Interleukin 9 (IL-9) exerts its pleiotropic effects through the IL-9 receptor (IL-9R) complex, which consists of the IL-9R α-chain, which determines the cytokine specificity, and the IL-2 receptor γ-chain. In the present study we used a modified yeast two-hybrid system to isolate cDNA species encoding proteins that interacted with the intracellular domain of the human IL-9R α-chain (hIL-9Rα). We have identified 14-3-3ζ as an hIL-9Rα-interacting protein. We also mapped residues 518-522 (Arg-Ser519-Trp-Thr521-Phe) in hIL-9Rα and helix I of 14-3-3ζ as being important for interaction. Moreover, peptide competition experi-ments suggested that interaction between hIL-9Rα and 14-3-3ζ requires the phosphorylation of Ser519 or Thr521. This is the first demonstration that 14-3-3 can interact with a non-tyrosine kinase receptor. The interaction between 14-3-3 and IL-9Rα but not IL-4Rα also suggests a potential role for 14-3-3 in determining cytokine specificity.

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