All Publications

  • In Vivo Measurement of Redox-Regulated TG2 Activity. Methods in molecular biology (Clifton, N.J.) Melkonian, A. V., Weng, N. n., Palanski, B. A., Khosla, C. n. 2019; 1967: 263–74


    Transglutaminase 2 (TG2) is a ubiquitous mammalian enzyme that is implicated in a variety of physiological processes and human diseases. Normally, extracellular TG2 is catalytically dormant due to formation of an allosteric disulphide bond between Cys370 and 371 of the enzyme. In this protocol, we describe a method to reduce this disulphide bond in living mice and to monitor the resulting in vivo TG2 activity. Briefly, exogenous thioredoxin-1 protein (TRX) is prepared and administered as a specific, physiologically relevant reductant of the Cys370-371 disulphide along with the small molecule 5-biotinamidopentylamine (5-BP) as a TG2 activity probe. Tissue cryosections are then analyzed by immunohistochemistry to ascertain the extent of 5-BP incorporation, which serves as a record of the redox state of TG2 in vivo. This protocol focuses on the modulation and measurement of TG2 in the small intestine, but we encourage investigators to evaluate it in their organ(s) of interest.

    View details for PubMedID 31069776

  • In Vivo Measurement of Redox-Regulated TG2 Activity FUNCTIONAL DISULPHIDE BONDS: METHODS AND PROTOCOLS Melkonian, A. V., Weng, N., Palanski, B. A., Khosla, C., Hogg, P. 2019; 1967: 263–74
  • Interleukin 4 is inactivated via selective disulfide-bond reduction by extracellular thioredoxin. Proceedings of the National Academy of Sciences of the United States of America Plugis, N. M., Weng, N., Zhao, Q., Palanski, B. A., Maecker, H. T., Habtezion, A., Khosla, C. 2018


    Thioredoxin 1 (TRX), an essential intracellular redox regulator, is also secreted by mammalian cells. Recently, we showed that TRX activates extracellular transglutaminase 2 via reduction of an allosteric disulfide bond. In an effort to identify other extracellular substrates of TRX, macrophages derived from THP-1 cells were treated with NP161, a small-molecule inhibitor of secreted TRX. NP161 enhanced cytokine outputs of alternatively activated macrophages, suggesting that extracellular TRX regulated the activity of interleukin 4 (IL-4) and/or interleukin 13 (IL-13). To test this hypothesis, the C35S mutant of human TRX was shown to form a mixed disulfide bond with recombinant IL-4 but not IL-13. Kinetic analysis revealed a kcat/KM value of 8.1 muM-1min-1 for TRX-mediated recognition of IL-4, which established this cytokine as the most selective partner of extracellular TRX to date. Mass spectrometry identified the C46-C99 bond of IL-4 as the target of TRX, consistent with the essential role of this disulfide bond in IL-4 activity. To demonstrate the physiological relevance of our biochemical findings, recombinant TRX was shown to attenuate IL-4-dependent proliferation of cultured TF-1 erythroleukemia cells and also to inhibit the progression of chronic pancreatitis in an IL-4-driven mouse model of this disease. By establishing that IL-4 is posttranslationally regulated by TRX-promoted reduction of a disulfide bond, our findings highlight a novel regulatory mechanism of the type 2 immune response that is specific to IL-4 over IL-13.

    View details for PubMedID 30104382

  • Thioredoxin-1 Selectively Activates Transglutaminase 2 in the Extracellular Matrix of the Small Intestine: IMPLICATIONS FOR CELIAC DISEASE. journal of biological chemistry Plugis, N. M., Palanski, B. A., Weng, C., Albertelli, M., Khosla, C. 2017; 292 (5): 2000-2008


    Transglutaminase 2 (TG2) catalyzes transamidation or deamidation of its substrates and is ordinarily maintained in a catalytically inactive state in the intestine and other organs. Aberrant TG2 activity is thought to play a role in celiac disease, suggesting that a better understanding of TG2 regulation could help to elucidate the mechanistic basis of this malady. Structural and biochemical analysis has led to the hypothesis that extracellular TG2 activation involves reduction of an allosteric disulfide bond by thioredoxin-1 (TRX), but cellular and in vivo evidence for this proposal is lacking. To test the physiological relevance of this hypothesis, we first showed that macrophages exposed to pro-inflammatory stimuli released TRX in sufficient quantities to activate their extracellular pools of TG2. By using the C35S mutant of TRX, which formed a metastable mixed disulfide bond with TG2, we demonstrated that these proteins specifically recognized each other in the extracellular matrix of fibroblasts. When injected into mice and visualized with antibodies, we observed the C35S TRX mutant bound to endogenous TG2 as its principal protein partner in the small intestine. Control experiments showed no labeling of TG2 knock-out mice. Intravenous administration of recombinant TRX in wild-type mice, but not TG2 knock-out mice, led to a rapid rise in intestinal transglutaminase activity in a manner that could be inhibited by small molecules targeting TG2 or TRX. Our findings support the potential pathophysiological relevance of TRX in celiac disease and establish the Cys(370)-Cys(371) disulfide bond of TG2 as one of clearest examples of an allosteric disulfide bond in mammals.

    View details for DOI 10.1074/jbc.M116.767988

    View details for PubMedID 28003361

    View details for PubMedCentralID PMC5290969

  • In vitro reconstitution and analysis of the 6-deoxyerythronolide B synthase. Journal of the American Chemical Society Lowry, B., Robbins, T., Weng, C., O'Brien, R. V., Cane, D. E., Khosla, C. 2013; 135 (45): 16809-16812


    Notwithstanding an extensive literature on assembly line polyketide synthases such as the 6-deoxyerythronolide B synthase (DEBS), a complete naturally occurring synthase has never been reconstituted in vitro from purified protein components. Here, we describe the fully reconstituted DEBS and quantitatively characterize some of the properties of the assembled system that have never been explored previously. The maximum turnover rate of the complete hexamodular system is 1.1 min(-1), comparable to the turnover rate of a truncated trimodular derivative (2.5 min(-1)) but slower than that of a bimodular derivative (21 min(-1)). In the presence of similar concentrations of methylmalonyl- and ethylmalonyl-CoA substrates, DEBS synthesizes multiple regiospecifically modified analogues, one of which we have analyzed in detail. Our studies lay the foundation for biochemically interrogating and rationally engineering polyketide assembly lines in an unprecedented manner.

    View details for DOI 10.1021/ja409048k

    View details for PubMedID 24161212

    View details for PubMedCentralID PMC3858836