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. 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. 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. Downstream signaling adapters and intermediates may mediate positive and/or negative feedback regulation (By similarity). The SH2 domains mediate the interaction of SYK with the phosphorylated ITAM domains of transmembrane proteins. Belongs to the protein kinase superfamily. Tyr protein kinase family. SYK/ZAP-70 subfamily.