Non-receptor tyrosine kinase which mediates signal transduction downstream of a variety of transmembrane receptors including classical immunoreceptors like the B-cell receptor (BCR). Regulates several biological processes including innate and adaptive immunity, cell adhesion, osteoclast maturation, platelet activation and vascular development (PubMed:33782605). Assembles into signaling complexes with activated receptors at the plasma membrane via interaction between its SH2 domains and the receptor tyrosine-phosphorylated ITAM domains. The association with the receptor can also be indirect and mediated by adapter proteins containing ITAM or partial hemITAM domains. The phosphorylation of the ITAM domains is generally mediated by SRC subfamily kinases upon engagement of the receptor. More rarely signal transduction via SYK could be ITAM-independent. Direct downstream effectors phosphorylated by SYK include VAV1, PLCG1, PI-3-kinase, LCP2 and BLNK. Initially identified as essential in B-cell receptor (BCR) signaling, it is necessary for the maturation of B-cells most probably at the pro-B to pre-B transition. Activated upon BCR engagement, it phosphorylates and activates BLNK an adapter linking the activated BCR to downstream signaling adapters and effectors. It also phosphorylates and activates PLCG1 and the PKC signaling pathway. It also phosphorylates BTK and regulates its activity in B-cell antigen receptor (BCR)-coupled signaling. In addition to its function downstream of BCR also plays a role in T-cell receptor signaling. Plays also a crucial role in the innate immune response to fungal, bacterial and viral pathogens. It is for instance activated by the membrane lectin CLEC7A. Upon stimulation by fungal proteins, CLEC7A together with SYK activates immune cells inducing the production of ROS. Also activates the inflammasome and NF-kappa-B-mediated transcription of chemokines and cytokines in presence of pathogens. Regulates neutrophil degranulation and phagocytosis through activation of the MAPK signaling cascade. Required for the stimulation of neutrophil phagocytosis by IL15 (By similarity). Also mediates the activation of dendritic cells by cell necrosis stimuli. Also involved in mast cells activation. Involved in interleukin-3/IL3-mediated signaling pathway in basophils (PubMed:19098920). Also functions downstream of receptors mediating cell adhesion. Relays for instance, integrin-mediated neutrophils and macrophages activation and P-selectin receptor/SELPG-mediated recruitment of leukocytes to inflammatory loci. Also plays a role in non-immune processes. It is for instance involved in vascular development where it may regulate blood and lymphatic vascular separation. It is also required for osteoclast development and function. Functions in the activation of platelets by collagen, mediating PLCG2 phosphorylation and activation. May be coupled to the collagen receptor by the ITAM domain-containing FCER1G. Also activated by the membrane lectin CLEC1B that is required for activation of platelets by PDPN/podoplanin. Involved in platelet adhesion being activated by ITGB3 engaged by fibrinogen. Together with CEACAM20, enhances production of the cytokine CXCL8/IL-8 via the NFKB pathway and may thus have a role in the intestinal immune response (PubMed:26195794). ATP + L-tyrosyl-[protein] = ADP + H(+) + O-phospho-L-tyrosyl-[protein] Autoinhibited. Intramolecular binding of the interdomains A and B (also called linker region) to parts of the catalytic domain keep the catalytic center in an inactive conformation. The phosphorylation of the interdomains or the binding of the SH2 domains with dually phosphorylated ITAM domains on transmembrane proteins disrupt those intramolecular interactions allowing the kinase domain to adopt an active conformation. The phosphorylation of SYK and of the ITAM domains which is responsible for SYK activation is essentially mediated by SRC subfamily kinases, like LYN, upon transmembrane receptors engagement. May also be negatively regulated by PTPN6 through dephosphorylation (By similarity). Downstream signaling adapters and intermediates like BLNK or RHOH may mediate positive and/or negative feedback regulation. Negatively regulated by CBL and CBLB through ubiquitination and probable degradation. Phosphorylates SH3BP2 which in turn may regulate SYK through LYN (By similarity). Interacts with LYN; phosphorylates SYK. Interacts with RHOH (phosphorylated); regulates mast cells activation. Interacts with NFAM1 (phosphorylated); probably involved in BCR signaling. Interacts with VAV1 (via SH2 domain); phosphorylates VAV1 upon BCR activation (By similarity). Interacts with GAB2 (phosphorylated); probably involved in IgE Fc receptor signaling. Interacts (via its SH2 domains) with CD79A (via its phosphorylated ITAM domain); the interaction stimulates SYK autophosphorylation and activation. Interacts (via SH2 domains) with FCER1G (via ITAM domain); activates SYK and mediates neutrophils and macrophages integrin-mediated activation. Interaction with FCER1G in basophils triggers IL3-induced IL4 production (PubMed:19098920). Interacts with ITGB2 and FGR; involved in ITGB2 downstream signaling. Interacts with ITGB3; upon activation by ITGB3 promotes platelet adhesion (By similarity). Interacts (via SH2 domains) with TYROBP (via ITAM domain); involved in neutrophils and macrophages integrin-mediated activation. Interacts with MSN and SELPLG; mediates the selectin-dependent activation of SYK by SELPLG (By similarity). Interacts with BLNK (via SH2 domain). Interacts (via the second SH2 domain) with USP25 (via C-terminus); phosphorylates USP25 and regulates USP25 intracellular levels (By similarity). Interacts (via SH2 domains) with CLEC1B (dimer) (By similarity). Interacts with CLEC7A; participates in leukocyte activation in presence of fungal pathogens. Interacts (phosphorylated) with SLA; may regulate SYK through CBL recruitment (By similarity). Interacts with YWHAG; attenuates BCR-induced membrane translocation and activation of SYK (By similarity). Interacts (via SH2 domains) with GCSAM; the interaction increases after B-cell receptor stimulation, resulting in enhanced SYK autophosphorylation and activity (By similarity). Interacts with TNS2; leading to the phosphorylation of SYK (By similarity). Interacts with FLNA (via filamin repeat 5); docks SYK to the plasma membrane (By similarity). Interacts with CEACAM1; lipopolysaccharide activated neutrophils induce phosphorylation of SYK resulting in the formation of a complex including TLR4 and the phosphorylated form of SYK and CEACAM1, which in turn, recruits PTPN6 that dephosphorylates SYK, reducing the production of reactive oxygen species (ROS) and lysosome disruption, leading to a reduction of the inflammasome activity (PubMed:22496641). Interacts (via SH2 domains) with CEACAM20 (phosphorylated form); the interaction further enhances CEACAM20 phosphorylation (PubMed:26195794). Interacts with IL15RA (By similarity). The SH2 domains mediate the interaction of SYK with the phosphorylated ITAM domains of transmembrane proteins. Some proteins like CLEC1B have a partial ITAM domain (also called hemITAM) containing a single YxxL motif. The interaction with SYK requires CLEC1B homodimerization (By similarity). Autophosphorylated. Phosphorylated on tyrosine residues by LYN following receptors engagement. Phosphorylation on Tyr-317 creates a binding site for CBL, an adapter protein that serves as a negative regulator of BCR-stimulated calcium ion signaling. Phosphorylation at Tyr-342 creates a binding site for VAV1 (By similarity). Phosphorylation on Tyr-342 and Tyr-346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of calcium ion mobilization via a phosphoinositide 3-kinase-independent pathway (By similarity). Phosphorylated on tyrosine residues in response to IL15 (By similarity). Phosphorylation on Ser-291 is very common, it peaks 5 minutes after BCR stimulation, and creates a binding site for YWHAG (By similarity). Phosphorylation at Tyr-624 creates a binding site for BLNK (By similarity). Dephosphorylated by PTPN6 (By similarity). Ubiquitinated by CBLB after BCR activation; which promotes proteasomal degradation. Embryos display severe systemic hemorrhage and mice are not viable dying perinatally. While T-cells development is not affected, the development of B-cells is impaired most probably at the pro-B to pre-B transition and mice lack mature B-cells. Belongs to the protein kinase superfamily. Tyr protein kinase family. SYK/ZAP-70 subfamily.