Tissue transglutaminase

Gene The human tTG gene is located on the 20th chromosome (20q11.2-q12). Protein TG2 is a multifunctional enzyme that belongs to transglutaminases which catalyze the crosslinking of proteins by epsilon-(gamma-glutamyl)lysine isopeptide bonds. Similarly to other transglutaminases, tTG consists of a GTP/ GDP binding site, a catalytic domain, two beta barrel and a beta-sandwich. Crystal structures of TG2 with bound GDP, GTP, or ATP have demonstrated that these forms of TG2 adopt a "closed" conformation, whereas TG2 with the active site occupied by an inhibitory gluten peptide mimic or other similar inhibitors adopts an "open" conformation. In the open conformation the four domains of TG2 are arranged in an extended configuration, allowing for catalytic activity, whereas in the closed conformation the two C-terminal domains are folded in on the catalytic core domain which includes the residue Cys-277. The N-terminal domain only shows minor structural changes between the two different conformations. ==Mechanism==

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. ==Regulation==

The expression of tTG is regulated at the transcriptional level depending on complex signal cascades. Once synthesized, most of the protein is found in the cytoplasm, plasma membrane and ECM, but a small fraction is translocated to the nucleus, where it participates in the control of its own expression through the regulation of transcription factors. Crosslinking activity by tTG requires the binding of Ca2+ ions. When this disulfide bond forms, the enzyme remains in an open confirmation but becomes catalytically inactive. The, 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-1 has been shown to activate extracellular tTG by reducing the disulfide bond. Another disulfide bond can form in tTG, between the residues Cys-230 and Cys-370. While this bond does not exist in the enzyme's native state, it appears when the enzyme is inactivated via oxidation. The presence of calcium protects against the formation of both disulfide bonds, thus making the enzyme more resistant to oxidation. 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 3) In the extracellular matrix, TG2 is "turned off", due primarily to the oxidizing activity of endoplasmic reticulum protein 57 (ERp57). Thus, tTG is allosterically regulated by two separate proteins, Erp57 and TRX-1. (See Figure 4). == Function ==

tTG is expressed ubiquitously and is present in various cellular compartments, such as the cytosol, the nucleus, and the plasma membrane. tTG is thought to be involved in the regulation of the cytoskeleton by crosslinking various cytoskeletal proteins including myosin, actin, and spectrin. 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. tTG also presents PDI (Protein Disulfide Isomerase) activity. Based on its PDI activity, tTG plays an important role in the regulation of proteostasis, by catalyzing the trimerization of HSF1 (Heat Shock Factor 1) and thus the body's response to heat shock. In the absence of tTG, the response to heat shock is impaired since the necessary trimer is not formed. == Clinical significance ==

tTG is the most comprehensively studied transglutaminase and has been associated with many diseases. However, none of these diseases are related to an enzyme deficiency. Indeed, thus far no disease has been attributed to the lack of tTG activity and this has been attested through the study of tTG knockout mice. Celiac Disease tTG is best known for its link with celiac disease. tTG specifically deamidates the glutamine residues creating epitopes that increase the binding affinity of the gluten peptide to the antigen presenting T cells, initiating an adaptive immune response. However, other studies Therapeutic It's still experimental to use tTG as a form of surgical glue. It is also being studied as an attenuator of metastasis in certain tumors. tTG inhibitors have also been shown to inhibit the formation of toxic inclusions related to neurodegenerative diseases. This indicates that tTG inhibitors could also serve as a tool to mitigate the progression of tTG brain related diseases. == Interactions ==

TG2 participates in both enzymatic and non-enzymatic interactions. Enzymatic interactions are formed between TG2 and its substrate proteins containing the glutamine donor and lysine donor groups in the presence of calcium. Substrates of TG2 are known to affect TG2 activity, which enables it to subsequently execute diverse biological functions in the cell. However, the importance of non-enzymatic interactions in regulating TG2 activities is yet to be revealed. Recent studies indicate that non-enzymatic interactions play physiological roles and enable diverse TG2 functions in a context-specific manner. ==Erp57==

Endoplasmic reticulum protein 57 (Erp57), is a chaperone molecule involved in loading peptide onto MHC class I molecules in the endoplasmic reticulum. Transglutaminase 2 (TG2) is a ubiquitously expressed (intracellular as well as extracellular) protein, with multiple modes of post-translational regulation, including an allosteric disulfide bond between Cys-370-Cys-371 that renders the enzyme inactive in the extracellular matrix. Endoplasmic reticulum (ER)-resident protein 57 (ERp57), a protein in the ER that promotes folding of nascent proteins and is also present in the extracellular environment, has the cellular and biochemical characteristics for inactivating TG2. We found that ERp57 colocalizes with extracellular TG2 in cultured human umbilical vein endothelial cells (HUVECs). ERp57 oxidized TG2 with a rate constant that was 400-2000-fold higher than those of the aforementioned small molecule oxidants. Moreover, its specificity for TG2 was also markedly higher than those of other secreted redox proteins, including protein disulfide isomerase (PDI), ERp72, TRX, and quiescin sulfhydryl oxidase 1 (QSOX1). == References ==