Interleukin-7 (IL7) signaling is believed to resemble that of other gammaC family receptors, based on detailed studies of the Interleukin-2 receptor. Extending this model to IL7 suggested a series of events that bring Tyrosine-protein kinase JAK1 (JAK1) and Tyrosine-protein kinase JAK3 (JAK3) into proximity within a complex IL7:IL7R:JAK1:IL2RG:JAK3. Cytoplasmic domains of the receptor chains re-orient so that their associated kinases (JAKs and possibly phosphatidylinositol 3-kinases) can phosphorylate sequence elements on the cytoplasmic domains (Jiang et al. 2005). Tyrosine-449 (Y449) in the cytoplasmic domain of Interleukin-7 receptor is required for T-cell development in vivo and for activation of the JAK/STAT5 and PI3K/Akt pathways (Jiang et al. 2004, Pallard et al. 1999).
It has been sugggested that JAK1 phosphorylates IL7R (Jiang et al. 2004) and it is believed that JAK3, associated with IL2RG, phosphorylates the tyrosine residues in the cytoplasmic portion of IL7R that lead to recruitment of STATs (Fry & Mackall 2002). This is consistent with the lack of intrinsic tyrosine kinase activity in IL7R:JAK1 in the absence of IL2RG:JAK3 (Lai et al. 1996). Phosphorylated Y449 is believed to be the docking site for STAT5 and possibly PI3K, which are then activated by JAKs (Lin et al. 1995, Jiang et al. 2004). T-cells from IL7R Y449F knock-in mice did not activate Signal transducer and activator of transcription A or B (STAT5A, STAT5B) (Osbourne et al. 2007), indicating that IL7 regulates STAT5 activity via this key tyrosine.