Requirement for an initial signal from the membrane-proximal region of the interleukin 2 receptor γ(c) chain for Janus kinase activation leading to T cell proliferation

The interleukin 2 receptor (IL-2R) generates proliferative signals in T lymphocytes by ligand-induced heterodimerization of two chains, IL-2Rβ and γ(c), which associate with the tyrosine kinases Jak1 and Jak3, respectively. Genetic and molecular studies have demonstrated that Jak3 is essential for mitogenic signaling by the γ(c) chain; because it is also the only molecule known to associate with γ(c), we speculated that Jak3 might be sufficient for signaling by this chain. Therefore, fusion proteins were constructed in which all or part of the cytoplasmic domain of γ(c) was replaced by Jak3. Signaling was evaluated in the IL-2-dependent T cell line CTLL-2 using chimeric IL-2Rβ and γ(c) chains that bind and are activated by the cytokine granulocyte–macrophage colony-stimulating factor. Chimeric γ(c) chains containing only Jak3 in the cytoplasmic domain failed to mediate proliferation of CTLL-2 cells, but addition of a conserved membrane-proximal (PROX) domain of γ(c) in tandem with Jak3 fully reconstituted γ(c) function. The requirement for the PROX domain reflected an essential role in the activation of Jak3 in vivo. Despite lacking defined catalytic motifs, PROX induced an early Jak-independent signal, including tyrosine phosphorylation of IL-2Rβ and the tyrosine phosphatase SHP-2. The results define the minimal signaling components of γ(c) and suggest a new mechanism by which the IL-2R initiates signaling in response to ligand.