In contrast to IL-7–mediated activation of IL-7R, Kit-mediated tyrosine phosphorylation of IL-7R is independent of Jak3. (A) IL-7–induced tyrosine phosphorylation of IL-7R is Jak3 dependent. 293T cells transiently expressing IL-7R and Jak3 (lanes 1,2) or IL-7R alone (lanes 3,4) were stimulated with IL-7 for 10 minutes and tyrosine phosphorylation and protein of γc (top 2 panels) and IL-7Rα (bottom 2 panels) were examined. Tyrosine phosphorylation of γc and IL-7Rα was detected only in IL-7–stimulated cells that express Jak3. (B) KL-induced tyrosine phosphorylation of IL-7R in absence of Jak3. 293T coexpressing Kit and IL-7R (lanes 1-4) in presence (lanes 1,2) or absence (lanes 3,4) of Jak3. Immunoprecipitations against γc (top 2 panels) and IL-7Rα were performed and analyzed for tyrosine phosphorylation and presence of proteins as shown. Specificity of IL-7Rα–associated activated Kit (arrow) was verified by immunoprecipitations from cells expressing only Kit (lanes 5,6).