The Extracellular and Transmembrane Domains of the γC and Interleukin (IL)-13 Receptor α1 Chains, Not Their Cytoplasmic Domains, Dictate the Nature of Signaling Responses to IL-4 and IL-13

Previously, we demonstrated that the gamma C subunit of type I IL-4 receptor was required for robust tyrosine phosphorylation of the downstream adapter protein, IRS-2, correlating with the expression of genes (ArgI, Retnla, and Chi3l3) characteristic of alternatively activated macrophages. We located an I4R-like motif (IRS-2 docking sequence) in the gamma C cytoplasmic domain but not in the IL-13R alpha 1. Thus, we predicted that the gamma C tail directed enhanced IRS-2 phosphorylation. To test this, IL-4 signaling responses were examined in a mutant of the key I4R motif tyrosine residue (Y325F) and different gamma C truncation mutants (gamma 285, gamma 308, gamma 318, gamma 323, and gamma FULL LENGTH (FL)) co-expressed in L-cells or CHO cells with wild-type (WT) IL-4R alpha. Surprisingly, IRS-1 phosphorylation was not diminished in Y325F L-cell mutants suggesting Tyr-325 was not required for the robust insulin receptor substrate response. IRS-2, STAT6, and JAK3 phosphorylation was observed in CHO cells expressing gamma 323 and gamma FL but not in gamma 318 and gamma 285 mutants. In addition, when CHO cells expressed gamma 318, gamma 323, or gamma FL with IL-2R beta, IL-2 induced phospho-STAT5 only in the gamma 323 and gamma FL clones. Our data suggest that a smaller (5 amino acid) interval than previously determined is necessary for JAK3 activation/gamma C-mediated signaling in response to IL-4 and IL-2. Chimeric receptor chains of the gamma C tail fused to the IL-13R alpha 1 extracellular and transmembrane domain did not elicit robust IRS-2 phosphorylation in response to IL-13 suggesting that the extracellular/transmembrane domains of the IL-4/IL-13 receptor, not the cytoplasmic domains, control signaling efficiency. Understanding this pathway fully will lead to rational drug design for allergic disease.