Syk is activated by phosphotyrosine-containing peptides representing the tyrosine-based activation motifs of the high affinity receptor for IgE.

Engagement of the high affinity receptor for immunoglobulin E (Fc epsilon RI) on the surface of mast cells induces tyrosine phosphorylation of numerous cellular proteins. Syk, one of several non-receptor protein tyrosine kinases implicated in Fc epsilon RI signaling, is activated following receptor cross-linking and associates with phosphorylated gamma subunits of Fc epsilon RI. We previously showed that the Src homology 2 (SH2) domains of Syk bind with high affinity to the conserved tyrosine-based activation motif (TAM) of the gamma subunit in vitro. In this report, we show that a tyrosine-phosphorylated gamma TAM peptide induced tyrosine phosphorylation of Syk in RBL-2H3 cell lysates and stimulated Syk kinase activity 10-fold in vitro, with half-maximal activation at 1-2 microM. A similar beta subunit TAM peptide showed much lower stimulation of Syk tyrosine phosphorylation and kinase activity. Phosphopeptide-induced activation was inhibited by an antiserum to the carboxyl-terminal tail of Syk, suggesting that those amino acids are also involved in Syk activation. These results indicate that the catalytic domain of Syk may be regulated by intramolecular interactions with adjacent domains and suggest that Syk binding to phosphorylated gamma subunits following Fc epsilon RI engagement in vivo stimulates Syk kinase activity.