ZAP-70 and Syk comprise a family of hematopoietic cell specific protein tyrosine kinases (PTKs) that are required for antigen and antibody receptor function. ZAP-70 is expressed in T and natural killer (NK) cells and Syk is expressed in B cells, mast cells, polymorphonuclear leukocytes, platelets, macrophages, and immature T cells. They are required for the proper development of T and B cells, immune receptors, and activating NK cells []. They consist of two N-terminal Src homology 2 (SH2) domains and a C-terminal kinase domain separated from the SH2 domains by a linker or hinge region. Phosphorylation of both tyrosine residues within the Immunoreceptor Tyrosine-based Activation Motifs (ITAM; consensus sequence Yxx[LI]x(7,8)Yxx[LI]) by the Src-family PTKs is required for efficient interaction of ZAP-70 and Syk with the receptor subunits and for receptor function []. ZAP-70 forms two phosphotyrosine binding pockets, one of which is shared by both SH2 domains. In Syk the two SH2 domains do not form such a phosphotyrosine-binding site. The SH2 domains here are believed to function independently. In addition, the two SH2 domains of Syk display flexibility in their relative orientation, allowing Syk to accommodate a greater variety of spacing sequences between the ITAM phosphotyrosines and singly phosphorylated non-classical ITAM ligands []. This entry contains the N terminus SH2 domains of both Syk and Zap70.
Phosphorylated immunoreceptor signalling motifs (ITAMs) exhibit unique abilities to bind and activate Lyn and Syk tyrosine kinases []. Motif may be dually phosphorylated on tyrosine that links antigen receptors to downstream signalling machinery.
IL-15 receptor (IL-15R) composed of IL-15Ralpha, IL-2/15Rbeta (CD122), and gamma(c) (CD132) subunits []. IL-15 is a cytokine that plays an important role in innate and adaptive immunity []. Syk kinase interacts with IL-15Ralpha and may play a key role in mediating IL-15-induced signal transduction [].
Hematopoietic cell signal transducer (HCST, also known as DAP10) is a transmembrane adaptor that associates with an activation receptor, NKG2D, which is found on NK and subsets of T cells. The ligands for this receptor include MHC class I chain-related (MIC) protein A and protein B and UL16-binding proteins []. In activated mouse natural killer (NK) cells, the NKG2D receptor associates with two intracellular adaptors, DAP10 and DAP12, which trigger phosphatidyl inositol 3 kinase (PI3K) and Syk family protein tyrosine kinases, respectively. It has been suggested that the DAP10-PI3K pathway is sufficient to initiate NKG2D-mediated killing of target cells [].
This entry represents the SH2 domain found in Drosophila shark protein and hydra protein HTK16. Shark and HTK16 are non-receptor protein-tyrosine kinases contain two SH2 domains, five ankyrin (ANK)-like repeats, and a potential tyrosine phosphorylation site in the carboxyl-terminal tail which resembles the phosphorylation site in members of the src family. Like, mammalian non-receptor protein-tyrosine kinases, ZAP-70 and syk proteins, they do not have SH3 domains. However, the presence of ANK makes these unique among protein-tyrosine kinases. Both tyrosine kinases and ANK repeats have been shown to transduce developmental signals, and SH2 domains are known to participate intimately in tyrosine kinase signaling [].Drosophila Shark transduces intracellularly the Crumbs, a protein necessary for proper organization of ectodermal epithelia, intercellular signal []. It is essential for Draper-mediated signalling [].
The adaptor protein 3BP2/SH3BP2 is a cytoplasmic adaptor that contributes to the regulation of immune responses []. The protein-tyrosine kinase Syk phosphorylates 3BP2 which results in the activation of Rac1 through the interaction with the SH2 domain of Vav1 and induces the binding to the SH2 domain of the upstream protein-tyrosine kinase Lyn and enhances its kinase activity []. 3BP2 has a positive regulatory role in IgE-mediated mast cell activation []. In lymphocytes, engagement of T cell or B cell receptors triggers tyrosine phosphorylation of 3BP2 []. 3BP2 is required for the proliferation of B cells and B cell receptor signaling. Mutations in the 3BP2 gene are responsible for cherubism resulting in excessive bone resorption in the jaw [].This entry represents the SH2 domain of SH3BP2.
Cortactin is a key regulator of actin polymerisation in response to tyrosine kinase signalling []. It was first identified as a tyrosine-phosphorylated protein in v-Src infected fibroblasts []. It contains several domains: an N-terminal acidic (NTA) domain, a central repeat region and a C-terminal Src homology 3 (SH3) domain. The central repeat region binds to actin filaments, the NTA domain binds to the Arp2/3 complex and the SH3 domain interacts with N-WASp, Arg and WIP []. When activated, cortactin can recruit Arp2/3 complex to existing actin filaments to nucleate a new actin filament. Cortactin is involved in the regulation of cell migration, lamellipodia formation, invadopodia formation and endocytosis []. Cortactin can be phosphorylated by Src at several sites, and also binds directly to the SH2 domain of SRC. The non-receptor kinases, such as Fyn, Syk and Fer may also play a role in cortactin tyrosine phosphorylation. The structure of cortactin has been solved [].
This entry represents the SH3 domain of UBASH3B. UBASH3B (also known as STS-1 or TULA-2), belongs to the TULA family, is an active phosphatase that is expressed ubiquitously []. The phosphatase activity of UBASH3B is essential for its roles in the suppression of TCR signaling []and the regulation of EGFR []. It also interacts with Syk and functions as a negative regulator of platelet glycoprotein VI signaling in platelets []. The TULA family includes two members termed p70/STS-1/TULA-2 and UBASH3A/STS-2/TULA/Cbl-InteractingProtein 4 (CLIP4). TULA proteins contain an N-terminal UBA domain, a central SH3 domain, and a C-terminal histidine phosphatase domain. They bind c-Cbl (a multidomain adaptor and an E3 ubiquitinligase) through the SH3 domain []and to ubiquitin via UBA [].
The adaptor protein 3BP2/SH3BP2 is a cytoplasmic adaptor that contributes to the regulation of immune responses []. The protein-tyrosine kinase Syk phosphorylates 3BP2 which results in the activation of Rac1 through the interaction with the SH2 domain of Vav1 and induces the binding to the SH2 domain of the upstream protein-tyrosine kinase Lyn and enhances its kinase activity []. 3BP2 has a positive regulatory role in IgE-mediated mast cell activation []. In lymphocytes, engagement of T cell or B cell receptors triggers tyrosine phosphorylation of 3BP2 []. 3BP2 is required for the proliferation of B cells and B cell receptor signaling. Mutations in the 3BP2 gene are responsible for cherubism resulting in excessive bone resorption in the jaw [].
Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity []:Serine/threonine-protein kinasesTyrosine-protein kinasesDual specificity protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)Protein kinase function is evolutionarily conserved from Escherichia coli to human []. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation []. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [].Tyrosine-protein kinases can transfer a phosphate group from ATP to a tyrosine residue in a protein. These enzymes can be divided into two main groups []:Receptor tyrosine kinases (RTK), which are transmembrane proteins involved in signal transduction; they play key roles in growth, differentiation, metabolism, adhesion, motility, death and oncogenesis []. RTKs are composed of 3 domains: an extracellular domain (binds ligand), a transmembrane (TM) domain, and an intracellular catalytic domain (phosphorylates substrate). The TM domain plays an important role in the dimerisation process necessary for signal transduction []. Cytoplasmic / non-receptor tyrosine kinases, which act as regulatory proteins, playing key roles in cell differentiation, motility, proliferation, and survival. For example, the Src-family of protein-tyrosine kinases [].This entry represents the non-receptor tyrosine kinases SYK and ZAP-70 [, , ]:SYK is a positive effector of BCR-stimulated responses. It couples the B-cell antigen receptor (BCR) to the mobilisation of calcium ion, either through a phosphoinositide 3-kinase-dependent pathway (when not phosphorylated on tyrosines of the linker region), or through a phospholipase C-gamma-dependent pathway (when phosphorylated on Tyr-342 and Tyr-346). Therefore, the differential phosphorylation of Syk can determine the pathway by which BCR is coupled to the regulation of intracellular calcium ion [, ].ZAP70 plays a role in T-cell development and lymphocyte activation. It is essential for TCR-mediated IL-2 production. Isoform 1 of ZAP70 induces TCR-mediated signal transduction, isoform 2 does not [, ].
This family consists of several Gammaherpesvirus latent membrane protein (LMP2) proteins. Epstein-Barr virus (strain GD1) (HHV-4) (Human herpesvirus 4) is a human gammaherpesvirus that infects and establishes latency in B lymphocytes in vivo. The latent membrane protein 2 (LMP2) gene is expressed in latently infected B cells and encodes two protein isoforms, LMP2A and LMP2B, that are identical except for an additional N-terminal 119 aa cytoplasmic domain which is present in the LMP2A isoform. LMP2A is thought to play a key role in either the establishment or the maintenance of latency and/or the reactivation of productive infection from the latent state. It modulates B-cell receptor signal transduction through its association with the cellular tyrosine kinases Lyn and Syk via specific motifs at the N-terminal tail domain. LMP2A also modulates the immune system reducing infected B cells recognition by EBV-specific CD8+ T cells []. The significance of LMP2B and its role in pathogenesis remain unclear. It is suggested that it may be a negative regulator of LMP2A [, ].
Lck is a member of the Src subfamily of proteins, which are cytoplasmic (or non-receptor) PTKs. Lck is expressed in T-cellsand natural killer cells. It plays a critical role in T-cell maturation, activation, and T-cell receptor (TCR) signaling [, ]. Lck phosphorylates ITAM (immunoreceptor tyr activation motif) sequences on several subunits of TCRs, leading to the activation of different second messenger cascades. Phosphorylated ITAMs serve as binding sites for other signaling factor such as Syk and ZAP-70, leading to their activation and propagation of downstream events []. In addition, Lck regulates drug-induced apoptosis by interfering with the mitochondrial death pathway. The apototic role of Lck is independent of its primary function in T-cell signaling []. Src kinases contain an N-terminal SH4 domain with a myristoylation site, followed by SH3 and SH2 domains, a tyr kinase domain, and a regulatory C-terminal region containing a conserved tyr. They are activated by autophosphorylation at the tyr kinase domain, but are negatively regulated by phosphorylation at the C-terminal tyr by Csk (C-terminal Src Kinase). The SH3 domain of Src kinases contributes to substrate recruitment by binding adaptor proteins/substrates, and regulation of kinase activity through an intramolecular interaction [, ].
Protein phosphorylation, which plays a key role in most cellular activities, is a reversible process mediated by protein kinases and phosphoprotein phosphatases. Protein kinases catalyse the transfer of the gamma phosphate from nucleotide triphosphates (often ATP) to one or more amino acid residues in a protein substrate side chain, resulting in a conformational change affecting protein function. Phosphoprotein phosphatases catalyse the reverse process. Protein kinases fall into three broad classes, characterised with respect to substrate specificity []:Serine/threonine-protein kinasesTyrosine-protein kinasesDual specificity protein kinases (e.g. MEK - phosphorylates both Thr and Tyr on target proteins)Protein kinase function is evolutionarily conserved from Escherichia coli to human []. Protein kinases play a role in a multitude of cellular processes, including division, proliferation, apoptosis, and differentiation []. Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins. The catalytic subunits of protein kinases are highly conserved, and several structures have been solved [], leading to large screens to develop kinase-specific inhibitors for the treatments of a number of diseases [].Tyrosine-protein kinases can transfer a phosphate group from ATP to a tyrosine residue in a protein. These enzymes can be divided into two main groups []:Receptor tyrosine kinases (RTK), which are transmembrane proteins involved in signal transduction; they play key roles in growth, differentiation, metabolism, adhesion, motility, death and oncogenesis []. RTKs are composed of 3 domains: an extracellular domain (binds ligand), a transmembrane (TM) domain, and an intracellular catalytic domain (phosphorylates substrate). The TM domain plays an important role in the dimerisation process necessary for signal transduction []. Cytoplasmic / non-receptor tyrosine kinases, which act as regulatory proteins, playing key roles in cell differentiation, motility, proliferation, and survival. For example, the Src-family of protein-tyrosine kinases [].SYK is a positive effector of B-cell antigen receptor (BCR) stimulated responses [, ]. ZAP-70 plays a role in T-cell development and lymphocyte activation. It is essential for TCR-mediated IL-2 production [, ].The N-terminal region of ZAP-70 consists of two SH2 domains that are connected by an helical region. The overall fold is Y shaped, with the intervening residues forming the stem []. This superfamily represents the inter-SH2 domain found in ZAP-70 and SYK kinases.
