Three-dimensional structure of the human transglutaminase 3 enzyme: binding of calcium ions changes structure for activation

Fungal transglutaminase domain-containing proteins are involved in hyphal protection at the septal pore against wounding

Transglutaminase (TG) is a ubiquitous enzyme that crosslinks substrates. In humans, TG participates in blood clotting and wound healing. However, the functions related to the cellular protection of microbial TG are unknown. In filamentous fungi, we previously identified SppB, which contains the transglutaminase core (TGc) domain and functions in hyphal protection at the septal pore upon wounding. Here, we further analyzed the cytokinesis-related protein Cyk3 and peptide N-glycanase Png1, as both contain the TGc domain. All three proteins exhibited functional importance in wound-related hyphal protection at the septal pore. Upon wounding, SppB and AoPng1 accumulated at the septal pore, whereas AoCyk3 and AoPng1 normally localized around the septal pore. The putative Cys-His-Asp catalytic triad of SppB is conserved with the human TGc domain-containing kyphoscoliosis peptidase. Catalytic triad disruptive mutants of SppB and AoCyk3 exhibited septal pore plugging defects. Similar to other TGs, SppB underwent wound-induced truncation of the N-terminal region. Notably, TG activity was detected in vivo at the septal pore of wounded hyphae using a fluorescent-labeled substrate; however, the activity was inhibited by the TG inhibitor cystamine. Our study suggests a conserved role for TGc domain-containing proteins in wound-related protection in fungi, similar to that in humans.

Autoantibody binding and unique enzyme-substrate intermediate conformation of human transglutaminase 3

Transglutaminase 3 (TG3), the autoantigen of dermatitis herpetiformis (DH), is a calcium dependent enzyme that targets glutamine residues in polypeptides for either transamidation or deamidation modifications. To become catalytically active TG3 requires proteolytic cleavage between the core domain and two C-terminal β-barrels (C1C2). Here, we report four X-ray crystal structures representing inactive and active conformations of human TG3 in complex with a TG3-specific Fab fragment of a DH patient derived antibody. We demonstrate that cleaved TG3, upon binding of a substrate-mimicking inhibitor, undergoes a large conformational change as a β-sheet in the catalytic core domain moves and C1C2 detaches. The unique enzyme-substrate conformation of TG3 without C1C2 is recognized by DH autoantibodies. The findings support a model where B-cell receptors of TG3-specific B cells bind and internalize TG3-gluten enzyme-substrate complexes thereby facilitating gluten-antigen presentation, T-cell help and autoantibody production.

Separate Gut Plasma Cell Populations Produce Auto-Antibodies against Transglutaminase 2 and Transglutaminase 3 in Dermatitis Herpetiformis

Dermatitis herpetiformis (DH) is an inflammatory skin disorder often considered as an extra intestinal manifestation of celiac disease (CeD). Hallmarks of CeD and DH are auto-antibodies to transglutaminase 2 (TG2) and transglutaminase 3 (TG3), respectively. DH patients have auto-antibodies reactive with both transglutaminase enzymes. Here it is reported that in DH both gut plasma cells and serum auto-antibodies are specific for either TG2 or TG3 with no TG2-TG3 cross reactivity. By generating monoclonal antibodies from TG3-specific duodenal plasma cells of DH patients, three conformational epitope groups are defined. Both TG2-specific and TG3-specific gut plasma cells have few immunoglobulin (Ig) mutations, and the two transglutaminase-reactive populations show distinct selection of certain heavy and light chain V-genes. Mass spectrometry analysis of TG3-specific serum IgA corroborates preferential usage of IGHV2-5 in combination with IGKV4-1. Collectively, these results demonstrate parallel induction of anti-TG2 and anti-TG3 auto-antibody responses involving separate B-cell populations in DH patients.

Identification and Classification of Papain-like Cysteine Proteinases

Papain-like cysteine peptidases form a big and highly diverse superfamily of proteins involved in many important biological functions, such as protein turnover, deubiquitination, tissue remodeling, blood clotting, virulence, defense, and cell wall remodeling. High sequence and structure diversity observed within these proteins hinders their comprehensive classification as well as the identification of new representatives. Moreover, in general protein databases, many families already classified as papain-like lack details regarding their mechanism of action or biological function. Here, we use transitive remote homology searches and 3D modeling to newly classify 21 families to the papain-like cysteine peptidase superfamily. We attempt to predict their biological function, and provide structural chacterization of 89 protein clusters defined based on sequence similarity altogether spanning 106 papain-like families. Moreover, we systematically discuss observed diversity in sequences, structures, and catalytic sites. Eventually, we expand the list of human papain-related proteins by seven representatives, including dopamine receptor-interacting protein (DRIP1) as potential deubiquitinase, and centriole duplication regulating CEP76 as retaining catalytically active peptidase-like domain. The presented results not only provide structure-based rationales to already existing peptidase databases but also may inspire further experimental research focused on peptidase-related biological processes.

PSMA2 knockdown impacts expression of proteins involved in immune and cellular stress responses in human lung cells

Proteasome subunit alpha type-2 (PSMA2) is a critical component of the 20S proteasome, which is the core particle of the 26S proteasome complex and is involved in cellular protein quality control by recognizing and recycling defective proteins. PSMA2 expression dysregulation has been detected in different human diseases and viral infections. No study yet has reported PSMA2 knockdown (KD) effects on the cellular proteome.

METHODS

We used SOMAScan, an aptamer-based multiplexed technique, to measure >1300 human proteins to determine the impact of PSMA2 KD on A549 human lung epithelial cells.

RESULTS

PSMA2 KD resulted in significant dysregulation of 52 cellular proteins involved in different bio-functions, including cellular movement and development, cell death and survival, and cancer. The immune system and signal transduction were the most affected cellular functions. PSMA2 KD caused dysregulation of several signaling pathways involved in immune response, cytokine signaling, organismal growth and development, cellular stress and injury (including autophagy and unfolded protein response), and cancer responses.

CONCLUSIONS

In summary, this study helps us better understand the importance of PSMA2 in different cellular functions, signaling pathways, and human diseases.

Structure, dynamics and assembly of the ankyrin complex on human red blood cell membrane

The cytoskeleton of a red blood cell (RBC) is anchored to the cell membrane by the ankyrin complex. This complex is assembled during RBC genesis and comprises primarily band 3, protein 4.2 and ankyrin, whose mutations contribute to numerous human inherited diseases. High-resolution structures of the ankyrin complex have been long sought-after to understand its assembly and disease-causing mutations. Here, we analyzed native complexes on the human RBC membrane by stepwise fractionation. Cryo-electron microscopy structures of nine band-3-associated complexes reveal that protein 4.2 stabilizes the cytoplasmic domain of band 3 dimer. In turn, the superhelix-shaped ankyrin binds to this protein 4.2 via ankyrin repeats (ARs) 6-13 and to another band 3 dimer via ARs 17-20, bridging two band 3 dimers in the ankyrin complex. Integration of these structures with both prior data and our biochemical data supports a model of ankyrin complex assembly during erythropoiesis and identifies interactions essential for the mechanical stability of RBC.

Transglutaminase 3 crosslinks the secreted gel-forming mucus component Mucin-2 and stabilizes the colonic mucus layer

The colonic mucus layer is organized as a two-layered system providing a physical barrier against pathogens and simultaneously harboring the commensal flora. The factors contributing to the organization of this gel network are not well understood. In this study, the impact of transglutaminase activity on this architecture was analyzed. Here, we show that transglutaminase TGM3 is the major transglutaminase-isoform expressed and synthesized in the colon. Furthermore, intrinsic extracellular transglutaminase activity in the secreted mucus was demonstrated in vitro and ex vivo. Absence of this acyl-transferase activity resulted in faster degradation of the major mucus component the MUC2 mucin and changed the biochemical properties of mucus. Finally, TGM3-deficient mice showed an early increased susceptibility to Dextran Sodium Sulfate-induced colitis. Here, we report that natural isopeptide cross-linking by TGM3 is important for mucus homeostasis and protection of the colon from inflammation, reducing the risk of colitis.

The colonic mucus layer is an organized system providing a physical barrier against pathogens and simultaneously harbouring the commensal flora. Here the authors report that transglutaminase 3 activity contributes to homeostasis of the colonic mucus layer and the lack of this enzymatic activity leads to increased susceptibility against DSS-induced colitis in mice.

A method to prevail false positive responses due to excess cations and viscous nature of Radiopharmaceuticals in Limulus Amebocyte Lysate Gel Clot test

OBJECTIVES

Bacterial endotoxin test (BET) for detection and quantification of endotoxin in radiopharmaceuticals (RPs), used for therapy or diagnosis, is prerequisite to administration in patients. Out of the two established methods used for this purpose (Kinetic Chromogenic Assay: KCM and Gel Clot Bacterial Endotoxin Test: GC-BET), GC-BET is recommended by pharmacopeias to evaluate the interferences exhibited during the assay due to presence of various ingredients in samples. In the present study, the influence of excess of cations in [¹⁷⁷Lu]Lu-DOTATATE, used for Peptide Receptor Radionuclide Therapy (PRRT), were studied and a protocol to negate the enhancement observed was developed. Additionally, a protocol for carrying out GC-BET for extremely viscous [¹³¹I]I-Lipiodol was standardized.

METHODS

GC-BET was performed for [¹⁷⁷Lu]Lu-DOTATATE and [¹³¹I]I-Lipiodol at maximum valid dilution (MVD), using LRW as a diluent. To negate the false positivity observed in case of [¹⁷⁷Lu]Lu-DOTATATE, various concentrations of calcium chloride (CaCl₂) were added and evaluated for the reversal of the interference observed initially. To prevail the difficulty in performing GC-BET for [¹³¹I]I-Lipiodol various modification in the protocols like orbital vortexing at different rpm and time intervals were performed. KCM assays were also performed for studied RPs at MVD.

RESULTS

It was observed that at MVD, [¹⁷⁷Lu]Lu-DOTATATE exhibited false positivity in GC-BET. However, all the individual reagents used in labeling of [¹⁷⁷Lu]Lu-DOTATATE did not show any false positivity. Finally, performing the assay with an addition of 2mM CaCl₂ (final concentration) nullified the false positivity. Further, intricacy in performing GC-BET for [¹³¹I]I-Lipiodol due to its viscosity was resolved by orbital vortexing at 3000 rpm for 5 minutes.

CONCLUSIONS

Our study proved that false positivity was observed in GC-BET for [¹⁷⁷Lu]Lu-DOTATATE due to the presence excess M³⁺ ions. Further, our study is the first of its kind which demonstrated methods for negating these false positive results by using modified protocol and hypothesizing the reason behind the enhancement. Additionally, ours is the first study which proved that a simple step of vortexing the viscous RPs like [¹³¹I]I-Lipiodol can resolved the problems encountered during performing GC-BET due to viscosity of RPs.

Transglutaminase 2 promotes tumorigenicity of colon cancer cells by inactivation of the tumor suppressor p53

Despite a high clinical need for the treatment of colorectal carcinoma (CRC) as the second leading cause of cancer-related deaths, targeted therapies are still limited. The multifunctional enzyme Transglutaminase 2 (TGM2), which harbors transamidation and GTPase activity, has been implicated in the development and progression of different types of human cancers. However, the mechanism and role of TGM2 in colorectal cancer are poorly understood. Here, we present TGM2 as a promising drug target.In primary patient material of CRC patients, we detected an increased expression and enzymatic activity of TGM2 in colon cancer tissue in comparison to matched normal colon mucosa cells. The genetic ablation of TGM2 in CRC cell lines using shRNAs or CRISPR/Cas9 inhibited cell expansion and tumorsphere formation. In vivo, tumor initiation and growth were reduced upon genetic knockdown of TGM2 in xenotransplantations. TGM2 ablation led to the induction of Caspase-3-driven apoptosis in CRC cells. Functional rescue experiments with TGM2 variants revealed that the transamidation activity is critical for the pro-survival function of TGM2. Transcriptomic and protein-protein interaction analyses applying various methods including super-resolution and time-lapse microscopy showed that TGM2 directly binds to the tumor suppressor p53, leading to its inactivation and escape of apoptosis induction.We demonstrate here that TGM2 is an essential survival factor in CRC, highlighting the therapeutic potential of TGM2 inhibitors in CRC patients with high TGM2 expression. The inactivation of p53 by TGM2 binding indicates a general anti-apoptotic function, which may be relevant in cancers beyond CRC.

Transglutaminase 3: The Involvement in Epithelial Differentiation and Cancer

Transglutaminases (TGMs) contribute to the formation of rigid, insoluble macromolecular complexes, which are essential for the epidermis and hair follicles to perform protective and barrier functions against the environment. During differentiation, epidermal keratinocytes undergo structural alterations being transformed into cornified cells, which constitute a highly tough outermost layer of the epidermis, the stratum corneum. Similar processes occur during the hardening of the hair follicle and the hair shaft, which is provided by the enzymatic cross-linking of the structural proteins and keratin intermediate filaments. TGM3, also known as epidermal TGM, is one of the pivotal enzymes responsible for the formation of protein polymers in the epidermis and the hair follicle. Numerous studies have shown that TGM3 is extensively involved in epidermal and hair follicle physiology and pathology. However, the roles of TGM3, its substrates, and its importance for the integument system are not fully understood. Here, we summarize the main advances that have recently been achieved in TGM3 analyses in skin and hair follicle biology and also in understanding the functional role of TGM3 in human tumor pathology as well as the reliability of its prognostic clinical usage as a cancer diagnosis biomarker. This review also focuses on human and murine hair follicle abnormalities connected with TGM3 mutations.

Characterisation of a novel cold-adapted calcium-activated transglutaminase: implications for medicine and food processing

Transglutaminases are a family of enzymes that catalyse the cross‐linking of proteins by forming covalent bonds between lysine and glutamine residues in various polypeptides. Cross‐linking reactions are involved in blood clots, skin formation, embryogenesis and apoptosis. Clinically, these enzymes appear to be implicated in neurodegenerative diseases, tumours and coeliac diseases. Transglutaminases have great potential for use in the food industry because of their ability to cross‐link proteins that are not normally linked. Here, a gene coding for transglutaminase from Atlantic cod was cloned into a bacterial expression vector and used to transform protein expression in a strain of Escherichia coli. The successful expression of recombinant transglutaminase protein from Atlantic cod (AcTG‐1) as a soluble protein upon induction at low temperature was confirmed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, immunoblotting and mass spectrometry analysis. Biochemical characterisation demonstrated that the transglutaminase was active between 0 and 65 °C, but was completely inactivated after 20‐min incubation at 70 °C. Interestingly, the enzyme displayed cold‐adapted features, such as temperature instability combined with high catalytic efficiency at low temperatures (8–16 °C). In addition, the enzyme had optimal activity at 50 °C, a new feature for a cold‐adapted enzyme. AcTG‐1 was active in the pH range from 6 to 9, with an optimum at pH 8, and required 5 mm calcium for maximum activity. Potential calcium‐binding sites in the enzyme were predictable, making the enzyme an appropriate model for studying structure–function relationships in the calcium‐dependent transglutaminase family. In vitro gel analysis revealed that transglutaminase cross‐linked casein, collagen and gelatin. The binding of fish fillets in the presence of recombinant AcTG‐1 provided further macroscopic proof for the potential application of AcTG‐1 as a biological cross‐linker in the food industry. Once binding occurred, fish fillets withstood further processing such as frying, boiling, freeze‐thawing and chilling. The low‐temperature activity and new enzymatic properties of AcTG‐1 appear to offer advantages over commercially available enzymatic glues in the food industry.

Competitive Binding of Magnesium to Calcium Binding Sites Reciprocally Regulates Transamidase and GTP Hydrolysis Activity of Transglutaminase 2

Transglutaminase 2 (TG2) is a Ca²⁺-dependent enzyme, which regulates various cellular processes by catalyzing protein crosslinking or polyamination. Intracellular TG2 is activated and inhibited by Ca²⁺ and GTP binding, respectively. Although aberrant TG2 activation has been implicated in the pathogenesis of diverse diseases, including cancer and degenerative and fibrotic diseases, the structural basis for the regulation of TG2 by Ca²⁺ and GTP binding is not fully understood. Here, we produced and analyzed a Ca²⁺-containing TG2 crystal, and identified two glutamate residues, E437 and E539, as Ca²⁺-binding sites. The enzymatic analysis of the mutants revealed that Ca²⁺ binding to these sites is required for the transamidase activity of TG2. Interestingly, we found that magnesium (Mg²⁺) competitively binds to the E437 and E539 residues. The Mg²⁺ binding to these allosteric sites enhances the GTP binding/hydrolysis activity but inhibits transamidase activity. Furthermore, HEK293 cells transfected with mutant TG2 exhibited higher transamidase activity than cells with wild-type TG2. Cells with wild-type TG2 showed an increase in transamidase activity under Mg²⁺-depleted conditions, whereas cells with mutant TG2 were unaffected. These results indicate that E437 and E539 are Ca²⁺-binding sites contributing to the reciprocal regulation of transamidase and GTP binding/hydrolysis activities of TG2 through competitive Mg²⁺ binding.

Structure functional insights into calcium binding during the activation of coagulation factor XIII A

The dimeric FXIII-A₂, a pro-transglutaminase is the catalytic part of the heterotetrameric coagulation FXIII-A₂B₂ complex that upon activation by calcium binding/thrombin cleavage covalently cross-links preformed fibrin clots protecting them from premature fibrinolysis. Our study characterizes the recently disclosed three calcium binding sites of FXIII-A concerning evolution, mutual crosstalk, thermodynamic activation profile, substrate binding, and interaction with other similarly charged ions. We demonstrate unique structural aspects within FXIII-A calcium binding sites that give rise to functional differences making FXIII unique from other transglutaminases. The first calcium binding site showed an antagonistic relationship towards the other two. The thermodynamic profile of calcium/thrombin-induced FXIII-A activation explains the role of bulk solvent in transitioning its zymogenic dimeric form to an activated monomeric form. We also explain the indirect effect of solvent ion concentration on FXIII-A activation. Our study suggests FXIII-A calcium binding sites could be putative pharmacologically targetable regions.

SpTGase plays an important role in the hemolymph clotting in mud crab (Scylla paramamosain)

Transglutaminase (TGase) is important in blood coagulation, a conserved immunological defense mechanism among invertebrates. This study is the first report of the TGase in mud crab (Scylla paramamosain) (SpTGase) with a 2304 bp ORF encoding 767 amino acids (molecular weight 85.88 kDa). SpTGase is acidic, hydrophilic, stable and thermostable, containing three transglutaminase domains, one TGase/protease-like homolog domain (TGc), one integrin-binding motif (Arg²⁷⁰, Gly²⁷¹, Asp²⁷²) and three catalytic sites (Cys³³³, His⁴⁰¹, Asp⁴²⁴) within the TGc. Neither a signal peptide nor a transmembrane domain was found, and the random coil is dominant in the secondary structure of SpTGase. Phylogenetic analysis revealed a close relation between SpTGase to its homolog EsTGase 1 from Chinese mitten crab (Eriocheir sinensis). Expression of SpTGase was investigated using qRT-PCR (1) in eight tissues from healthy mud crabs, with the highest expression in hemocytes, and (2) in response to various immune challenges (Vibrio parahaemolyticus, lipopolysaccharide (LPS) or Poly I:C infection), revealing a major up-regulation in hemocytes, skin, and hepatopancreas during the 96-h post injection. The recombinant SpTGase showed a capacity of agglutination activities on both Gram-negative bacteria and yeast. SpTGase was found to directly interact with another important blood coagulation component clip domain serine protease (SpcSP). Moreover, knockdown of SpTGase resulted in a decreased expression of both clotting protein precursor (SppreCP) and SpcSP and an increase of duration time in the blood coagulation. Taken together, the findings of this study suggest SpTGase play an important role in the hemolymph clotting in mud crab S. paramamosain.

Structural and functional characterization of TgpA, a critical protein for the viability of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen associated with severe diseases, such as cystic fibrosis. During an extensive search for novel essential genes, we identified tgpA (locus PA2873) in P. aeruginosa PAO1, as a gene playing a critical role in bacterial viability. TgpA, the translated protein, is an internal membrane protein with a periplasmic soluble domain, predicted to be endowed with a transglutaminase-like fold, hosting the Cys404, His448, and Asp464 triad. We report here that Cys404 mutation hampers the essential role of TgpA in granting P. aeruginosa viability. Moreover, we present the crystal structure of the TgpA periplasmic domain at 1.6 Å resolution as a first step towards structure-activity analysis of a new potential target for the discovery of antibacterial compounds.

The clotting system in decapod crustaceans: History, current knowledge and what we need to know beyond the models

Hemolymph coagulation is among the major arms of the humoral immune response in crustaceans. According to the current model, hemolymph clotting in decapod crustacean relies mostly on the polymerization of the plasmatic clotting protein (CP) which is directly promoted by calcium-depended transglutaminase (TGase) released from hemocytes upon microbial stimulus or injury. However, the type of hemocytes containing TGase, and hence how the TGase is released, might vary among species. Thus, we discourse here about possible mechanisms for clotting initiation. On the other hand, the initiation of coagulation reaction in the absence of microbial elicitors is poorly understood and seems to involve hemocytes lability, yet the mechanism remains unknown. A cellular clottable protein called coagulogen, different to the plasma CP, occurs in several species and could be related with the immune response, but the biological relevance of this protein is unknown. It is also demonstrated that the clotting response is actively involved in defense against pathogens. In addition, both TGase and the CP show pleiotropic functions, and although both proteins are relatively conserved, some of their physic-chemical properties vary significantly. The occurrence of differences in the clotting system in crustaceans is conceivable given the high number of species and their diverse ecology. Results from still non-studied decapods may provide explanation for some of the issues presented here from an evolutionary perspective.

Opening up about Tissue Transglutaminase: When Conformation Matters More than Enzymatic Activity

Tissue transglutaminase (tTG), also referred to as type 2 transglutaminase or Gα_h, can bind and hydrolyze GTP, as well as function as a protein crosslinking enzyme. tTG is widely expressed and can be detected both inside cells and in the extracellular space. In contrast to many enzymes, the active and inactive conformations of tTG are markedly different. The catalytically inactive form of tTG adopts a compact “closed-state” conformation, while the catalytically active form of the protein adopts an elongated “open-state” conformation. tTG has long been appreciated as an important player in numerous diseases, including celiac disease, neuronal degenerative diseases, and cancer, and its roles in these diseases often depend as much upon its conformation as its catalytic activity. While its ability to promote these diseases has been traditionally thought to be dependent on its protein crosslinking activity, more recent findings suggest that the conformational state tTG adopts is also important for mediating its effects. In particular, we and others have shown that the closed-state of tTG is important for promoting cell growth and survival, while maintaining tTG in the open-state is cytotoxic. In this review, we examine the two unique conformations of tTG and how they contribute to distinct biological processes. We will also describe how this information can be used to generate novel therapies to treat diseases, with a special focus on cancer.

Structure of natural variant transglutaminase 2 reveals molecular basis of gaining stability and higher activity

Multi-functional transglutaminase 2 (TG2), which possesses protein cross-linking and GTP hydrolysis activities, is involved in various cellular processes, including apoptosis, angiogenesis, wound healing, and neuronal regeneration, and is associated with many human diseases, including inflammatory disease, celiac disease, neurodegenerative disease, diabetes, tissue fibrosis, and cancers. Although most biochemical and cellular studies have been conducted with the TG2 (G224) form, the TG2 (G224V) form has recently emerged as a putative natural variant of TG2. In this study, we characterized the putative natural form of TG2, TG2 (G224V), and through a new crystal structure of TG2 (G224V), we revealed how TG2 (G224V) gained stability and higher Ca²⁺-dependent activity than an artificial variant of TG2 (G224).

Structural aspects of transglutaminase 2: functional, structural, and regulatory diversity

Transglutaminase 2 (TG2) is a multi-functional protein that has both protein cross-linking and guanosine 5'-triphosphate (GTP) hydrolysis activities. The activities of this protein are controlled by many cellular factors, including calcium (Ca²⁺) and GTP, and have been implicated in several physiological activities, including apoptosis, angiogenesis, wound healing, cellular differentiation, neuronal regeneration, and bone development. TG2 is linked to many human diseases such as inflammatory disease, celiac disease, neurodegenerative disease, diabetes, tissue fibrosis, and various cancers and is one of the most dynamic enzymes in terms of its functions, structures, and regulatory mechanisms. The aim of this review was to summarize the functional, structural, and regulatory diversity of TG2, with a particular focus on the structure of TG2.

Genomic variants reveal differential evolutionary constraints on human transglutaminases and point towards unrecognized significance of transglutaminase 2

Transglutaminases (TGMs) catalyze Ca2+-dependent transamidation of proteins with specified roles in blood clotting (F13a) and in cornification (TGM1, TGM3). The ubiquitous TGM2 has well described enzymatic and non-enzymatic functions but in-spite of numerous studies its physiological function in humans has not been defined. We compared data on non-synonymous single nucleotide variations (nsSNVs) and loss-of-function variants on TGM1-7 and F13a from the Exome aggregation consortium dataset, and used computational and biochemical analysis to reveal the roles of damaging nsSNVs of TGM2. TGM2 and F13a display rarer damaging nsSNV sites than other TGMs and sequence of TGM2, F13a and TGM1 are evolutionary constrained. TGM2 nsSNVs are predicted to destabilize protein structure, influence Ca2+ and GTP regulation, and non-enzymatic interactions, but none coincide with conserved functional sites. We have experimentally characterized six TGM2 allelic variants detected so far in homozygous form, out of which only one, p.Arg222Gln, has decreased activities. Published exome sequencing data from various populations have not uncovered individuals with homozygous loss-of-function variants for TGM2, TGM3 and TGM7. Thus it can be concluded that human transglutaminases differ in harboring damaging variants and TGM2 is under purifying selection suggesting that it may have so far not revealed physiological functions.

Interplay between transglutaminases and heparan sulphate in progressive renal scarring

Transglutaminase-2 (TG2) is a new anti-fibrotic target for chronic kidney disease, for its role in altering the extracellular homeostatic balance leading to excessive build-up of matrix in kidney. However, there is no confirmation that TG2 is the only transglutaminase involved, neither there are strategies to control its action specifically over that of the conserved family-members. In this study, we have profiled transglutaminase isozymes in the rat subtotal nephrectomy (SNx) model of progressive renal scarring. All transglutaminases increased post-SNx peaking at loss of renal function but TG2 was the predominant enzyme. Upon SNx, extracellular TG2 deposited in the tubulointerstitium and peri-glomerulus via binding to heparan sulphate (HS) chains of proteoglycans and co-associated with syndecan-4. Extracellular TG2 was sufficient to activate transforming growth factor-β1 in tubular epithelial cells, and this process occurred in a HS-dependent way, in keeping with TG2-affinity for HS. Analysis of heparin binding of the main transglutaminases revealed that although the interaction between TG1 and HS is strong, the conformational heparin binding site of TG2 is not conserved, suggesting that TG2 has a unique interaction with HS within the family. Our data provides a rationale for a novel anti-fibrotic strategy specifically targeting the conformation-dependent TG2-epitope interacting with HS.

Structural and Functional Characterization of an Ancient Bacterial Transglutaminase Sheds Light on the Minimal Requirements for Protein Cross-Linking

Transglutaminases are best known for their ability to catalyze protein cross-linking reactions that impart chemical and physical resilience to cellular structures. Here, we report the crystal structure and characterization of Tgl, a transglutaminase from the bacterium Bacillus subtilis. Tgl is produced during sporulation and cross-links the surface of the highly resilient spore. Tgl-like proteins are found only in spore-forming bacteria of the Bacillus and Clostridia classes, indicating an ancient origin. Tgl is a single-domain protein, produced in active form, and the smallest transglutaminase characterized to date. We show that Tgl is structurally similar to bacterial cell wall endopeptidases and has an NlpC/P60 catalytic core, thought to represent the ancestral unit of the cysteine protease fold. We show that Tgl functions through a unique partially redundant catalytic dyad formed by Cys116 and Glu187 or Glu115. Strikingly, the catalytic Cys is insulated within a hydrophobic tunnel that traverses the molecule from side to side. The lack of similarity of Tgl to other transglutaminases together with its small size suggests that an NlpC/P60 catalytic core and insulation of the active site during catalysis may be essential requirements for protein cross-linking.

Transglutaminase as a therapeutic target for celiac disease

INTRODUCTION

The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development.

AREAS COVERED

The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach.

EXPERT OPINION

Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

Activity-regulating structural changes and autoantibody epitopes in transglutaminase 2 assessed by hydrogen/deuterium exchange

The multifunctional enzyme transglutaminase 2 (TG2) is the target of autoantibodies in the gluten-sensitive enteropathy celiac disease. In addition, the enzyme is responsible for deamidation of gluten peptides, which are subsequently targeted by T cells. To understand the regulation of TG2 activity and the enzyme's role as an autoantigen in celiac disease, we have addressed structural properties of TG2 in solution by using hydrogen/deuterium exchange monitored by mass spectrometry. We demonstrate that Ca(2+) binding, which is necessary for TG2 activity, induces structural changes in the catalytic core domain of the enzyme. Cysteine oxidation was found to abolish these changes, suggesting a mechanism whereby disulfide bond formation inactivates the enzyme. Further, by using TG2-specific human monoclonal antibodies generated from intestinal plasma cells of celiac disease patients, we observed that binding of TG2 by autoantibodies can induce structural changes that could be relevant for the pathogenesis. Detailed mapping of two of the main epitopes targeted by celiac disease autoantibodies revealed that they are located adjacent to each other in the N-terminal part of the TG2 molecule.

Protein structure as a means to triage proposed PTM sites

PTMs such as phosphorylation are often important actors in protein regulation and recognition. These functions require both visibility and accessibility to other proteins; that the modification is located at the surface of the protein. Currently, many repositories provide information on PTMs but structural information is often lacking. This study, which focuses on phosphorylation sites available in UniProtKB/Swiss-Prot, illustrates that most phosphorylation sites are indeed found at the surface of the protein, but that some sites are found buried in the core of the protein. Several of these identified buried phosphorylation sites can easily become accessible upon small conformational changes while others would require the whole protein to unfold and are hence most unlikely modification sites. Subsequent analysis of phosphorylation sites available in PRIDE demonstrates that taking the structure of the protein into account would be a good guide in the identification of the actual phosphorylated positions in phophoproteomics experiments. This analysis illustrates that care must be taken when simply accepting the position of a PTM without first analyzing its position within the protein structure if the latter is available.

Activation of Extracellular Transglutaminase 2 by Mechanical Force in the Arterial Wall

Inward remodeling of small arteries occurs after prolonged vasoconstriction, low blood flow, and in several models of hypertension. The cross-linking enzyme, transglutaminases 2 (TG2), is able to induce inward remodeling and stiffening of arteries. The activity of TG2 is dependent on its conformation, which can be open or closed, and on its redox state. Several factors have been shown to be involved in modulating TG2 activity, including Ca²⁺ and GTP/GDP concentrations, as well as the redox state of the environment. This review introduces the hypothesis that mechanical force could be involved in regulating the activity of TG2 during inward remodeling by promoting its open and reduced active state. Several aspects of TG2, such as its structure and localization, are assessed in order to provide arguments that support the hypothesis. We conclude that a direct activation of TG2 by mechanical force exerted by smooth muscle cells may explain the link between smooth muscle activation and inward remodeling, as observed in several physiological and pathological conditions.

Regulation of the activities of the mammalian transglutaminase family of enzymes

Mammalian transglutaminases catalyze post-translational modifications of glutamine residues on proteins and peptides through transamidation or deamidation reactions. Their catalytic mechanism resembles that of cysteine proteases. In virtually every case, their enzymatic activity is modulated by elaborate strategies including controlled gene expression, allostery, covalent modification, and proteolysis. In this review, we focus on our current knowledge of post-translational regulation of transglutaminase activity by physiological as well as synthetic allosteric agents. Our discussion will primarily focus on transglutaminase 2, but will also compare and contrast its regulation with Factor XIIIa as well as transglutaminases 1 and 3. Potential structure-function relationships of known mutations in human transglutaminases are analyzed.

A single conformational transglutaminase 2 epitope contributed by three domains is critical for celiac antibody binding and effects

The multifunctional, protein cross-linking transglutaminase 2 (TG2) is the main autoantigen in celiac disease, an autoimmune disorder with defined etiology. Glutamine-rich gliadin peptides from ingested cereals, after their deamidation by TG2, induce T-lymphocyte activation accompanied by autoantibody production against TG2 in 1-2% of the population. The pathogenic role and exact binding properties of these antibodies to TG2 are still unclear. Here we show that antibodies from different celiac patients target the same conformational TG2 epitope formed by spatially close amino acids of adjacent domains. Glu153 and 154 on the first alpha-helix of the core domain and Arg19 on first alpha-helix of the N-terminal domain determine the celiac epitope that is accessible both in the closed and open conformation of TG2 and dependent on the relative position of these helices. Met659 on the C-terminal domain also can cooperate in antibody binding. This composite epitope is disease-specific, recognized by antibodies derived from celiac tissues and associated with biological effects when passively transferred from celiac mothers into their newborns. These findings suggest that celiac antibodies are produced in a surface-specific way for which certain homology of the central glutamic acid residues of the TG2 epitope with deamidated gliadin peptides could be a structural basis. Monoclonal mouse antibodies with partially overlapping epitope specificity released celiac antibodies from patient tissues and antagonized their harmful effects in cell culture experiments. Such antibodies or similar specific competitors will be useful in further functional studies and in exploring whether interference with celiac antibody actions leads to therapeutic benefits.

Transglutaminase 6: a protein associated with central nervous system development and motor function

Transglutaminases (TG) form a family of enzymes that catalyse various post-translational modifications of glutamine residues in proteins and peptides including intra- and intermolecular isopeptide bond formation, esterification and deamidation. We have characterized a novel member of the mammalian TG family, TG6, which is expressed in a human carcinoma cell line with neuronal characteristics and in mouse brain. Besides full-length protein, alternative splicing results in a short variant lacking the second β-barrel domain in man and a variant with truncated β-sandwich domain in mouse. Biochemical data show that TG6 is allosterically regulated by Ca²⁺ and guanine nucleotides. Molecular modelling indicates that TG6 could have Ca²⁺ and GDP-binding sites related to those of TG3 and TG2, respectively. Localization of mRNA and protein in the mouse identified abundant expression of TG6 in the central nervous system. Analysis of its temporal and spatial pattern of induction in mouse development indicates an association with neurogenesis. Neuronal expression of TG6 was confirmed by double-labelling of mouse forebrain cells with cell type-specific markers. Induction of differentiation in mouse Neuro 2a cells with NGF or dibutyryl cAMP is associated with an upregulation of TG6 expression. Familial ataxia has recently been linked to mutations in the TGM6 gene. Autoantibodies to TG6 were identified in immune-mediated ataxia in patients with gluten sensitivity. These findings suggest a critical role for TG6 in cortical and cerebellar neurons.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Crystal structure and inhibition studies of transglutaminase from Streptomyces mobaraense

The crystal structure of the microbial transglutaminase (MTGase) zymogen from Streptomyces mobaraense has been determined at 1.9-Å resolution using the molecular replacement method based on the crystal structure of the mature MTGase. The overall structure of this zymogen is similar to that of the mature form, consisting of a single disk-like domain with a deep active cleft at the edge of the molecule. A major portion of the prosequence (45 additional amino acid residues at the N terminus of the mature transglutaminase) folds into an L-shaped structure, consisting of an extended N-terminal segment linked with a one-turn short helix and a long α-helix. Two key residues in the short helix of the prosequence, Tyr-12 and Tyr-16, are located on top of the catalytic triad (Cys-110, Asp-301, and His-320) to block access of the substrate acyl donors and acceptors. Biochemical characterization of the mature MTGase, using N-α-benzyloxycarbonyl-L-glutaminylglycine as a substrate, revealed apparent K(m) and k(cat)/K(m) values of 52.66 mM and 40.42 mM(-1) min(-1), respectively. Inhibition studies using the partial prosequence SYAETYR and homologous sequence SQAETYR showed a noncompetitive inhibition mechanism with IC(50) values of 0.75 and 0.65 mM, respectively, but no cross-linking product formation. Nevertheless, the prosequence homologous oligopeptide SQAETQR, with Tyr-12 and Tyr-16 each replaced with Gln, exhibited inhibitory activity with the formation of the SQAETQR-monodansylcadaverine fluorophore cross-linking product (SQAETQR-C-DNS). MALDI-TOF tandem MS analysis of SQAETQR-C-DNS revealed molecular masses corresponding to those of (N)SQAETQ(C)-C-DNS and C-DNS-(N)QR(C) sequences, suggesting the incorporation of C-DNS onto the C-terminal Gln residue of the prosequence homologous oligopeptide. These results support the putative functional roles of both Tyr residues in substrate binding and inhibition.

Reversible activation of cellular factor XIII by calcium

Factor XIII (FXIII) is a pro-transglutaminase found in the plasma as well as intracellularly in platelets and macrophages. Plasma FXIII is activated by thrombin cleavage (FXIIIa*) and acts in the final stages of blood coagulation cascade. In contrast, the function and activation of cellular FXIII are less characterized. Cellular FXIII relies on a conformational activation of the protein. The nonproteolytic activation of FXIII to FXIIIa° induced by Ca(2+) alone is well known, but up until now it has been discussed under which conditions the process can be induced and whether it can be reversed. Here, we study the nature of the Ca(2+)-induced FXIII activation. Previously used methods to evaluate FXIII activity detect both FXIIIa* and FXIIIa° because they rely on occurrence of enzyme activity or on active site Cys-314 solvent accessibility. Therefore, an analytical HPLC method was developed that separates zymogen recombinant FXIII (rFXIII) from rFXIIIa°. The data demonstrate that nonproteolytic activation and deactivation are highly dependent on Ca(2+) concentration, buffer, and salt components. Moreover, it is established that Ca(2+) activation of rFXIII is fully reversible, and only 2-5 mm CaCl(2) is sufficient to retain full rFXIIIa° activity. However, below 2 mm CaCl(2) the rFXIIIa° molecule deactivates. The deactivated molecule can subsequently undergo a new activation round. Furthermore, it is demonstrated that thermal stress of freeze-dried rFXIII can induce a new predisposed form that activates faster than nonstressed rFXIII.

Factor XIII: novel structural and functional aspects

Factor (F)XIII is a protransglutaminase that, in addition to maintaining hemostasis, has multiple plasmatic and intracellular functions. Its plasmatic form (pFXIII) is a tetramer of two potentially active A (FXIII-A) and two inhibitory/carrier B (FXIII-B) subunits, whereas its cellular form (cFXIII) is a dimer of FXIII-A. FXIII-A belongs to the family of transglutaminases (TGs), which show modest similarity in the primary structure, but a high degree of conservatism in their domain and sub-domain secondary structure. FXIII-A consists of an activation peptide, a β-sandwich, a catalytic and two β-barrel domains. FXIII-B is a glycoprotein consisting of 10 repetitive sushi domains each held together by two internal disulfide bonds. The structural elements of FXIII-A involved in the interaction with FXIII-B have not been elucidated; in FXIII-B the first sushi domain seems important for complex formation. In the circulation pFXIII is bound to the fibrinogen γ'-chain through its B subunit. In the process of pFXIII activation first thrombin cleaves off the activation peptide from FXIII-A, then in the presence of Ca(2+) FXIII-B dissociates and FXIII-A becomes transformed into an active transglutaminase (FXIIIa). The activation is highly accelerated by the presence of fibrin(ogen). cFXIII does not require proteolysis for intracellular activation. The three-dimensional structure of FXIIIa has not been resolved. Based on analogies with transglutaminase-2, a three-dimensional structure of FXIIIa was developed by molecular modeling, which shows good agreement with the drastic structural changes demonstrated by biochemical studies. The structural requirements for enzyme-substrate interaction and for transglutaminase activity are also reviewed.

Thermodynamics of binding of regulatory ligands to tissue transglutaminase

The transamidating activity of tissue transglutaminase is regulated by the ligands calcium and GTP, via conformational changes which facilitate or interfere with interaction with the peptidyl-glutamine substrate. We have analysed binding of these ligands by calorimetric and computational approaches. In the case of GTP we have detected a single high affinity site (K (D) approximately 1 microM), with moderate thermal effects suggestive that binding GTP involves replacement of GDP, normally bound to the protein. On line with this possibility no significant binding was observed during titration with GDP and computational studies support this view. Titration with calcium at a high cation molar excess yielded a complex binding isotherm with a number of "apparent binding sites" in large excess over those detectable by equilibrium dialysis (6 sites). This binding pattern is ascribed to occurrence of additional thermal contributions, beyond those of binding, due to the occurrence of conformational changes and to catalysis itself (with protein self-crosslinking). In contrast only one site for binding calcium with high affinity (K (D) approximately 0.15 microM) is observed with samples of enzyme inactivated by alkylation at the active site (to prevent enzyme crosslinkage and thermal effects of catalysis). These results indicate an intrinsic ability of tissue transglutaminase to bind calcium with high affinity and the necessity of careful reassessment of the enzyme regulatory pattern in relation to the concentrations of ligands in living cells, taking also in account effects of ligands on protein subcellular compartimentation.

Functional significance of five noncanonical Ca2+-binding sites of human transglutaminase 2 characterized by site-directed mutagenesis

The multifunctional tissue transglutaminase 2 (TG2) has a four-domain structure with several Ca(2+)-regulated biochemical activities, including transglutamylation and GTP hydrolysis. The structure of the Ca(2+)-binding form of the human enzyme is not known, and its Ca(2+)-binding sites have not been fully characterized. By mutagenesis, we have targeted its active site Cys, three sites based on homology to Ca(2+)-binding residues of epidermal transglutaminase and factor XIIIa (S1-S3), and two regions with negative surface potentials (S4 and S5). CD spectroscopy, antibody-binding assay and GTPase activity measurements indicated that the amino acid substitutions did not cause major structural alterations. Calcium-45 equilibrium dialysis and isothermal calorimetric titration showed that both wild-type and active site-deleted enzymes (C277S) bind six Ca(2+). Each of the S1-S5 mutants binds fewer than six Ca(2+), S1 is a strong Ca(2+)-binding site, and mutation of one site resulted in the loss of more than one bound Ca(2+), suggesting cooperativity among sites. All mutants were deficient in transglutaminase activity, and GTP inhibited remnant activities. Like those of the wild-type enzyme, the GTPase activities of the mutants were inhibited by Ca(2+), except in the case of the S4 and S5 mutants, which exhibited increased activity. TG2 is the major autoantigen in celiac disease, and testing the reactivity of mutants with autoantibodies from celiac disease patients revealed that S4 strongly determines antigenicity. It can be concluded that five of the Ca(2+)-binding sites of TG2 influence its transglutaminase activity, two sites are involved in the regulation of GTPase activity, and one determines antigenicity for autoantibodies in celiac patients.

Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

The treatment of collagen fibrils by tissue transglutaminase to promote vascular smooth muscle cell contractile signaling

The enzyme tissue transglutaminase 2 (TG2) appears to play an important role in several physiological processes such as wound healing, the progression of cancer and of vascular disease. Additionally, TG2 has been proposed as a means of stabilizing collagen extracellular matrix (ECM) scaffolds for tissue engineering applications. In this report, we examined the effect of TG2 treatment on the mechanical properties of the ECM, and associated cell responses. Using a model ECM of fibrillar collagen, we quantitatively examined vascular smooth muscle cell (vSMC) response to untreated, or TG2 treated collagen. We show that cells respond to TG2 treated collagen with increased spreading, an increase in contractile response as indicated by elevated F-actin polymerization and myosin light chain phosphorylation, and increased proliferation, without apparent changes in integrin specificity or matrix topography. Comparative atomic force microscopy loading studies indicate that TG2 treated fibrils are 3 times more resistant to shearing force from an AFM tip than untreated fibrils. The data suggest that TG2 treatment of collagen increases matrix mechanical stiffness, which apparently alters the contractile and proliferative response of vSMC.

Transglutaminase activation in neurodegenerative diseases

The following review examines the role of calcium in promoting the in vitro and in vivo activation of transglutaminases in neurodegenerative disorders. Diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease exhibit increased transglutaminase activity and rises in intracellular calcium concentrations, which may be related. The aberrant activation of transglutaminase by calcium is thought to give rise to a variety of pathological moieties in these diseases, and the inhibition has been shown to have therapeutic benefit in animal and cellular models of neurodegeneration. Given the potential clinical relevance of transglutaminase inhibitors, we have also reviewed the recent development of such compounds.

Photolabeling of tissue transglutaminase reveals the binding mode of potent cinnamoyl inhibitors

We have recently developed a new class of cinnamoyl derivatives as potent tissue transglutaminase (TG2) inhibitors. Herein, we report the synthesis of a diazirine derivative of these inhibitors and its application to the photolabeling of its binding site on guinea pig liver transglutaminase. Two novel homology models were generated for this commonly studied TG2, which differ in the conformational state they represent. Tryptic digest and mass spectrometric analysis of the photolabeling experiment showed that only residue Cys230 was labeled, and our homology models were used to visualize these results. This visualization suggested that Cys230 is somewhat more solvent-exposed in the "closed" conformation of TG2, compared to the "open" conformation. Docking experiments suggested binding modes consistent with the labeling pattern that would block access to the tunnel leading to the active site, consistent with the observed mode of inhibition. However, while these modeling simulations favored the closed conformation as the target of our cinnamoyl inhibitors, native PAGE experiments indicated the open conformation of the enzyme in fact predominates in the presence of our photolabeling derivative. These results are important for understanding the binding modes of TG2 inhibitors in general and will be critical for the structure-based design of future inhibitors.