Catalytic domain of Spleen Tyrosine Kinase-like Protein Tyrosine Kinases
PTKs catalyze the transfer of the gamma-phosphoryl group from ATP to tyrosine (tyr) residues in protein substrates. The Syk-like subfamily is composed of Syk, ZAP-70, Shark, and similar proteins. They are cytoplasmic (or nonreceptor) PTKs containing two Src homology 2 (SH2) domains N-terminal to the catalytic tyr kinase domain. They are involved in the signaling downstream of activated receptors (including B-cell, T-cell, and Fc receptors) that contain ITAMs (immunoreceptor tyr activation motifs), leading to processes such as cell proliferation, differentiation, survival, adhesion, migration, and phagocytosis. Syk is important in B-cell receptor signaling, while Zap-70 is primarily expressed in T-cells and NK cells, and is a crucial component in T-cell receptor signaling. Syk also plays a central role in Fc receptor-mediated phagocytosis in the adaptive immune system. Shark is exclusively expressed in ectodermally derived epithelia, and is localized preferentially to the apical surface of the epithelial cells, it may play a role in a signaling pathway for epithelial cell polarity. The Syk-like subfamily is part of a larger superfamily that includes the catalytic domains of other kinases such as protein serine/threonine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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