transmembrane region. In contrast, ligand-independent constitutive endocytosis of γc occurred more slowly and was dependent upon a PEST sequence in a more membrane-proximal region of the cytoplasmic tail of γc. Thus, this receptor subunit may use distinct sorting signals for its constitutive regulation and ligand-induced endocytosis. Rapid endocytosis of IL-2 was inhibited by the tyrosine kinase inhibitor genistein, implicating a role for a signal transduction pathway in IL-2 internalization. However, one T cell line bearing a mutant γc exhibited impaired endocytosis of IL-2, despite normal IL-2-induced Jak/STAT activation. Furthermore, inefficient endocytosis of IL-2 was noted after transfection of the COS7 epithelial cell line with the IL-2R, and further reconstitution of these cells with Jak/STAT proteins did not enhance this internalization. Collectively, these latter findings indicate that rapid endocytosis of IL-2 is dependent upon cellular signaling in lymphoid cell environment that is not solely a consequence of the presence of the Jak/STAT pathway.

The common ␥-chain (␥ c ) that functions both in ligand binding and signal transduction is a shared subunit of the multichain receptors for interleukin (IL)-2, IL-4, IL-7, IL-9, IL-15, and IL-21. The structural basis by which the ectodomain of ␥ c contributes to binding six distinct cytokines is only partially defined. In the present study, epitope mapping of antagonistic anti-␥ c monoclonal antibodies led to the identification of Asn-128 of mouse ␥ c that represents another potential contact residue that is required for binding IL-2, IL-7, and IL-15 but not IL-4. In addition, Tyr-103, Cys-161, Cys-210, and Cys-211, previously identified to contribute to binding IL-2 and IL-7, were also found to be involved in binding IL-4 and IL-15. Collectively, these data favor a model in which ␥ c utilizes a common mechanism for its interactions with multiple cytokines, and the binding sites are largely overlapping but not identical. Asn-128 and Tyr-103 likely act as contact residues whereas Cys-161, Cys-210, and Gly-211 may stabilize the structure of the proposed ligandinteracting surface formed by the two extracytoplasmic domains.

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