IL-4 induces serine phosphorylation of the STAT6 transactivation domain in B lymphocytes

The binding of IL-4 to its receptor results in rapid tyrosine phosphorylation of STAT6 by IL-4R-associated Jak kinases. Phosphorylated STAT6 dimerizes and translocates to the nucleus where it acts as a transcription factor to regulate a number of important immune response-related genes in a variety of cell types. Studies of other STAT proteins have demonstrated a role for serine phosphorylation in addition to tyrosine phosphorylation in the regulation of STAT-mediated gene transcription. In this study, phosphoamino acid analysis and two-dimensional phosphopeptide mapping of STAT6 from mouse splenic B cells demonstrated that IL-4 induces phosphorylation of STAT6 on multiple serines. Expression and analysis of a mutant STAT6 protein in which tyrosine 641 (Y641) was replaced with phenylalanine demonstrated that Y641 is necessary for tyrosine phosphorylation of STAT6, but that tyrosine phosphorylation is not necessary for serine phosphorylation. Analysis of STAT6 deletion mutants localized the majority of serine phosphorylation sites to a region between residues 719 and 789, within the previously described transactivation domain. IL-4-stimulated serine phosphorylation of STAT6 was resistant to H7 and HA1004, inhibitors of many serine/threonine kinases including PKC. Serine phosphorylation was also resistant to Wortmannin and LY294002, demonstrating that the IRS/PI 3-kinase pathway is also not required. These data, coupled with previous studies showing that IL-4 does not activate MAPK pathways in lymphocytes, suggest that IL-4 may induce serine phosphorylation of STAT6 by a novel-signaling pathway.