Edited by Ruma Banerjee ?, Transglutaminase 2 (TG2) is a ubiquitously expressed,

Question

Edited by Ruma Banerjee ?, Transglutaminase 2 (TG2) is a ubiquitously expressed, intra cellular 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 inac tive in the extracellular matribe Although recent studies have established that extracellular TG2 is switched "on" by the redox J cofactor protein thioredoxin-1 (TRX), it is unclear how TG2 is switched "off." Here, we demonstrate that TG2 oxidation by small-molecule biological oxidants, including glutathione, cys- tine, and hydrogen peroxide, is unlikely to be the inactivation mechanism. Instead, endoplasmic reticulum (ER)-resident pro- tein 57 (ERp57), a protein in the ER that promotes folding of nascent proteins and is also present in the extracellular environ- ment, has the cellular and biochemical characteristics for inac- tivating TG2. We found that ERp57 colocalizes with extracellu- lar 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). Lastly, siRNA-mediated ERp57 knockdown in HUVECs increased TG2-catalyzed trans- amidation in the extracellular environment. We conclude that, to the best of our knowledge, the disulfide bond switch in human TG2 represents the first example of a post-translational redox regulatory mechanism that is reversibly and allosterically mod- ulated by two distinct proteins (ERp57 and TRX).

Explanation / Answer

Transglutaminase 2 (TG2) is a highly expressed protein in both intra and extracellular manner, it has an allosteric disulfide bond between Cys-370-Cys-371 this bond it is involve in the activation and desactivation of the protein. The first process, the activation of extracellular TG2, is mediated by redox cofactor protein thioredoxin-1 (TRX); while the desactivation process, the oxidation of TG2 extracellular, is mediated by the endoplasmic reticulum (ER)-resident protein 57 (ERp57).

Experiments realized in cultured human umbilical vein endothelial cells (HUVECs), the oxidation of TG2 by ERp57 was 400-2000-fold higher than the oxidation mediated by glutathione, cystine and hidrogen peroxide, demonstrating that the inactivation process is regulated by ERp57. Additionally, the ERp57 is highly specific for TG2 than other redox proteins as protein disulfide isomerase (PDI), ERp72, TRX and quiescin sulfhydryl oxidase 1 (QSOX1). Finally, the TG2-catalyzed transamidation was increased in siRNA-mediated ERp57 knockdown HUVECs, all of these in order to show the importance of the ERp57 protein in the TG2 regulation.

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