Tyrosine-phosphorylated forms of Ig beta, CD22, TCR zeta and HOSS are major ligands for tandem SH2 domains of Syk.

The protein tyrosine kinase Syk plays an important role in signal transduction from the B cell antigen receptor and possibly also from the TCR. We have examined the binding specificity of Syk-derived SH2 domains in vitro and found that the tandem SH2 domains have two major ligands in activated Ramos B cells as well as in activated Jurkat T cells. The SH2-binding proteins in Ramos B cells were identified as the tyrosine-phosphorylated forms of the Ig alpha beta heterodimer and of CD22. Binding to the Ig alpha beta heterodimer seems to occur predominantly via Ig beta, indicating that the two receptor components might couple to distinct signaling pathways. In Jurkat T cells one of the SH2-binding proteins represents the tyrosine-phosphorylated TCR zeta chain. The identity of the second SH2 ligand, called HOSS, is not known. HOSS is discussed as a putative member of the receptor family characterized by the immunoreceptor tyrosine-based activation motif.