Interleukin‐7 induces the association of phosphatidylinositol 3‐kinase with the α chain of the interleukin‐7 receptor

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Abstract
The recently characterized receptor for interleukin (IL)‐7 (IL‐7R) includes a unique α chain as well as a common γ chain shared with the receptors for IL‐2 and IL‐4. Engagement of the IL‐7R activates the intracellular enzyme phosphatidylinositol (PtdIns) 3‐kinase but the mechanism of PtdIns 3‐kinase activation and the molecular basis of its interaction with IL‐7R are not known. Here we show that IL‐7 causes the 85‐kDa regulatory subunit of PtdIns 3‐kinase (p85), and PtdIns 3‐kinase activity, to associate with the IL‐7R. This interaction can be ascribed to ligand‐induced phosphorylation of a single Tyr residue in the receptor's unique α chain. Herbimycin A, a specific protein tyrosine kinase inhibitor, suppresses not only tyrosine phosphorylation of the IL‐7R but also its association with p85. A phosphopeptide corresponding to the sequence surrounding Tyr449 in the cytoplasmic tail of the IL‐7R α chain, but not its non‐phosphorylated analogue or phosphopeptides coincident with the sequences surrounding other α chain Tyr residues, efficiently competes out p85 binding. Replacement of Tyr449 with Phe results in a loss of p85 binding. Finally, soluble forms of the src homology 2 domains of p85, which bind directly to phosphotyrosyl peptides, specifically inhibit the association of p85 with the IL‐7R. Thus, PtdIns 3‐kinase recruitment occurs through a single, phosphotyrosine dependent recognition motif surrounding Tyr449 in the IL‐7R α chain. This motif corresponds to a canonical sequence for p85 binding, Tyr(P)‐X‐X‐Met. Since the closely related IL‐2R and IL‐4R also activate PtdIns 3‐kinase but are devoid of such canonical motifs, our results suggest that the mechanism by which IL‐7R recruits and activates PtdIns 3‐kinase differs fundamentally from that used by the other receptors. PtdIns 3‐kinase may, therefore, play a unique and important role in the biological response to IL‐7.