SH2B adapter protein 3 (SH2B3), also known as lymphocyte adapter protein (LNK), is a protein that in humans is encoded by the SH2B3 gene on chromosome 12. [5] [6] SH2B3 is ubiquitously expressed in many tissues and cell types. [7] LNK functions as a regulator in signaling pathways relating to hematopoiesis, inflammation, and cell migration. [8] As a result, it is involved in blood diseases, autoimmune disorders, and vascular disease. [9] The SH2B3 gene also contains one of 27 SNPs associated with increased risk of coronary artery disease. [10]
The SH2B3 gene resides on chromosome 12 at the band 12q24 and contains 12 exons. [6]
This protein belongs to the Src homology 2-B (SH2B) adapter family. [8] [11] LNK contains 3 functional domains: a C-terminal Src homology 2 (SH2) domain, [12] a pleckstrin homology (PH) domain, and a dimerization domain. The SH2 domain spans approximately 100 amino acid residues and binds phosphotyrosine-containing proteins such as kinases. The PH domain spans approximately 120 amino acid residues and binds the phosphatidylinositol lipids found in the cell membrane. Thus, it is proposed to target the protein to the cell membrane, where LNK performs its regulatory function. The dimerization domain spans approximately 70 amino acid residues and contains a central phenylalanine zipper motif, which is formed by stacking of the aromatic side chains from 10 phenylalanine residues. This motif is responsible for facilitating the homo- or heterodimerization of SH2-B family proteins as a mechanism for regulating signal transduction. In addition to these domains, LNK possesses a proline-rich region that contains a minimal consensus sequence of Pro-X-X-Pro, which is recognized by the SH3 domain of another protein, as well as putative tyrosine phosphorylation motifs. [8]
LNK is widely expressed in human tissues, with the highest expression in hematopoietic cells. LNK negatively controls the activation of several receptors, including stem cell factor receptor (c-kit), [13] thrombopoietin receptor (MPL), [14] erythropoietin receptor (EPOR), [15] platelet-derived growth factor receptor (PDGFR), [16] macrophage colony-stimulating factor receptor (c-Fms), [17] and their related pathways. LNK is a negative regulator of signaling in endothelial cells, such as the TNF signaling pathway, especially in inflammation. LNK has been found to function as a negative regulator in lymphopoiesis, megakaryopoiesis, erythropoiesis as well as HSC expansion by moderating growth factor and cytokine receptor-mediated signaling. [8] The overexpression of LNK led to the inhibition of anti-CD3 mediated NF-AT-Luc activation, indicating that LNK is involved in the mechanism of T cell-negative regulation. [18] In addition to its role in progenitor cell growth and commitment, LNK appears to be involved in cell motility and cellular interactions. LNK modulates crosstalk between integrin- and cytokine-mediated signals, thus controlling thrombopoiesis. [19] LNK facilitates integrin aIIbb3 phosphorylation and signaling in order to promote platelet cytoskeleton rearrangement and spreading, and thus stabilizes thrombosis formation. [20]
In humans, genetic linkage analyses, genome-wide association studies of single nucleotide polymorphisms, copy number variation surveys, and mutation screenings found the human chromosomal 12q24 locus, with the SH2B3 gene at its core, to be associated with an exceptionally wide spectrum of disease susceptibilities. For example, hematopoietic traits of red and white blood cells (like erythrocytosis and myeloproliferative disease), autoimmune disorders, and vascular pathology have been reported. [9] Moreover, co-expression of the interleukin-7 receptor together with LNK was carefully studied, and it was concluded that interleukin-7 receptor expression was significantly more highly expressed than LNK in B-cell acute leukemic lymphoma. This observation distinguished a novel subset of high-risk B-cell acute lymphoblastic lymphoma [22] with a potential therapy targeting the interleukin-7 signaling pathway. Another study indicated that LNK can suppress the interleukin-7/JAK/STAT signaling pathway to restrict pre B-cell progenitor expansion and leukemia development, which provided a pathogenic mechanism and a potential therapeutic approach for B-cell acute lymphoblastic leukemia with SH2B3 gene mutations. [23]
A multi-locus genetic risk score study based on a combination of 27 loci, including the SH2B3 gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22). [10]
Growth factor receptor-bound protein 2, also known as Grb2, is an adaptor protein involved in signal transduction/cell communication. In humans, the GRB2 protein is encoded by the GRB2 gene.
Proto-oncogene c-KIT is the gene encoding the receptor tyrosine kinase protein known as tyrosine-protein kinase KIT, CD117 or mast/stem cell growth factor receptor (SCFR). Multiple transcript variants encoding different isoforms have been found for this gene. KIT was first described by the German biochemist Axel Ullrich in 1987 as the cellular homolog of the feline sarcoma viral oncogene v-kit.
Son of sevenless homolog 1 is a protein that in humans is encoded by the SOS1 gene.
Adapter molecule crk also known as proto-oncogene c-Crk is a protein that in humans is encoded by the CRK gene.
Lymphocyte cytosolic protein 2, also known as LCP2 or SLP-76, is a signal-transducing adaptor protein expressed in T cells and myeloid cells and is important in the signaling of T-cell receptors (TCRs). As an adaptor protein, SLP-76 does not have catalytic functions, primarily binding other signaling proteins to form larger signaling complexes. It is a key component of the signaling pathways of receptors with immunoreceptor tyrosine-based activation motifs (ITAMs) such as T-cell receptors, its precursors, and receptors for the Fc regions of certain antibodies. SLP-76 is expressed in T-cells and related lymphocytes like natural killer cells.
The Linker for activation of T cells, also known as linker of activated T cells or LAT, is a protein involved in the T-cell antigen receptor signal transduction pathway which in humans is encoded by the LAT gene. Alternative splicing results in multiple transcript variants encoding different isoforms.
GRB2-associated-binding protein 2 also known as GAB2 is a protein that in humans is encoded by the GAB2 gene.
Mitogen-activated protein kinase kinase kinase kinase 1 is a protein kinase that in humans is encoded by the MAP4K1 gene. It is also known as HPK1. The protein has been shown to play a role in JNK activation.
Cbl is a mammalian gene family. CBL gene, a part of the Cbl family, encodes the protein CBL which is an E3 ubiquitin-protein ligase involved in cell signalling and protein ubiquitination. Mutations to this gene have been implicated in a number of human cancers, particularly acute myeloid leukaemia.
Rap guanine nucleotide exchange factor 1 is a protein that in humans is encoded by the RAPGEF1 gene.
AP-1 complex subunit gamma-1 is a protein that in humans is encoded by the AP1G1 gene.
Cytoplasmic protein NCK1 is a protein that in humans is encoded by the NCK1 gene.
Src homology 2 (SH2) domain containing inositol polyphosphate 5-phosphatase 1(SHIP1) is an enzyme with phosphatase activity. SHIP1 is structured by multiple domain and is encoded by the INPP5D gene in humans. SHIP1 is expressed predominantly by hematopoietic cells but also, for example, by osteoblasts and endothelial cells. This phosphatase is important for the regulation of cellular activation. Not only catalytic but also adaptor activities of this protein are involved in this process. Its movement from the cytosol to the cytoplasmic membrane, where predominantly performs its function, is mediated by tyrosine phosphorylation of the intracellular chains of cell surface receptors that SHIP1 binds. Insufficient regulation of SHIP1 leads to different pathologies.
B-cell linker (BLNK) protein is expressed in B cells and macrophages and plays a large role in B cell receptor signaling. Like all adaptor proteins, BLNK has no known intrinsic enzymatic activity. Its function is to temporally and spatially coordinate and regulate downstream signaling effectors in B cell receptor (BCR) signaling, which is important in B cell development. Binding of these downstream effectors is dependent on BLNK phosphorylation. BLNK is encoded by the BLNK gene and is also known as SLP-65, BASH, and BCA.
GRB2-related adapter protein 2 also known as GRB2-related adaptor downstream of Shc (GADS) is a 37 kDa protein that in humans is encoded by the GRAP2 gene.
Cytoplasmic protein NCK2 is a protein that in humans is encoded by the NCK2 gene.
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SH2B adapter protein 2 is a protein that in humans is encoded by the SH2B2 gene.
SH2 domain containing 3C, also known as SH2D3C, is a protein that in humans is encoded by the SH2D3C gene.
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