A reevaluation of the spleen tyrosine kinase (SYK) activation mechanism

J Biol Chem. 2019 May 10;294(19):7658-7668. doi: 10.1074/jbc.RA119.008045. Epub 2019 Mar 28.

Abstract

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 S-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.

Keywords: ITAM (immune-receptor tyrosine activation motif); autophosphorylation; cell signaling; conformational change; enzyme activation; enzyme kinetics; immunity; non-receptor tyrosine kinase (nRTK); phosphorylation; spleen tyrosine kinase (SYK); tonic signalling.

MeSH terms

  • Amino Acid Motifs
  • Enzyme Activation
  • Humans
  • Models, Chemical*
  • Phosphorylation
  • Proto-Oncogene Mas
  • Syk Kinase / chemistry*
  • Syk Kinase / metabolism
  • src Homology Domains
  • src-Family Kinases / chemistry
  • src-Family Kinases / metabolism

Substances

  • MAS1 protein, human
  • Proto-Oncogene Mas
  • SYK protein, human
  • Syk Kinase
  • lyn protein-tyrosine kinase
  • src-Family Kinases