Tissue transglutaminase drawn from PDB 1KV3.
|Symbols||; G-ALPHA-h; GNAH; HEL-S-45; TG2; TGC|
|External IDs||ChEMBL: GeneCards:|
|RNA expression pattern|
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
Tissue transglutaminase (abbreviated as tTG or TG2) is a 78-kDa, calcium dependent enzyme (EC 188.8.131.52) of the protein-glutamine γ-glutamyltransferases family (or simply transglutaminase family). Like other transglutaminases, it crosslinks proteins between an ε-amino group of a lysine residue and a γ-carboxamide group of glutamine residue, creating an inter- or intramolecular bond that is highly resistant to proteolysis (protein degradation). Aside from its crosslinking function, tTG catalyzes other types of reactions including deamidation, GTP-binding/hydrolyzing, and isopeptidase activities. Unlike other members of the transglutaminase family, tTG can be found both in the intracellular and the extracellular spaces of various types of tissues and is found in many different organs including the heart, the liver, and the small intestine. Intracellular tTG is abundant in the cytosol but smaller amounts can also be found in the nucleus and the mitochondria. Intracellular tTG is thought to play an important role in apoptosis. In the extracellular space, tTG binds to proteins of the extracellular matrix (ECM), binding particularly tightly to fibronectin. Extracellular tTG has been linked to cell adhesion, ECM stabilization, wound healing, receptor signaling, cellular proliferation, and cellular motility.
tTG is particularly notable for being the autoantigen in coeliac disease, a lifelong illness in which the consumption of dietary gluten causes a pathological immune response resulting in the inflammation of the small intestine and subsequent villous atrophy.
The catalytic mechanism for crosslinking in human tTG involves the thiol group from a Cys residue in the active site of tTG. The thiol group attacks the carboxamide of a glutamine residue on the surface of a protein or peptide substrate, releasing ammonia, and producing a thioester intermediate. The thioester intermediate can then be attacked by the surface amine of a second substrate (typically from a lysine residue). The end product of the reaction is a stable isopeptide bond between the two substrates (i.e. crosslinking). Alternatively, the thioester intermediate can be hydrolyzed, resulting in the net conversion of the glutamine residue to glutamic acid (i.e. deamidation). The deamidation of glutamine residues catalyzed by tTG is thought to be linked to the pathological immune response to gluten in celiac disease. A schematic for the crosslinking and the deamidation reactions is provided in Figure 1.
Crosslinking activity by tTG requires the binding of Ca2+ ions. Multiple Ca2+ can bind to a single tTG molecule. In contrast, the binding of one molecule of GTP or GDP inhibits the crosslinking activity of the enzyme. Therefore, intracellular tTG is mostly inactive due to the relatively high concentration of GTP/GDP and the low levels of calcium inside the cell. Although extracellular tTG is expected to be active due to the low concentration of guanine nucleotides and the high levels of calcium in the extracellular space, evidence has shown that extracellular tTG is mostly inactive. Recent studies suggest that extracellular tTG is kept inactive by the formation of a disulfide bond between two vicinal Cys residues. Therefore, oxidation/reduction of the disulfide bond serves as a third allosteric regulatory mechanism (along with GTP/GDP and Ca2+) for the activation of tTG. Thioredoxin has been shown to activate extracellular tTG by reducing the disulfide bond. Recent studies have suggested that interferon-γ may serve as an activator of extracellular tTG in the small intestine; these studies have a direct implication to the pathogenesis of celiac disease. Activation of tTG has been shown to be accompanied by large conformational changes, switching from a compact (inactive) to an extended (active) conformation. (see Figure 2)
The human tTG gene is located on the 20th chromosome (20q11.2-q12).
tTG is expressed ubiquitously. It requires calcium as a cofactor for transamidation activity. Transcription is increased by retinoic acid. Among its many supposed functions, it appears to play a role in wound healing, apoptosis, and extracellular matrix development
tTG is thought to be involved in the regulation of the cytoskeleton by crosslinking various cytoskeletal proteins including myosin, actin, and spectrin. Evidence shows that intracellular tTG crosslinks itself to myosin. It is also believed that tTG may stabilize the structure of the dying cells during apoptosis by polymerizing the components of the cytoskeleton, therefore preventing the leakage of the cellular contents into the extracellular space.
tTG also has GTPase activity: In the presence of GTP, it suggested to function as a G protein participating in signaling processes. Besides its transglutaminase activity, tTG is proposed to also act as kinase, and protein disulfide isomerase, and deamidase. This latter activity is important in the deamidation of gliadin peptides, thus playing important role in the pathology of coeliac disease.
Role in disease
tTG is best known for its link with celiac disease. Anti-transglutaminase antibodies (ATA) result in a form of gluten sensitivity in which a cellular response to Triticeae glutens that are crosslinked to tTG are able to stimulate transglutaminase specific B-cell responses that eventually result in the production of ATA IgA and IgG.
tTG is believed to be involved in several neurodegenerative disorders including Alzheimer, Parkinson and Huntington diseases. Such neurological diseases are characterized in part by the abnormal aggregation of proteins due to the increased activity of protein crosslinking in the affected brain. Additionally, specific proteins associated with these disorders have been found to be in vivo and in vitro substrates of tTG. Although tTG is up regulated in the areas of the brain affected by Huntington's disease, a recent study showed that increasing levels of tTG do not affect the onset and/or progression of the disease in mice.
Recent studies suggest that tTG also plays a role in inflammation, and tumor biology. tTG expression is elevated in multiple cancer cell types and is implicated in drug resistance and metastasis due to its ability to promote mesenchymal transition and stem cell like properties.
Serology for anti-tTG antibodies has superseded older serological tests (anti-endomysium, anti-gliadin, and anti-reticulin) and has a strong sensitivity (99%) and specificity (>90%) for identifying coeliac disease. Modern anti-tTG assays rely on a human recombinant protein as an antigen.
|Mouse Mutant Alleles for Tgm2|
|Marker Symbol for Mouse Gene. This symbol is assigned to the genomic locus by the MGI||Tgm2|
|Mutant Mouse Embryonic Stem Cell Clones. These are the known targeted mutations for this gene in a mouse.||Tgm2tm1a(KOMP)Wtsi|
|Example structure of targeted conditional mutant allele for this gene|
|These Mutant ES Cells can be studied directly or used to generate mice with this gene knocked out. Study of these mice can shed light on the function of Tgm2: see Knockout mouse|
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- Endomysial antibodies
- A collection of substrates and interaction partners of TG2 is accessible in the TRANSDAB, an interactive transglutaminase substrate database.