The core domain of the tissue transglutaminase Gh hydrolyzes GTP and ATP

Transglutaminase 2 Facilitates Murine Wound Healing in a Strain-Dependent Manner

Transglutaminase 2 (TG2) plays a role in cellular processes that are relevant to wound healing, but to date no studies of wound healing in TG2 knockout mice have been reported. Here, using 129T2/SvEmsJ (129)- or C57BL/6 (B6)-backcrossed TG2 knockout mice, we show that TG2 facilitates murine wound healing in a strain-dependent manner. Early healing of in vivo cutaneous wounds and closure of in vitro scratch wounds in murine embryonic fibroblast (MEF) monolayers were delayed in 129, but not B6, TG2 knockouts, relative to their wild-type counterparts, with wound closure in 129 being faster than in B6 wild-types. A single dose of exogenous recombinant wild-type TG2 to 129 TG2-/- mice or MEFs immediately post-wounding accelerated wound closure. Neutrophil and monocyte recruitment to 129 cutaneous wounds was not affected by Tgm2 deletion up to 5 days post-wounding. Tgm2 mRNA and TG2 protein abundance were higher in 129 than in B6 wild-types and increased in abundance following cutaneous and scratch wounding. Tgm1 and factor XIIA (F13A) mRNA abundance increased post-wounding, but there was no compensation by TG family members in TG2-/- relative to TG2+/+ mice in either strain before or after wounding. 129 TG2+/+ MEF adhesion was greater and spreading was faster than that of B6 TG2+/+ MEFs, and was dependent on syndecan binding in the presence, but not absence, of RGD inhibition of integrin binding. Adhesion and spreading of 129, but not B6, TG2-/- MEFs was impaired relative to their wild-type counterparts and was accelerated by exogenous addition or transfection of TG2 protein or cDNA, respectively, and was independent of the transamidase or GTP-binding activity of TG2. Rho-family GTPase activation, central to cytoskeletal organization, was altered in 129 TG2-/- MEFs, with delayed RhoA and earlier Rac1 activation than in TG2+/+ MEFs. These findings indicate that the rate of wound healing is different between 129 and B6 mouse strains, correlating with TG2 abundance, and although not essential for wound healing, TG2 facilitates integrin- and syndecan-mediated RhoA- and Rac1-activation in fibroblasts to promote efficient wound contraction.

Sulforaphane covalently interacts with the transglutaminase 2 cancer maintenance protein to alter its structure and suppress its activity

Type 2 transglutaminase (TG2) functions as an important cancer cell survival protein in a range of cancers including epidermal squamous cell carcinoma. TG2 exists in open and closed conformations each of which has a distinct and mutually exclusive activity. The closed conformation has GTP-binding/GTPase activity while the open conformation functions as a transamidase to catalyze protein-protein crosslinking. GTP-binding/GTPase activity is required for TG2 maintenance of the aggressive cancer phenotype. Thus, identifying agents that convert TG2 from the closed to the open GTP-binding/GTPase inactive conformation is an important cancer prevention/treatment strategy. Sulforaphane (SFN) is an important diet-derived cancer prevention agent that is known to possess a reactive isothiocyanate group and has potent anticancer activity. Using a biotin-tagged SFN analog (Biotin-ITC) and kinetic analysis we show that SFN covalently and irreversibly binds to recombinant TG2 to inhibit transamidase activity and shift TG2 to an open/extended conformation, leading to a partial inhibition of GTP binding. We also show that incubation of cancer cells or cancer cell extract with Biotin-ITC results in formation of a TG2/Biotin-ITC complex and that SFN treatment of cancer cells inhibits TG2 transamidase activity and shifts TG2 to an open/extended conformation. These findings identify TG2 as a direct SFN anticancer target in epidermal squamous cell carcinoma.

Transglutaminase 2: Development of therapeutic antibodies reveals four inhibitory epitopes and confirms extracellular function in fibrotic remodelling

BACKGROUND AND PURPOSE

Transglutaminase type 2 (TG2) catalyses formation of ε-(γ-glutamyl)-lysine bonds between proteins, including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlies tissue scarring and fibrosis. Many transglutaminase inhibitors exist and allowed for proof-of-concept studies in disease models, but their lack of specificity for the TG2 isoform, and/or poor pharmacokinetic/pharmacodynamic properties have limited their clinical application. We hypothesised that a high affinity TG2-specific antibody could be developed to specifically inhibit extracellular TG2 activity, with characteristics suitable for therapeutic development.

EXPERIMENTAL APPROACH

Individual human TG2 domains were used to immunise mice and generate hybridomas. Supernatants were screened for inhibition of recombinant human TG2 activity, with TG2 specificity determined by ELISA.

KEY RESULTS

Thirteen TG2-specific supernatants inhibited human transamidation activity. Each hybridoma was cloned and antibody mapped to an epitope in the TG2 core domain, using phage display panning of a TG2 fragment library. Four distinct inhibitory epitopes were determined. The most effective antibodies (AB1, DC1 and BB7) bound to amino acids 313-327 (catalytic core), with an IC₅₀ of approximately 10 nM. The antibodies inhibit TG2 in human cells and block ECM accumulation in a primary human proximal tubular epithelial cell model of fibrosis, only 7 antibodies inhibited rat TG2, and all with higher IC₅₀ values.

CONCLUSIONS AND IMPLICATIONS

We identified a preferred inhibitory epitope in human TG2, developed antibodies with required characteristics for clinical development, and established that targeted inhibition of extracellular TG2 transamidation activity is sufficient to modify fibrotic remodelling.

Spotlight on the Transglutaminase 2-Heparan Sulfate Interaction

Heparan sulfate proteoglycans (HSPGs), syndecan-4 (Sdc4) especially, have been suggested as potential partners of transglutaminase-2 (TG2) in kidney and cardiac fibrosis, metastatic cancer, neurodegeneration and coeliac disease. The proposed role for HSPGs in the trafficking of TG2 at the cell surface and in the extracellular matrix (ECM) has been linked to the fibrogenic action of TG2 in experimental models of kidney fibrosis. As the TG2-HSPG interaction is largely mediated by the heparan sulfate (HS) chains of proteoglycans, in the past few years a number of studies have investigated the affinity of TG2 for HS, and the TG2 heparin binding site has been mapped with alternative outlooks. In this review, we aim to provide a compendium of the main literature available on the interaction of TG2 with HS, with reference to the pathological processes in which extracellular TG2 plays a role.

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.

Mineralization-inhibiting effects of transglutaminase-crosslinked polymeric osteopontin

Osteopontin (OPN) belongs to the SIBLING family (Small, Integrin-Binding LIgand N-linked Glycoproteins) of mineral-binding matrix proteins found in bones and teeth. OPN is a well-known inhibitor of matrix mineralization, and enzymatic modification of OPN can affect this inhibitory function. In bone, OPN exists both as a monomer and as a high-molecular-weight polymer - the latter is formed by transglutaminase-mediated crosslinking of glutamine and lysine residues in OPN to create homotypic protein assemblies. OPN can be covalently crosslinked by transglutaminase 2 (TG2) and Factor XIII-A. Polymeric OPN has increased binding to collagen and promotes osteoblast adhesion, but despite these initial observations, its role in mineralization is not clear. In this study, we investigated the effect of polymerized OPN on mineralization using a hydroxyapatite crystal growth assay and mineralizing MC3T3-E1 osteoblast cultures. In the cultures, endogenous polymeric OPN was detected after mineralization occurred. In cell-free conditions, TG2 was used to crosslink bovine OPN into its polymeric form, and atomic force microscopy and dynamic light scattering revealed variably-sized, large branched aggregates ranging across hundreds of nanometers. These OPN polymers inhibited the growth of hydroxyapatite crystals in solution at concentrations similar to monomeric OPN, although the crosslinking slightly reduced its inhibitory potency. When added to MC3T3-E1 osteoblast cultures, this exogenous polymeric OPN essentially did not inhibit mineralization when given during the later mineralization stages of culture; however, cultures treated early and then continuously with polymeric OPN throughout both the matrix assembly and mineral deposition stages showed reduced mineralization. Immunoblotting of protein extracts from these continuously treated cultures revealed exogenous OPN polymers incorporated into mature matrix that had not yet mineralized. These results suggest that in bone, the increased size and branched structure of crosslinked inhibitory polymeric OPN near the mineralization front could hinder it from accessing focal mineralization sites in the dense collagen-rich matrix, suggesting that OPN-crosslinking into polymers may represent a way to fine-tune the inhibitory potency of OPN on bone mineralization.

Transglutaminase 2 has opposing roles in the regulation of cellular functions as well as cell growth and death

Transglutaminase 2 (TG2) is primarily known as the most ubiquitously expressed member of the transglutaminase family with Ca²⁺-dependent protein crosslinking activity; however, this enzyme exhibits multiple additional functions through GTPase, cell adhesion, protein disulfide isomerase, kinase, and scaffold activities and is associated with cell growth, differentiation, and apoptosis. TG2 is found in the extracellular matrix, plasma membrane, cytosol, mitochondria, recycling endosomes, and nucleus, and its subcellular localization is an important determinant of its function. Depending upon the cell type and stimuli, TG2 changes its subcellular localization and biological activities, playing both anti- and pro-apoptotic roles. Increasing evidence indicates that the GTP-bound form of the enzyme (in its closed form) protects cells from apoptosis but that the transamidation activity of TG2 (in its open form) participates in both facilitating and inhibiting apoptosis. A difficulty in the study and understanding of this enigmatic protein is that opposing effects have been reported regarding its roles in the same physiological and/or pathological systems. These include neuroprotective or neurodegenerative effects, hepatic cell growth-promoting or hepatic cell death-inducing effects, exacerbating or having no effect on liver fibrosis, and anti- and pro-apoptotic effects on cancer cells. The reasons for these discrepancies have been ascribed to TG2's multifunctional activities, genetic variants, conformational changes induced by the immediate environment, and differences in the genetic background of the mice used in each of the experiments. In this article, we first report that TG2 has opposing roles like the protagonist in the novel Dr. Jekyll and Mr. Hyde, followed by a summary of the controversies reported, and finally discuss the possible reasons for these discrepancies.

Transglutaminase is a tumor cell and cancer stem cell survival factor

Recent studies indicate that cancer cells express elevated levels of type II transglutaminase (TG2), and that expression is further highly enriched in cancer stem cells derived from these cancers. Moreover, elevated TG2 expression is associated with enhanced cancer stem cell marker expression, survival signaling, proliferation, migration, invasion, integrin-mediated adhesion, epithelial-mesenchymal transition, and drug resistance. TG2 expression is also associated with formation of aggressive and metastatic tumors that are resistant to conventional therapeutic intervention. This review summarizes the role of TG2 as a cancer cell survival factor in a range of tumor types, and as a target for preventive and therapeutic intervention. The literature supports the idea that TG2, in the closed/GTP-binding/signaling conformation, drives cancer cell and cancer stem cell survival, and that TG2, in the open/crosslinking conformation, is associated with cell death.

Phosphorylation of transglutaminase 2 (TG2) at serine-216 has a role in TG2 mediated activation of nuclear factor-kappa B and in the downregulation of PTEN

Background

Transglutaminase 2 (TG2) and its phosphorylation have been consistently found to be upregulated in a number of cancer cell types. At the molecular level, TG2 has been associated with the activation of nuclear factor-kappa B (NF-κB), protein kinase B (PKB/Akt) and in the downregulation of phosphatase and tensin homologue deleted on chromosome 10 (PTEN). However, the underlying mechanism involved is not known. We have reported that protein kinase A (PKA) induced phosphorylation of TG2 at serine-216 (Ser²¹⁶) regulates TG2 function and facilitates protein-protein interaction. However, the role of TG2 phosphorylation in the modulation of NF-κB, Akt and PTEN is not explored.

Methods

In this study we have investigated the effect of TG2 phosphorylation on NF-κB, Akt and PTEN using embryonic fibroblasts derived from TG2 null mice (MEF^tg2-/-) overexpressing native TG2 or mutant-TG2 (m-TG2) lacking Ser²¹⁶ phosphorylation site with and without dibutyryl cyclic-AMP (db-cAMP) stimulation. Functional consequences on cell cycle and cell motility were determined by fluorescence activated cell sorting (FACS) analysis and cell migration assay respectively.

Results

PKA activation in TG2 overexpressing MEF^tg2-/- cells resulted in an increased activation of NF-κB and Akt phosphorylation in comparison to empty vector transfected control cells as determined by the reporter-gene assay and immunoblot analysis respectively. These effects were not observed in MEF^tg2-/- cells overexpressing m-TG2. Similarly, a significant downregulation of PTEN at both, the mRNA and protein levels were found in cells overexpressing TG2 in comparison to empty vector control and m-TG2 transfected cells. Furthermore, Akt activation correlated with the simultaneous activation of NF-κB and a decrease in PTEN suggesting that the facilitatory effect of TG2 on Akt activation occurs in a PTEN-dependent manner. Similar results were found with MCF-7 and T-47D breast cancer cells overexpressing TG2 and m-TG2 further supporting the role of TG2 phosphorylation in NF-κB activation and in the downregulation of PTEN.

Conclusions

Collectively, these data suggest that phosphorylation of TG2 at Ser²¹⁶ plays a role in TG2 mediated activation of NF-κB, Akt and in the downregulation of PTEN. Blocking TG2 phosphorylation may provide a novel strategy to attenuate NF-κB activation and downregulation of PTEN in TG2 overexpressing cancers.

Functional characterization of naturally occurring transglutaminase 2 mutants implicated in early-onset type 2 diabetes

Transglutaminase 2 (TG2) is an enzyme with diverse biological functions. TG2 catalyzes transamidation reactions, has intrinsic kinase activity, and acts as a G-protein in intracellular signaling. TG2 (Tgm2)-null mice are glucose intolerant and have impaired glucose-stimulated insulin secretion (GSIS). Furthermore, three naturally occurring missense mutations in the human TGM2 gene, corresponding to amino acid substitutions of Met330Arg, Ile331Asn, and Asn333Ser in the TG2 protein, have been reported and found to be associated with early-onset type 2 diabetes. However, their effect on TG2 function is not fully understood. To determine this, we have reproduced naturally occurring mutations in TG2 using site-directed mutagenesis. Overexpression of Myc-TG2 mutants in INS-1E cells resulted in a reduction of GSIS in comparison with cells overexpressing wild-type Myc-TG2 (WT-TG2). The maximum reduction was found in cells overexpressing Ile331Asn-TG2 (32%) followed by Met330Arg-TG2 (20%), and the least in Asn333Ser-TG2 (7%). Enzymatic analysis revealed that TG2 mutants have impaired transamidation and kinase activities in comparison with WT-TG2. GTP-binding assays showed that TG2 mutants also have altered GTP-binding ability, which is found to be modulated in response to glucose stimulation. Collectively, these data suggest that naturally occurring mutations in TG2 affect transamidation, kinase, and GTP-binding functions of TG2. While reduced insulin secretion, as a result of naturally occurring mutations in TG2, is due to the impairment of more than one biological function of TG2, it is the transamidation function that appears to be impaired during the first phase, whereas the GTP-binding function affects the second phase of insulin secretion.

"Activation of tissue tranglutsaminase by removal of carboxyl-terminal peptides"

Tissue transglutaminase (TGC or TG2) functions as transglutaminase (cross-linking), deamidase, kinase, and disulfide isomerase and its activities are implicated in the pathogenesis of several human diseases. Proteolytic activation of zymogens in the transglutaminase family is not unusual. Plasma transglutaminase (FXIIIa), epidermal transglutaminase (TG 3), transglutaminase-5, and microbial transglutaminase (MTG) can be subjected to proteolysis from specific proteases to generate the active functional enzyme. In the present study, calcium or GTP was essential for activation of TGC cross-linking activity by trypsin in membrane fractions from human RBC and was accompanied by the conversion of TGC (80 kDa) to a smaller TG form (55 kDa). While bacterially expressed TGC showed no activity, bacterial expression of C-terminal domain deletion constructs with carboxy-terminal ends ranging from lysine 464 (TG464) to glycine 480 (TG480) produced enzymes that were highly active in cross-linking activity. The product of a construct with a coding region ended at proline 446 (TG446), which interrupted the calcium-binding domain, exhibited weak cross-linking activity. TG480 and TG512 were characterized by about 80% and 10%, respectively, of the cross-linking activities of TG464. This may indicate that the longer the peptide after the calcium binding domain, the less the enzymatic activity expressed, possibly because the folding of such peptide which interfere with the calcium binding site or the catalytic site. Western analysis of MCF7 and T47D human breast cancer cells transfected with TGC showed TGC as a major protein and TG as a minor fragment. Incubation of lysate from transfected cells with serum resulted in the conversion of the TGC to TG, a condition that may be comparable to injury or wounds that lead to rapid enzymatic transamidation activation.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Tissue-specific responses to loss of transglutaminase 2

Of the eight catalytic transglutaminases (TGs), transglutaminase 2 (TG2) has been the most comprehensively studied due to its ubiquitous expression in multiple cell types. Despite the observed critical role for this enzyme in multiple biological processes in vitro, TG2 knockout mouse models have shown no severe developmental phenotypes, suggesting compensation by other TGs. To begin characterization of the compensating mechanisms, we analyzed total transamidating activity and expression patterns of all catalytically active TGs in seven different tissues/organs from wild-type and TG2 knockout mice. Inhibitory analysis with TG2-specific inhibitor KCC-009 suggests that relative contribution of TG2 in total transamidating activity differs in various tissues. Accordingly, our data indicate tissue-specific mechanisms of compensation for the loss of TG2, including transcriptional compensation in heart and liver versus functional compensation in aorta, kidney and skeletal/cartiagenous tissues. On the contrary, no compensation has been detected in skeletal muscle, suggesting a limited role for the TG2-mediated transamidation in normal development of this tissue.

Transglutaminase 2: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

Crystal structure of human transglutaminase 2 in complex with adenosine triphosphate

Transglutaminase 2 (TG2) is a calcium-dependent multifunctional protein associated with various human diseases. We determined the crystal structure of human TG2 in complex with adenosine triphosphate (ATP). The ATP molecule binds to the previously identified guanosine diphosphate (GDP) binding pocket but has different hydrogen bonds and ion interaction with protein. The four residues Arg476, Arg478, Val479 and Tyr583, all of which are involved in both ATP and GDP binding by hydrogen bonds, might play important roles in the stabilization of TG2 by ATP or GDP. However, Ser482 and Arg580, which are involved in GDP binding, do not form hydrogen bond with ATP. Additionally, we newly discovered an intramolecular disulfide bond between Cys230 and Cys370, which formation might regulate the enzymatic activity of TG2.

Regulation of ATPase activity of transglutaminase 2 by MT1-MMP: implications for mineralization of MC3T3-E1 osteoblast cultures

A pro-mineralization function for transglutaminase 2 (TG2) has been suggested in numerous studies related to bone, cartilage, and vascular calcification. TG2 is an enzyme which can perform protein crosslinking functions, or act as a GTPase/ATPase depending upon different stimuli. We have previously demonstrated that TG2 can act as an ATPase in a Ca(2+)-rich environment and that it can regulate phosphate levels in osteoblast cultures. In this study, we investigate the role MT1-MMP in regulating the ATPase activity of TG2. We report that proteolytic cleavage of TG2 by MT1-MMP in vitro results in nearly a 3-fold increase in the ATPase activity of TG2 with a concomitant reduction in its protein-crosslinking activity. We show that MC3T3-E1 osteoblasts secreted full-length TG2 and major smaller fragments of 66 and 56 kDa, the latter having ATP-binding abilities. MT1-MMP inhibition by a neutralizing antibody suppressed mineralization of osteoblast cultures to 35% of control, and significantly reduced phosphate levels in conditioned medium (CM). Furthermore, MT1-MMP inhibition abolished two of TG2 fragments in the cultures, one of which, the 56-kDa fragment, has ATPase activity. Neutralization of MT1-MMP at early phases of mineralization significantly reduced mineral deposition, but had no effect in later phases implying MT1-MMP and TG2 might contribute to the initiation of mineralization. The cleavage of TG2 by MT1-MMP likely occurs on the cell surface/pericellular matrix where MT1-MMP and TG2 were co-localized. Based on these data, we propose that MT1-MMP modulates the extracellular function TG2 as part of a regulatory mechanism activates the pro-mineralization function of TG2.

TG2 protects neuroblastoma cells against DNA-damage-induced stress, suppresses p53 activation

Tissue transglutaminase (TG2) is a multifunctional member of the transglutaminase (TGase) family (E.C.2.3.2.13), which catalyzes in a calcium-dependent reaction the formation of covalent bonds between the gamma-carboxamide groups of peptide-bound glutamine residues and various primary amines. Here, we investigated the role of TG2 in a response of the neuroblastoma SH-SY5Y cells to topoisomerase II inhibitor etoposide, known to trigger DNA-damage cell response. We found an early and transient (approximately 2 h) increase of the TG2 protein in SH-SY5Y cells treated with etoposide, along with the increase of phosphorylated and total levels of the p53 protein. Next, we showed that SH-SY5Y cells, which overexpress wild-type TG2 were significantly protected against etoposide-induced cell death. The TG2 protective effect was associated only with the transamidation active form of TG2, because overexpression the wild-type TG2, but not its transamidation inactive C277S form, resulted in a pronounced suppression of caspase-3 activity as well as p53 phosphorylation during the etoposide-induced stress. In addition, exacerbation of cell death with a significant increase in caspase-3 and p53 activation was observed in SH/anti-TG2 cells, in which expression of the endogenous TG2 protein has been greatly reduced by the antisense cDNA construct. Though the cell signaling and molecular mechanisms of the TG2-driven suppression of the cell death machinery remain to be investigated, our findings strongly suggest that TG2 plays an active role in the response of neuroblastoma cells to DNA-damage-induced stress by exerting a strong protective effect, likely by the suppression of p53 activation and p53-driven cell signaling events.

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.

Nucleolar trafficking of nucleostemin family proteins: common versus protein-specific mechanisms

The nucleolus has begun to emerge as a subnuclear organelle capable of modulating the activities of nuclear proteins in a dynamic and cell type-dependent manner. It remains unclear whether one can extrapolate a rule that predicts the nucleolar localization of multiple proteins based on protein sequence. Here, we address this issue by determining the shared and unique mechanisms that regulate the static and dynamic distributions of a family of nucleolar GTP-binding proteins, consisting of nucleostemin (NS), guanine nucleotide binding protein-like 3 (GNL3L), and Ngp1. The nucleolar residence of GNL3L is short and primarily controlled by its basic-coiled-coil domain, whereas the nucleolar residence of NS and Ngp1 is long and requires the basic and the GTP-binding domains, the latter of which functions as a retention signal. All three proteins contain a nucleoplasmic localization signal (NpLS) that prevents their nucleolar accumulation. Unlike that of the basic domain, the activity of NpLS is dynamically controlled by the GTP-binding domain. The nucleolar retention and the NpLS-regulating functions of the G domain involve specific residues that cannot be predicted by overall protein homology. This work reveals common and protein-specific mechanisms underlying the nucleolar movement of NS family proteins.

Tissue transglutaminase clusters soluble A-type ephrins into functionally active high molecular weight oligomers

The Eph receptors and their ligands, the ephrins, are thought to act at points of close cell-cell contact to elicit bi-directional signaling in receptor and ligand expressing cells. However, when cultured in vitro, some A-type ephrins are released from the cell surface and it is unclear if these soluble ephrins participate in Eph receptor activation. We show that soluble ephrin A5 is subject to oligomerization. Ephrins A1 and A5 are substrates for a cross-linking enzyme, tissue transglutaminase, which mediates the formation of oligomeric ephrin. Transglutaminase-cross-linked ephrin binds to A-type Eph receptors, stimulates Eph kinase activity, and promotes invasion and migration of HeLa cells. Transglutaminase-mediated oligomerization of soluble ephrin potentially represents a novel mechanism of forward signaling through Eph receptors and may extend the influence of A-type ephrins beyond cell contact mediated signaling.

Transglutaminase 2 kinase activity facilitates protein kinase A-induced phosphorylation of retinoblastoma protein

Transglutaminase 2 (TG2, tissue transglutaminase) is a multifunctional protein involved in cross-linking a variety of proteins, including retinoblastoma protein (Rb). Here we show that Rb is also a substrate for the recently identified serine/threonine kinase activity of TG2 and that TG2 phosphorylates Rb at the critically important Ser780 residue. Furthermore, phosphorylation of Rb by TG2 destabilizes the Rb.E2F1 complex. TG2 phosphorylation of Rb was abrogated by high Ca2+ concentrations, whereas TG2 transamidating activity was inhibited by ATP. TG2 was itself phosphorylated by protein kinase A (PKA). Phosphorylation of TG2 by PKA attenuated its transamidating activity and enhanced its kinase activity. Activation of PKA in mouse embryonic fibroblasts (MEF) with dibutyryl-cAMP enhanced phosphorylation of both TG2 and Rb by a process that was inhibited by the PKA inhibitor H89. Treatment with dibutyryl-cAMP enhanced Rb phosphorylation in MEFtg2+/+ cells but not in MEFtg2-/- cells. These data indicate that Rb is a substrate for TG2 kinase activity and suggest that phosphorylation of Rb, which results from activation of PKA in fibroblasts, is indirect and requires TG2 kinase activity.

Type 2 transglutaminase differentially modulates striatal cell death in the presence of wild type or mutant huntingtin

Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt), is characterized by extensive loss of striatal neurons. The dysregulation of type 2 transglutaminase (TG2) has been proposed to contribute to the pathogenesis in HD as TG2 is up-regulated in HD brain and knocking out TG2 in mouse models of HD ameliorates the disease process. To understand the role of TG2 in the pathogenesis of HD, immortalized striatal cells established from mice in which mutant htt with a polyglutamine stretch of 111 Gln had been knocked-in and wild type (WT) littermates, were stably transfected with human TG2 in a tetracycline inducible vector. Overexpression of TG2 in the WT striatal cells resulted in significantly greater cell death under basal conditions as well as in response to thapsigargin treatment, which causes increased intracellular calcium concentrations. Furthermore, in WT striatal cells TG2 overexpression potentiated mitochondrial membrane depolarization, intracellular reactive oxygen species production, and apoptotic cell death in response to thapsigargin. In contrast, in mutant striatal cells, TG2 overexpression did not increase cell death, nor did it potentiate thapsigargin-induced mitochondrial membrane depolarization or intracellular reactive oxygen species production. Instead, TG2 overexpression in mutant striatal cells attenuated the thapsigargin-activated apoptosis. When in situ transglutaminase activity was quantitatively analyzed in these cell lines, we found that in response to thapsigargin treatment TG2 was activated in WT, but not mutant striatal cells. These data suggest that mutant htt alters the activation of TG2 in response to certain stimuli and therefore differentially modulates how TG2 contributes to cell death processes.

Mechanism of allosteric regulation of transglutaminase 2 by GTP

Allosteric regulation is a fundamental mechanism of biological control. Here, we investigated the allosteric mechanism by which GTP inhibits cross-linking activity of transglutaminase 2 (TG2), a multifunctional protein, with postulated roles in receptor signaling, extracellular matrix assembly, and apoptosis. Our findings indicate that at least two components are involved in functionally coupling the allosteric site and active center of TG2, namely (i) GTP binding to mask a conformationally destabilizing switch residue, Arg-579, and to facilitate interdomain interactions that promote adoption of a compact, catalytically inactive conformation and (ii) stabilization of the inactive conformation by an uncommon H bond between a cysteine (Cys-277, an active center residue) and a tyrosine (Tyr-516, a residue located on a loop of the beta-barrel 1 domain that harbors the GTP-binding site). Although not essential for GTP-mediated inhibition of cross-linking, this H bond enhances the rate of formation of the inactive conformer.

Mutation of a critical arginine in the GTP-binding site of transglutaminase 2 disinhibits intracellular cross-linking activity

Transglutaminase type 2 (TG2; also known as G(h)) is a multifunctional protein involved in diverse cellular processes. It has two well characterized enzyme activities: receptor-stimulated signaling that requires GTP binding and calcium-activated transamidation or cross-linking that is inhibited by GTP. In addition to the GDP binding residues identified from the human TG2 crystal structure (Liu, S., Cerione, R. A., and Clardy, J. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 2743-2747), we have previously implicated Ser171 in GTP binding, as binding is lost with glutamate substitution (Iismaa, S. E., Wu, M.-J., Nanda, N., Church, W. B., and Graham, R. M. (2000) J. Biol. Chem. 275, 18259-18265). Here, we have shown that alanine substitution of homologous residues in rat TG2 (Phe174 in the core domain or Arg476, Arg478, or Arg579 in barrel 1) does not affect TG activity but reduces or abolishes GTP binding and GTPgammaS inhibition of TG activity in vitro, indicating that these residues are important in GTP binding. Alanine substitution of Ser171 does not impair GTP binding, indicating this residue does not interact directly with GTP. Arg579 is particularly important for GTP binding, as isothermal titration calorimetry demonstrated a 100-fold reduction in GTP binding affinity by the R579A mutant. Unlike wild-type TG2 or its S171E or F174A mutants, which are sensitive to both trypsin and mu-calpain digestion, R579A is inherently more resistant to mu-calpain, but not trypsin, digestion, indicating reduced accessibility and/or flexibility of this mutant in the region of the calpain cleavage site(s). Basal TG activity of intact R579A stable SH-SY5Y neuroblastoma cell transfectants was slightly increased relative to wild-type transfectants and, in contrast to the TG activity of the latter, was further stimulated by muscarinic receptor-activated calcium mobilization. Thus, loss of GTP binding sensitizes TG2 to intracellular calcium concentrations. These findings are consistent with the notion that intracellularly, under physiological conditions, TG2 is maintained largely as a latent enzyme, its calcium-activated cross-linking activity being suppressed allosterically by guanine nucleotide binding.

Tissue transglutaminase interacts with protein kinase A anchor protein 13 in prostate cancer

We have previously described that tissue transglutaminase (tTG) is a high level phenotypic biomarker in prostate cancer, which is down regulated in prostate cancer and surrounding premalignant field compared to benign prostate glands. To understand the function of tTG in prostate cancer, we sought to identify proteins that interact with the transglutaminase moiety of tTG using a human prostate cancer complementary deoxyribonucleic acid library in a Yeast 2-Hybrid system. The Yeast 2-Hybrid experiments identified a strong and novel interaction between the transglutaminase moiety and protein kinase A anchor protein 13 (AKAP13), which was quantified by beta-galactosidase assay, confirmed in vitro by immunoprecipitation experiments using PC3 prostate cancer cell lysates, and in vivo colocalization was confirmed by immunofluorescence studies in PC3 cells. Because AKAP plays a major role in protein kinase A and Rho protein mediated signaling, functional studies are underway to elucidate the significance of tTG-AKAP13 interaction in prostate cancer.

The p53 oncoprotein is a substrate for tissue transglutaminase kinase activity

Increased expression and activity of the ubiquitous enzyme, tissue transglutaminase (TG2), is consistently seen in a variety of models of apoptosis. The p53 oncoprotein is also involved in apoptosis. Here we investigated the interaction of TG2 with p53 and show that the p53 is a substrate for the recently identified serine/threonine kinase activity of TG2. Phosphospecific antibodies indicated that TG2 phosphorylated p53 at Ser(15) and Ser(20), residues that are critically important in the interaction of p53 with Mdm2. The TG2-induced phosphorylation was abrogated by high Ca(2+) concentrations and inhibited by cystamine, a known inhibitor of TG2 cross-linking activity. Furthermore, we demonstrate that TG2-induced phosphorylation of p53 reduces the ability of p53 to interact with Mdm2. Although TG2 cross-linking activity has been clearly implicated in apoptosis, our observations reported here suggest TG2 modification of p53 could be an additional mechanism whereby TG2 could facilitate apoptosis.

A multistep, GTP-driven mechanism controlling the dynamic cycling of nucleostemin

Nucleostemin (NS) was identified as a stem cell– and cancer cell–enriched nucleolar protein that controls the proliferation of these cells. Here, we report the mechanism that regulates its dynamic shuttling between the nucleolus and nucleoplasm. The nucleolar residence of nucleostemin involves a transient and a long-term binding by the basic and GTP-binding domains, and a dissociation mechanism mediated by the COOH-terminal region. This cycle is propelled by the GTP binding state of nucleostemin. We propose that a rapid nucleostemin cycle is designed to translate extra- and intra-cellular signals into the amount of nucleostemin in the nucleolus in a bidirectional and fast manner.

Transglutaminase function in epidermis

Surface epithelial cells, such as the epidermal keratinocyte, undergo a process of terminal cell differentiation that results in the construction of a multilayered epithelium. This epithelium functions to protect the organism from the environment. Transglutaminases, enzymes that catalyze the formation of isopeptide protein-protein cross-links, are key enzymes involved in the construction of this structure. This brief review will focus on the role of these enzymes in constructing the epidermal surface.

Tissue transglutaminase is a multifunctional BH3-only protein

Tissue transglutaminase (TG2) protein accumulates to high levels in cells during early stages of apoptosis both in vivo and in vitro. The analysis of the TG2 primary sequence showed the presence of an eight amino acid domain, sharing 70% identity with the Bcl-2 family BH3 domain. Cell-permeable peptides, mimicking the domain sequence, were able to induce Bax conformational change and translocation to mitochondria, mitochondrial depolarization, release of cytochrome c, and cell death. Moreover, we found that the TG2-BH3 peptides as well as TG2 itself were able to interact with the pro-apoptotic Bcl-2 family member Bax, but not with anti-apoptotic members Bcl-2 and Bcl-X(L). Mutants in the TG2-BH3 domain failed to sensitize cells toward apoptosis. In TG2-overexpressing cells about half of the protein is localized on the outer mitochondrial membrane where, upon cell death induction, it cross-links many protein substrates including Bax. TG2 is the first member of a new subgroup of multifunctional BH3-only proteins showing a large mass size (80 kDa) and enzymatic activity.

Characterization of sea urchin transglutaminase, a protein regulated by guanine/adenine nucleotides

Transglutaminases (TGs) are calcium-dependent enzymes that catalyze the transamidation of glutamine residues to form intermolecular isopeptide bonds. Nine distinct TGs have been identified in mammals, and three of them (types 2, 3, and 5) are regulated by GTP/ATP and are able to hydrolyze GTP, working as bifunctional enzymes. We have isolated a cDNA clone encoding a TG from a cDNA library prepared from the blastula stage of sea urchin Paracentrotus lividus (PlTG). The cDNA sequence has an open reading frame coding for a protein of 738 amino acids, including a Cys active site and two other residues critical for catalytic activity, His and Asp. We have studied its expression pattern by in situ hybridization and have also demonstrated that the in vitro expressed PlTG had GTP- and ATP-hydrolyzing activity; moreover, GTP inhibited the transamidating activity of this enzyme as it does that of human TG2, TG3, and TG5.

Alpha1B-adrenoceptor signaling and cell motility: GTPase function of Gh/transglutaminase 2 inhibits cell migration through interaction with cytoplasmic tail of integrin alpha subunits

A multifunctional enzyme, G(h), is a GTP-binding protein that couples to the alpha(1B)-adrenoreceptor and stimulates phospholipase C-delta1 but also displays transglutaminase 2 (TG2) activity. G(h)/TG2 has been implicated to play a role in cell motility. In this study we have examined which function of G(h)/TG2 is involved in this cellular response and the molecular basis. Treatment of human aortic smooth muscle cell with epinephrine inhibits migration to fibronectin and vitronectin, and the inhibition is blocked by the alpha(1)-adrenoreceptor antagonist prazosin or chloroethylclonidine. Up-regulation or overexpression of G(h)/TG2 in human aortic smooth muscle cells, DDT1-MF2, or human embryonic kidney cells, HEK 293 cells, results in inhibition of the migratory activity, and stimulation of the alpha(1B)-adrenoreceptor with the alpha(1) agonist further augments the inhibition of migration of human aortic smooth muscle cells and DDT1-MF2. G(h)/TG2 is coimmunoprecipitated by an integrin alpha(5) antibody and binds to the cytoplasmic tail peptide of integrins alpha(5), alpha(v), and alpha(IIb) subunits in the presence of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS). Mutation of Lys-Arg residues in the GFFKR motif, present in the alpha(5)-tail, significantly reduces the binding of GTPgammaS-G(h)/TG2. Moreover, the motif-containing integrin alpha(5)-tail peptides block G(h)/TG2 coimmunoprecipitation and reverse the inhibition of the migratory activity of HEK 293 cells caused by overexpression G(h)/TG2. These results provide evidence that G(h) function initiates the modulation of cell motility via association of GTP-bound G(h)/TG2 with the GFFKR motif located in integrin alpha subunits.

Molecular cloning and characterization of tiger shrimp (Penaeus monodon) transglutaminase

Transglutaminases (TG) are important for blood coagulation and post-translation remodeling of proteins. Using a plaque screening assay, we isolated cDNA encoding a novel TG from a shrimp (Penaeus monodon) hemocyte cDNA library. The TG cDNA consists of 2988 bp with an open reading frame of 2271 bp. The deduced protein has 757 amino acid residues, a calculated molecular mass of 84,713 Da and an isoelectric point of 5.56. Neither a typical hydrophobic leader sequence nor a transmembrane domain could be identified from the deduced sequence. Thus, shrimp TG may be a typical cytoplasmic protein. The sequence of shrimp TG was similar to crayfish, other invertebrate and vertebrate TG sequences. Enzyme activity was detected in all organs tested. This is consistent with the widespread, low-level expression of TG mRNA. However, high levels of TG expression were detected in hematopoietic tissue. TG signals were stronger in mitotic cells, indicating that cell proliferation and TG synthesis are associated. Preliminary data showed that recombinant TG existed the enzyme activity but lacked coagulation activity.

Evolutionary specialization of a tryptophan indole group for transition-state stabilization by eukaryotic transglutaminases

Covalent posttranslational protein modifications by eukaryotic transglutaminases proceed by a kinetic pathway of acylation and deacylation. Ammonia is released as the acylenzyme is formed, whereas the cross-linked product is released later in the deacylation step. Superposition of the active sites of transglutaminase type 2 (TG2) and the structurally related cysteine protease, papain, indicates that in the formation of tetrahedral intermediates, the backbone nitrogen of the catalytic Cys-277 and the N1 nitrogen of Trp-241 of TG2 could contribute to transition-state stabilization. The importance of this Trp-241 side chain was demonstrated by examining the kinetics of dansylcadaverine incorporation into a model peptide. Although substitution of the Trp-241 side chain with Ala or Gly had only a small effect on the Michaelis constant Km (1.5-fold increase), it caused a >300-fold lowering of the catalytic rate constant kcat. The wild-type and mutant TG2-catalyzed release of ammonia showed kinetics similar to the kinetics for the formation of cross-linked product, indicating that transition-state stabilization in the acylation step was rate-limiting. In papain, a Gln residue is at the position of TG2-Trp-241. The conservation of Trp-241 in all eukaryotic transglutaminases and the finding that W241Q-TG2 had a much lower kcat than wild-type enzyme suggest evolutionary specialization in the use of the indole group. This notion is further supported by the observation that transition-state-stabilizing side chains of Tyr and His that operate in some serine and metalloproteases only partially substituted for Trp.

In vivo cross-linking of nucleosomal histones catalyzed by nuclear transglutaminase in starfish sperm and its induction by egg jelly triggering the acrosome reaction

A histone heterodimer, designated as p28, which contains an Nepsilon(gamma-glutamyl)lysine cross-link between Gln9 of histone H2B and Lys5 or Lys12 of histone H4, is present in starfish (Asterina pectinifera) sperm. Treatment of sperm nuclei with micrococcal nuclease produced soluble chromatin, which was size-fractionated by sucrose-gradient centrifugation to give p28-containing oligonucleosome and p28-free mononucleosome fractions, indicating that the cross-link is internucleosomal. When sperm nuclei were incubated with monodansylcadaverine, a fluorescent amine, in the presence or absence of Ca(2+), histone H2B was modified only in the presence of Ca(2+). Gln9, in the N-terminal region, was modified, but the other Gln residues located in the internal region were not, suggesting that the modification takes place on the surface of the nucleosome core by the in situ action of a Ca(2+)-dependent nuclear transglutaminase. Treatment of sperm with the egg jelly, which activates Ca(2+) influx to induce the acrosome reaction, resulted in a significant elevation of the p28 content in the nucleus. This is the first demonstration of an in vivo activation of transglutaminase leading to the formation of a cross-link in intracellular proteins.

Injury-induced "switch" from GTP-regulated to novel GTP-independent isoform of tissue transglutaminase in the rat spinal cord

We recently found that alternative transcripts of tissue transglutaminase (tTG or TG2) were present in hippocampal brain regions of Alzheimer's disease (AD), but not in control, non-demented, age-matched brains. Since antecedent non-severe trauma has been implicated in AD and other neurodegenerative diseases, such as Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), we were interested in whether alternative transcripts might be detected in a model of neurotrauma, controlled-contusion spinal cord injury (SCI) in the rat. Implicated in diverse roles from growth and differentiation to apoptotic cell death, only bifunctional tTG, of the nine member TG family, has dual catalytic activities: guanine trinucleotide (GTP) hydrolyzing activity (GTPase), as well as protein cross-linking. These functions imply two physiological functions: programmed cell life and death. These may have profound roles in the nervous system since studies in cultured astrocytes found tTG short (S) mRNA transcripts induced by treatment with injury-related cytokines. In the developing rat spinal cord, tTG activity is concentrated in ventral horn alpha motoneurons, but neither studies of spinal cord tTG gene expression, nor evaluation of the GTP-regulated isoforms in tissues, have been reported. We now report increased tTG protein and gene expression occurring rapidly after SCI. In parallel, novel appearance of a second, short form transcript, in addition to the normal long (L) isoform, occurs by 8 h of injury. Up-regulation of tTG message and activity following neural injury. with appearance of a truncated GTP-unregulated S form, may represent new approaches to drug targets in neurotrauma.

Molecular characterization of a novel nuclear transglutaminase that is expressed during starfish embryogenesis

We report the constitution and molecular characterization of a novel transglutaminase (EC 2.3.2.13) that starts to accumulate specifically in the nucleus in the starfish (Asterina pectinifera) embryo after progression through the early blastula stage. The cDNA for the nuclear transglutaminase was cloned and the cDNA-deduced sequence defines a single open reading frame encoding a protein with 737 amino acids and a predicted molecular mass of 83 kDa. A comparison of this transglutaminase with other members of the gene family revealed an overall sequence identity of 33-41%. A special sequence feature of this transglutaminase, which is not found in other transglutaminases, is the presence of nuclear localization signal-like sequences in the N-terminal region. Microinjection of hybrid constructs that encode the N-terminal segment fused to reporter proteins into the germinal vesicle of an oocyte produced chimeric proteins by transcription-coupled translation. It was found that the N-terminal segment alone was sufficient to effect nuclear accumulation of an otherwise cytoplasmic protein. These results suggest that the nuclear accumulation of the transglutaminase may play an important role in nuclear remodeling during early starfish embryogenesis.

Conserved tryptophan in the core domain of transglutaminase is essential for catalytic activity

Transglutaminase 2 (TG2) is a distinctive member of the family of Ca2+-dependent enzymes recognized mostly by their abilities to catalyze the posttranslational crosslinking of proteins. TG2 uniquely binds and hydrolyzes GTP; binding GTP inhibits its crosslinking activity but allows it to function in signal transduction (hence the G(h) designation). The core domain of TG2 (residues 139-471, rat) comprises the papain-like catalytic triad and the GTP-binding domain (residues 159-173) and contains almost all of the conserved tryptophans of the protein. Examining point mutations at Trp positions 180, 241, 278, 332, and 337 showed that, upon binding 2'-(or 3')-O-(N-methylanthraniloyl)GTP (mantGTP), the Phe-332 mutant was the weakest (35% less than wild type) in resonance energy transfer from the protein (lambda(exc, max) = 290 nm) to the mant fluorophore (lambda(em) = 444 nm) and had a reduced affinity for mantGTP. Trp-332, situated near the catalytic center and the nucleotide-binding area of TG2, may be part of the allosteric relay machinery that transmits negative effector signals from nucleotide binding to the active center of TG2. A most important observation was that, whereas no enzyme activity could be detected when Trp-241 was replaced with Ala or Gln, partial preservation of catalytic activity was seen with substitutions by Tyr > Phe > His. The results indicate that Trp-241 is essential for catalysis, possibly by stabilizing the transition states by H-bonding, quadrupole-ion, or van der Waals interactions. This contrasts with the evolutionarily related papain family of cysteine proteases, which uses Gln-19 (papain) for stabilizing the transition state.

Does tissue transglutaminase play a role in Huntington's disease?

Tissue transglutaminase (tTG) likely plays a role in numerous processes in the nervous system. tTG posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines (Glns). This reaction results in the incorporation of polyamines into substrate proteins or the formation of protein crosslinks, modifications that likely have significant effects on neural function. Huntington's disease is a genetic disorder caused by an expansion of the polyglutamine domain in the huntingtin protein. Because a polypeptide bound Gln is the determining factor for a tTG substrate, and mutant huntingtin aggregates have been found in Huntington's disease brain, it has been hypothesized that tTG may contribute to the pathogenesis of Huntington's disease. In vitro, polyglutamine constructs and huntingtin are substrates of tTG. Further, the levels of tTG and TG activity are elevated in Huntington's disease brain and immunohistochemical studies have demonstrated that there is an increase in tTG reactivity in affected neurons in Huntington's disease. These findings suggest that tTG may play a role in Huntington's disease. However in situ, neither wild type nor mutant huntingtin is modified by tTG. Further, immunocytochemical analysis revealed that tTG is totally excluded from the huntingtin aggregates, and modulation of the expression level of tTG had no effect on the frequency of the aggregates in the cells. Therefore, tTG is not required for the formation of huntingtin aggregates, and likely does not play a role in this process in Huntington's disease brain. However, tTG interacts with truncated huntingtin, and selectively polyaminates proteins that are associated with mutant truncated huntingtin. Given the fact that the levels of polyamines in cells is in the millimolar range and the crosslinking and polyaminating reactions catalyzed by tTG are competing reactions, intracellularly polyamination is likely to be the predominant reaction. Polyamination of proteins is likely to effect their function, and therefore it can be hypothesized that tTG may play a role in the pathogenesis of Huntington's disease by modifying specific proteins and altering their function and/or localization. Further research is required to define the specific role of tTG in Huntington's disease.

Protein crosslinking, tissue transglutaminase, alternative splicing and neurodegeneration

Increasing interest and awareness of protein aggregation as being implicated in neurodegenerative processes has developed in recent years. One novel mechanism for this may be transglutaminase (TGase)-mediated protein crosslinking, that is involved in a variety of natural processes ranging from the stabilization of fibrin clots to production of the epidermal cell envelope and the fluid barrier of the skin. TGases are also implicated in both function and dysfunction of the central (CNS) and peripheral (PNS) nervous systems. The most ubiquitously expressed member of the TGase family, known as tissue TGase (tTG) or TG2, which, in addition to catalyzing the production of epsilon-lysine to gamma-glutaminyl isodipeptide bonds, serves a dual function as the G-protein Galpha(h) and is both expressed and active in PNS and CNS. It differs from other members of the TGase gene family in this regard and may implicate it in 'switches' from life or trophic signaling to those associated with apoptosis. In this regard, recent data indicate that one or more TGases are involved in neurodegenerative disorders such as the Qn/CAG repeat disorders, as well as Alzheimer's and Parkinson's diseases. As do many genes, particularly those highly expressed in the CNS, tTG undergoes alternative processing. Elevated expression and alternative splicing, resulting in a short (S) isoform of tTG with more active crosslinking activity, are associated with increased neuronal loss in affected regions in the demented brain. Our recent and novel data indicate that tTG mRNA, protein, and TGase activity are elevated in certain neurodegenerative diseases, and are accompanied by transcription of this S splice variant that results in unregulated crosslinking, unique to neurodegenerative disorders.

Calcium binding of transglutaminases: a 43Ca NMR study combined with surface polarity analysis

Transglutaminases (TGases) form cross-links between glutamine and lysine side-chains of polypeptides in a Ca2+-dependent reaction. The structural basis of the Ca2+-effect is poorly defined. 43Ca NMR, surface polarity analysis combined with multiple sequence alignment and the construction of a new homology model of human tissue transglutaminase (tTGase) were used to obtain structural information about Ca2+ binding properties of factor XIII-A2, tTGase and TGase 3 (each of human origin). 43Ca NMR provided higher average dissociation constants titrating on a wide Ca2+-concentration scale than previous studies with equilibrium dialysis performed in shorter ranges. These results suggest the existence of low affinity Ca2+ binding sites on both FXIII-A and tTGase in addition to high affinity ones in accordance with our surface polarity analysis identifying high numbers of negatively charged clusters. Upon increasing the salt concentration or activating with thrombin, FXIII-A2 partially lost its original Ca2+ affinity; the NMR data suggested different mechanisms for the two activation processes. The NMR provided structural evidence of GTP-induced conformational changes on the tTGase molecule diminishing all of its Ca2+ binding sites. NMR data on the Ca2+ binding properties of the TGase 3 are presented here; it binds Ca2+ the most tightly, which is weakened after its proteolytic activation. The investigated TGases seem to have very symmetric Ca2+ binding sites and no EF-hand motifs.

Phospholipase Cdelta1 is a guanine nucleotide exchanging factor for transglutaminase II (Galpha h) and promotes alpha 1B-adrenoreceptor-mediated GTP binding and intracellular calcium release

Effectors involved in G protein-coupled receptor signaling modulate activity of GTPases through GTPase-activating protein or guanine nucleotide exchanging factor (GEF). Phospholipase Cdelta1 (PLCdelta1) is an effector in tissue transglutaminase (TGII)-mediated alpha1B-adrenoreceptor (alpha(1B)AR) signaling. We investigated whether PLCdelta1 modulates TGII activity. PLCdelta1 stimulated GDP release from TGII in a concentration-dependent manner, resulting in an increase in GTPgammaS binding to TGII. PLCdelta1 also inhibited GTP hydrolysis by TGII that was independent from the alpha(1B)AR. These results indicate that PLCdelta1 is GEF for TGII and stabilizes the GTP.TGII complex. When GEF function of PLCdelta1 was compared with that of the alpha(1B)AR, the alpha(1B)AR-mediated GTPgammaS binding to TGII was greater than PLCdelta1-mediated binding and was accelerated in the presence of PLCdelta1. Thus, the alpha(1B)AR is the prime GEF for TGII, and GEF activity of PLCdelta1 promotes coupling efficacy of this signaling system. Overexpression of TGII and its mutants with and without PLCdelta1 resulted in an increase in alpha(1B)AR-stimulated Ca2+ release from intracellular stores in a TGII-specific manner. We conclude that PLCdelta1 assists the alpha(1B)AR function through its GEF action and is primarily activated by the coupling of TGII to the cognate receptors.

Intron-exon swapping of transglutaminase mRNA and neuronal Tau aggregation in Alzheimer's disease

In order to understand the mechanism for insoluble neurotoxic protein polymerization in Alzheimer's disease (AD) brain neurons, we examined protein and gene expression for transglutaminase (TGase 2; tissue transglutaminase (tTG)) in hippocampus and isocortex. We found co-localization of tTG protein and activity with tau-positive neurofibrillary tangles, whereas mRNA and sequence analysis indicated an absolute increase in tTG synthesized. Although apoptosis in AD hippocampus is now an established mode of neuronal cell death, no definite underlying mechanism(s) is known. Since TGase-mediated protein aggregation is implicated in polyglutamine ((CAG)(n)/Q(n) expansion) disorder apoptosis, and expanded Q(n) repeats are excellent TGase substrates, a role for TGase in AD is possible. However, despite such suggestions almost 20 years ago, the molecular mechanism remained elusive. We now present one possible molecular mechanism for tTG-mediated, neurotoxic protein polymerization leading to neuronal apoptosis in AD that involves not its substrates (like Q(n) repeats) but rather the unique presence of alternative transcripts of tTG mRNA. In addition to a full-length (L) isoform in aged non-demented brains, we found a short isoform (S) lacking a binding domain in all AD brains. Our current results identify intron-exon "switching" between L and S isoforms, implicating G-protein-coupled signaling pathways associated with tTG that may help to determine the dual roles of this enzyme in neuronal life and death processes.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Nucleotide binding by the erythrocyte transglutaminase/Gh protein, probed with fluorescent analogs of GTP and GDP

GTP is known to be a potent inhibitor of the protein crosslinking activity of transglutaminase (TG), probably the most abundant G protein in the human red cell. Nucleotide binding to TG was examined by fluorescence spectroscopy and anisotropy in mixtures of TG with methylanthraniloyl analogs of GTP and GDP. A characteristic feature was the appearance of a major energy transfer band (lambda(exc, max) = 290 nm, lambda(em) = 444 nm) from protein tryptophans to the bound nucleotides. Quenching of the bound fluorophore (lambda(exc) = 360 nm, lambda(em) = 444 nm) by acrylamide was barely different from that of free ligand. However, major changes were observed in anisotropy, which was used to demonstrate a facile exchange between bound and free nucleotides and to evaluate affinity constants for the binding of methylanthraniloyl GTP and GDP to TG.