Spleen tyrosine kinase (SYK) is a signaling node in many immune pathways and comprises two tandem Src homology (SH) 2 domains， an SH2-kinase linker， and a C-terminal tyrosine kinase domain. Two prevalent models of SYK activation exist. The "OR-gate" model contends that SYK can be fully activated by phosphorylation or binding of its SH2 domains to a dual-phosphorylated immune-receptor tyrosine-based activation motif (ppITAM). An alternative model proposes that SYK activation requires ppITAM binding and phosphorylation of the SH2-kinase linker by a SRC family kinase such as LYN proto-oncogene， SRC family tyrosine kinase (LYN). To evaluate these two models， we generated directly comparable unphosphorylated... More
Spleen tyrosine kinase (SYK) is a signaling node in many immune pathways and comprises two tandem Src homology (SH) 2 domains， an SH2-kinase linker， and a C-terminal tyrosine kinase domain. Two prevalent models of SYK activation exist. The "OR-gate" model contends that SYK can be fully activated by phosphorylation or binding of its SH2 domains to a dual-phosphorylated immune-receptor tyrosine-based activation motif (ppITAM). An alternative model proposes that SYK activation requires ppITAM binding and phosphorylation of the SH2-kinase linker by a SRC family kinase such as LYN proto-oncogene， SRC family tyrosine kinase (LYN). To evaluate these two models， we generated directly comparable unphosphorylated (upSYK) and phosphorylated (pSYK) proteins with or without an N-terminal glutathione -transferase (GST) tag， resulting in monomeric or obligatory dimeric SYK， respectively. We assessed the ability of a ppITAM peptide and LYN to activate these SYK proteins. The ppITAM peptide strongly activated GST-SYK but was less effective in activating upSYK untagged with GST. LYN alone activated untagged upSYK to a greater extent than did ppITAM， and inclusion of both proteins rapidly and fully activated upSYK. Using immunoblot and phosphoproteomic approaches， we correlated the kinetics and order of site-specific SYK phosphorylation. Our results are consistent with the alternative model， indicating that ppITAM binding primes SYK for rapid LYN-mediated phosphorylation of Tyr-352 and then Tyr-348 of the SH2-kinase linker， which facilitates activation loop phosphorylation and full SYK activation. This gradual activation mechanism may also explain how SYK maintains ligand-independent tonic signaling， important for B-cell development and survival.
