# Tyrosine-protein kinase SYK
**Tyrosine-protein kinase SYK** (spleen tyrosine kinase) is a cytoplasmic, nonreceptor protein-tyrosine kinase (EC 2.7.10.2) that initiates signaling downstream of immune receptors, most prominently the [B cell](https://www.edgepedia.org/b-cell) antigen receptor (BCR), and also acts as a signaling adapter.[1](https://pubmed.ncbi.nlm.nih.gov/19909371/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) It is one of the two members of the Syk kinase family, the other being ZAP-70, which serves the analogous role in T cells.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2857167/)
| Key fact | Detail |
|---|---|
| Protein class | Nonreceptor tyrosine kinase of the Syk family, with two N-terminal tandem SH2 domains and a C-terminal kinase domain[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2857167/) |
| Recruitment | Binds dually phosphorylated ITAM motifs on receptor subunits through its tandem SH2 domains, recruiting it to clustered antigen receptors[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/) |
| Resting state | Autoinhibited: interdomain A and interdomain B bind the kinase domain, holding the catalytic center inactive[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) |
| Activation inputs | Binding of both SH2 domains to a dually phosphorylated ITAM, or phosphorylation of linker tyrosines (Tyr-348 and/or Tyr-352 in the interdomain linker), proposed as a logic "OR" gate[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/) |
| Src-family dependence | Can phosphorylate ITAMs and downstream targets independently of Src-family kinases, unlike ZAP70, which requires Lck[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC13317156/) |
| Intrinsic activity | Syk and SykB autophosphorylate and phosphorylate erythrocyte band 3 at least 100-fold more than Zap-70 in immune complex kinase reactions[7](https://www.jbc.org/article/S002192581961820X/pdf) |
| Therapeutic interest | SYK inhibitors are under development for autoimmune diseases and hematologic cancers[8](https://pubmed.ncbi.nlm.nih.gov/41396692/) |
## Structure and autoinhibition
SYK's regulatory region contains tandem SH2 (tSH2) domains separated by a helical "hinge" segment; in the inactive enzyme these associate with the kinase domain, keeping SYK inactive.[9](https://www.sciencedirect.com/science/article/pii/S0969212624004246) In the autoinhibited conformation, interdomain A and interdomain B bind the kinase domain, particularly its C-terminal end, holding the catalytic center in an inactive geometry.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) High-resolution X-ray structures of SYK and ZAP70, together with cryo-EM and computational studies, show a compact "linker-kinase sandwich" in which the interdomain linker is brought near the region between the two SH2 domains and the C-terminal helix of the kinase domain; the inactive protein is minimally phosphorylated and shows a low level of kinase activity.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/) Electron microscopy of SYK suggests an autoinhibited conformation comparable to that of ZAP-70.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/)
## Activation mechanism
Activation is triggered when the tSH2 domains bind a phosphorylated immunoreceptor tyrosine-based activation motif (ITAM) on receptor tails.[9](https://www.sciencedirect.com/science/article/pii/S0969212624004246) [Engagement](https://www.edgepedia.org/engagement) of the tandem SH2 domains by a dually phosphorylated ITAM reorients the domains and disrupts the autoinhibitory contacts, allowing the kinase domain to adopt an active conformation.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) Biophysical measurements show that pITAM binding clamps the SH2 domains and, allosterically through their linker, prevents kinase-domain binding, releasing the kinase domain.[9](https://www.sciencedirect.com/science/article/pii/S0969212624004246)
**Two proposed models.** Based on in vitro studies with GST-tagged SYK, Tsang et al. proposed the "OR-gate" mechanism, in which full and equivalent activation occurs through either ppITAM binding or phosphorylation at Tyr-348 and/or Tyr-352 in the interdomain linker.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/) An alternative model holds that ppITAM binding first induces a conformational change that relieves autoinhibition, exposing interdomain linker residues for phosphorylation by SRC family kinases (LYN for SYK) or by autophosphorylation, which then drives full activation.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/) A reevaluation using directly comparable tag-free and GST-tagged SYK preparations in both phosphorylated and unphosphorylated states supports a graduated activation mechanism: a partially activated, ppITAM-bound intermediate that is primed for full activation by LYN.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/)
Once active, SYK autophosphorylates on multiple residues, which releases it from the ITAM and sustains activation; phosphorylated tyrosines then recruit binding partners such as the p85α regulatory subunit of PI3K, triggering downstream signaling. SYK-mediated phosphorylation of ITAM tyrosines also creates a positive feedback loop that recruits more SYK to the receptor.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/)
## Role in B cell receptor signaling
The BCR comprises a membrane-bound immunoglobulin plus the disulfide-linked Ig-α (CD79a) and Ig-β (CD79b) heterodimer that transduces antigen binding into intracellular signaling.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/) [Phosphorylation](https://www.edgepedia.org/phosphorylation) of the two tyrosines within the ITAM recruits Syk to the clustered receptor through its tandem SH2 domains.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/) Syk plays a central role in amplifying and diversifying BCR signals, and depending on its interactions and localization it can signal opposing cell fates, such as proliferation or differentiation of B cells.[1](https://pubmed.ncbi.nlm.nih.gov/19909371/)
A key Syk substrate is the adaptor B-cell linker protein (BLNK/SLP-65), which when phosphorylated serves as a scaffold for phospholipase C gamma (PLCγ), Bruton's tyrosine kinase (Btk), GRB2/VAV, and Son of Sevenless (SOS).[10](https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2010.09.029) Binding of Syk to BLNK at the plasma membrane also provides a mechanism for its prolonged activation; sustained Syk activity for more than one hour after receptor ligation is required for events such as NFAT transcription factor activation.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/) Syk also phosphorylates ITAM tyrosines and a non-ITAM tyrosine (Y204) on Ig-α, residues involved in receptor internalization: phosphorylated receptors that bind Syk are retained at the cell surface while nonphosphorylated receptors are internalized.[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/)
## Comparison with ZAP70
ZAP-70 and Syk are the only two members of the Syk kinase family; both consist of two tandem N-terminal SH2 domains and a C-terminal kinase domain, and their tandem SH2 domains share 57% sequence identity.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC2857167/) Their expression is largely segregated: ZAP70 is mostly confined to T and NK cell lineages, while SYK is of primary importance in B cell receptor signaling, although both proteins contribute during thymocyte development and show redundancy in mature hematopoietic cell types.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/) • [11](https://rupress.org/jem/article/207/5/911/40703/Peripheral-T-cell-lymphoma-new-model-new)
The kinases differ in enzymatic autonomy. Syk is more capable of phosphorylating ITAMs and downstream targets independently of Src-family kinase activity, whereas ZAP70 cannot phosphorylate ITAMs or its own activation loop and is highly dependent on Lck to initiate downstream signaling.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC13317156/) Consistently, the measured autophosphorylation and substrate phosphorylation capacity of Syk and SykB was at least 100-fold greater than that of Zap-70 in immune complex kinase reactions.[7](https://www.jbc.org/article/S002192581961820X/pdf) This stronger intrinsic activity has been suggested as a possible reason for the change in kinase dependence during T-cell development.[12](https://www.sciencedirect.com/science/article/pii/S0898656822000924)
## Disease and drug targeting
Aberrant co-expression of ZAP70 with SYK was first identified in B-cell chronic lymphocytic leukemia (CLL) and is considered a biomarker of aggressive disease and poor clinical outcomes; ZAP70 co-expression also functions as an oncogenic driver in CLL and in other B-cell malignancies, including acute lymphoblastic leukemia (B-ALL) and mantle cell lymphoma.[12](https://www.sciencedirect.com/science/article/pii/S0898656822000924) In CLL, ZAP70 diverts SYK from NFAT activation toward tonic PI3K signaling, promoting survival of pathological B cells instead of cell death.[12](https://www.sciencedirect.com/science/article/pii/S0898656822000924)
Since SYK becomes active through a multi-step process, drugs targeting it are not restricted to ATP-competitive kinase inhibitors; inhibiting its activation may also be possible by preventing association with ppITAM sequences, hindering auto- or transphosphorylation, or stopping the structural change from the autoinhibited conformation to the active one.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/) Recent reviews of SYK inhibitor design focus on their therapeutic potential in autoimmune diseases and hematologic cancers.[8](https://pubmed.ncbi.nlm.nih.gov/41396692/)
## Evolutionary distribution
Mammals express SYK and the related ZAP70, but the [Caenorhabditis elegans](https://www.edgepedia.org/caenorhabditis-elegans) genome contains no SYK-related tyrosine kinases.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/)
## References
1. [Autoinhibition and adapter function of Syk (PubMed, 2009)](https://pubmed.ncbi.nlm.nih.gov/19909371/)
2. [The SYK tyrosine kinase: a crucial player in diverse biological functions (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC4782221/)
3. [ZAP-70: An Essential Kinase in T-cell Signaling (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2857167/)
4. [Syk and pTyr'd: Signaling through the B cell antigen receptor (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2700185/)
5. [A reevaluation of the spleen tyrosine kinase (SYK) activation mechanism (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6514621/)
6. [ZAP-70 in signaling, biology, and disease (PMC)](https://pmc.ncbi.nlm.nih.gov/articles/PMC13317156/)
7. [Differential Intrinsic Enzymatic Activity of Syk and Zap-70 Protein-tyrosine Kinases (JBC)](https://www.jbc.org/article/S002192581961820X/pdf)
8. [Recent Progress of Molecular Design and Activities of Spleen Tyrosine Kinase (SYK) Inhibitors (PubMed)](https://pubmed.ncbi.nlm.nih.gov/41396692/)
9. [The mechanism of allosteric activation of SYK kinase derived from multiple phospho-ITAM-bound structures (Structure, 2024)](https://www.sciencedirect.com/science/article/pii/S0969212624004246)
10. [The conundrum of inhibitory signaling by ITAM-containing immunoreceptors (FEBS Letters, 2010)](https://febs.onlinelibrary.wiley.com/doi/10.1016/j.febslet.2010.09.029)
11. [Peripheral T cell lymphoma: new model + new insight (J Exp Med)](https://rupress.org/jem/article/207/5/911/40703/Peripheral-T-cell-lymphoma-new-model-new)
12. [SYK and ZAP70 kinases in autoimmunity and lymphoid malignancies (Cellular Signalling, 2022)](https://www.sciencedirect.com/science/article/pii/S0898656822000924)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Protein families and complexes › Kinase and phosphatase families › Protein kinase families › Protein tyrosine kinases › Syk/ZAP70 family*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license