A 50 KDa protein modulates guanine nucleotide binding of transglutaminase II

Current Developments on the Role of α1-Adrenergic Receptors in Cognition, Cardioprotection, and Metabolism

The α₁-adrenergic receptors (ARs) are G-protein coupled receptors that bind the endogenous catecholamines, norepinephrine, and epinephrine. They play a key role in the regulation of the sympathetic nervous system along with β and α₂-AR family members. While all of the adrenergic receptors bind with similar affinity to the catecholamines, they can regulate different physiologies and pathophysiologies in the body because they couple to different G-proteins and signal transduction pathways, commonly in opposition to one another. While α₁-AR subtypes (α_1A, α_1B, α_1C) have long been known to be primary regulators of vascular smooth muscle contraction, blood pressure, and cardiac hypertrophy, their role in neurotransmission, improving cognition, protecting the heart during ischemia and failure, and regulating whole body and organ metabolism are not well known and are more recent developments. These advancements have been made possible through the development of transgenic and knockout mouse models and more selective ligands to advance their research. Here, we will review the recent literature to provide new insights into these physiological functions and possible use as a therapeutic target.

Oláh A, Fésüs L, Mádi A. Transglutaminase 2 Has Metabolic and Vascular Regulatory Functions Revealed by In Vivo Activation of Alpha1-Adrenergic Receptor

The multifunctional tissue transglutaminase has been demonstrated to act as α1-adrenergic receptor-coupled G protein with GTPase activity in several cell types. To explore further the pathophysiological significance of this function we investigated the in vivo effects of the α1-adrenergic receptor agonist phenylephrine comparing responses in wild type and TG2-/- mice. Injection of phenylephrine, but not a beta3-adrenergic agonist (CL-316,243), resulted in the long-term decline of the respiratory exchange ratio and lower lactate concentration in TG2-/- mice indicating they preferred to utilize fatty acids instead of glucose as fuels. Measurement of tail blood pressure revealed that the vasoconstrictive effect of phenylephrine was milder in TG2-/- mice leading to lower levels of lactate dehydrogenase (LDH) isoenzymes in blood. LDH isoenzyme patterns indicated more damage in lung, liver, kidney, skeletal, and cardiac muscle of wild type mice; the latter was confirmed by a higher level of heart-specific CK-MB. Our data suggest that TG2 as an α1-adrenergic receptor-coupled G protein has important regulatory functions in alpha1-adrenergic receptor-mediated metabolic processes and vascular functions.

Transglutaminase in Receptor and Neurotransmitter-Regulated Functions

Transglutaminases (TGs) and especially TG2 play important roles in neurotransmitter and receptor signaling pathways. Three different mechanisms by which TG2 interacts with neurotransmitter and receptor signaling systems will be discussed in this review. The first way in which TG2 interacts with receptor signaling is via its function as a guanine nucleotide binding protein (G-protein) coupling to G-protein coupled receptors (GPCRs) to activate down-stream signaling pathways. TG2 can exist in a least two conformations, a closed GTP-bound conformation and an open calcium-bound conformation. In the closed GTP-bound conformation, TG2 is capable of functioning as a G-protein for GPCRs. In the open calcium-bound conformation, TG2 catalyzes a transamidation reaction cross-linking proteins or catalyzing the covalent binding of a mono- or polyamine to a protein. The second mechanism is regulation of the transamidation reaction catalyzed by TG2 via receptor stimulation which can increase local calcium concentrations and thereby increase transamidation reactions. The third way in which TG2 plays a role in neurotransmitter and receptor signaling systems is via its use of monoamine neurotransmitters as a substrate. Monoamine neurotransmitters including serotonin can be substrates for transamidation to a protein often a small G-protein (also known as a small GTPase) resulting in activation of the small G-protein. The transamidation of a monoamine neurotransmitter or serotonin has been designated as monoaminylation or more specifically serotonylation, respectively. Other proteins are also targets for monoaminylation such as fibronectin and cytoskeletal proteins. These receptor and neurotransmitter-regulated reactions by TG2 play roles in physiological and key pathophysiological processes.

Physiological, pathological, and structural implications of non-enzymatic protein-protein interactions of the multifunctional human transglutaminase 2

Transglutaminase 2 (TG2) is a ubiquitously expressed member of an enzyme family catalyzing Ca(2+)-dependent transamidation of proteins. It is a multifunctional protein having several well-defined enzymatic (GTP binding and hydrolysis, protein disulfide isomerase, and protein kinase activities) and non-enzymatic (multiple interactions in protein scaffolds) functions. Unlike its enzymatic interactions, the significance of TG2's non-enzymatic regulation of its activities has recently gained importance. In this review, we summarize all the partners that directly interact with TG2 in a non-enzymatic manner and analyze how these interactions could modulate the crosslinking activity and cellular functions of TG2 in different cell compartments. We have found that TG2 mostly acts as a scaffold to bridge various proteins, leading to different functional outcomes. We have also studied how specific structural features, such as intrinsically disordered regions and embedded short linear motifs contribute to multifunctionality of TG2. Conformational diversity of intrinsically disordered regions enables them to interact with multiple partners, which can result in different biological outcomes. Indeed, ID regions in TG2 were identified in functionally relevant locations, indicating that they could facilitate conformational transitions towards the catalytically competent form. We reason that these structural features contribute to modulating the physiological and pathological functions of TG2 and could provide a new direction for detecting unique regulatory partners. Additionally, we have assembled all known anti-TG2 antibodies and have discussed their significance as a toolbox for identifying and confirming novel TG2 regulatory functions.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

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.

Identification of a 42-kDa Group IV cPLA2-activating protein, cPLAPγ, as a GTP-binding protein in the bovine brain

Brain tissue contains multiple forms of Phospholipase A(2) (PLA(2)) whose activities are involved in intracellular and intercellular signalling related to normal functions such as long-term potentiation, neurotransmitter release, cell growth and differentiation. Among them, we focused on regulatory mechanism of cPLA(2)α (Group IVA cytosolic PLA(2)) in brain tissue. In the present study, we report the identification of a cPLA(2)-activating protein (cPLAP) in the bovine brain. cPLAP activity appeared as two major peaks with molecular masses of 200 and 42 kDa in a Superose 12 gel filtration FPLC column. The 42-kDa form of cPLAP, designated cPLAPγ, was further purified using a Mono S FPLC column to near homogeneity and characterized to as a GTP-binding protein (G protein). Metabolic labelling and immunoprecipitation studies revealed that cPLAPγ associates with cPLA(2) in vitro and co-immunoprecipitates with [(35)S]-cPLA(2). Notably, cPLAPγ rendered cPLA(2) fully activated at submicromolar concentrations of Ca(2+). These results suggest that cPLAPγ may act as a G protein, activating cPLA(2)α prior to reaching full intracellular Ca(2+) concentrations.

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.

Transglutaminases: crosslinking enzymes with pleiotropic functions

Blood coagulation, skin-barrier formation, hardening of the fertilization envelope, extracellular-matrix assembly and other important biological processes are dependent on the rapid generation of covalent crosslinks between proteins. These reactions--which are catalysed by transglutaminases--endow the resulting supramolecular structure with extra rigidity and resistance against proteolytic degradation. Some transglutaminases function as molecular switches in cytoskeletal scaffolding and modulate protein-protein interactions. Having knowledge of these enzymes is essential for understanding the aetiologies of diverse hereditary diseases of the blood and skin, and various autoimmune, inflammatory and degenerative conditions.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

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.

GTP binding and signaling by Gh/transglutaminase II involves distinct residues in a unique GTP-binding pocket

G(h) is a dual function protein. It has receptor signaling activity that requires GTP binding and Ca(2+)-activated transglutaminase (TGase) activity that is inhibited by GTP binding. G(h) shows no homology with other GTP-binding proteins, and its GTP-binding site has not been defined. Based on sequence analysis of [alpha-(32)P]GTP-photolabeled and proteolytically released internal peptide fragments, we report localization of GTP binding to a 15-residue segment ((159)YVLTQQGFIYQGSVK(173)) of the G(h) core domain. This was confirmed by site-directed mutagenesis; a G(h)/fXIIIA chimera (in which residues 162-179 of G(h) were substituted with the equivalent but nonhomologous region of the non-GTP-binding TGase factor XIIIA) and a G(h) point mutant, S171E, retained TGase activity but failed to bind and hydrolyze GTP and did not support alpha(1B)-adrenergic receptor signaling. Slight impairment of GTP binding (1.5-fold) and hydrolysis (10-fold) in the absence of altered TGase activity did not affect signaling by the mutant K173N. However, greater impairment of GTP binding (6-fold) and hydrolysis (50-fold) abolished signaling by the mutant K173L. Mutant S171C exhibited enhanced GTP binding and signaling. Thus, residues Ser(171) and Lys(173) are critical for both GTP binding and signaling but not TGase activity. Mutagenesis of residues N-terminal to Gly(170) impaired both GTP binding and TGase activity. From computer modeling of G(h), it is evident that the GTP-binding region identified here is distinct from, but interacts with, the TGase active site. Together with structural considerations of G(h) versus other GTP-binding proteins, these findings indicate that G(h) has a unique GTP-binding pocket and provide for the first time a mechanism for GTP-mediated regulation of the TGase activity of G(h).

Interactions of G(h)/transglutaminase with phospholipase Cdelta1 and with GTP

The inositol phosphate hydrolyzing activity of human phospholipase Cdelta1 (PLCdelta1) is markedly inhibited when the enzyme is coexpressed with the human heart G(h)/transglutaminase (TG) in human embryonic kidney cells. Because the cotransfection does not affect the amount of PLCdelta1 in the cells, the depression of phospholipase activity probably is a result of a direct interaction between the two proteins. An ELISA procedure was employed to document the associations of purified TG preparations from a variety of tissues (human red cells, rabbit lens, guinea pig liver) with PLCdelta1. Nucleotides (GTP > GDP > ATP > GMP = ADP, in order of decreasing efficiency) interfered with the formation of the PLCdelta1:TG complex. A conformational change in the TG partner, occurring with nucleotide binding, is thought to be responsible for dissociating the two proteins. The structural rearrangement produces a remarkable shift in the anodic mobility of TG in electrophoresis: TG(slow) + GTP -->/<-- [TG:GTP](fast). Altogether, our findings indicate that GTP controls PLCdelta1 activity by releasing this protein from an inhibitory association with G(h)/transglutaminase.

Calreticulin down-regulates both GTP binding and transglutaminase activities of transglutaminase II

Enzyme regulation is an important mechanism for controlling cell proliferation and differentiation in response to extracellular signaling molecules. We have previously reported that a approximately 50 kDa protein (termed Gbetah) consistently copurified with Galphah (transglutaminase II, TGII) and that Gbetah down-regulates the GTPase function of TGII by associating with GDP-bound TGII [Baek et al. (1996) Biochemistry 35, 2651-2657]. In this study, we examined the identity of Gbetah by partial amino acid sequencing and immunological characterizations. The results strongly suggest that Gbetah is a protein known as calreticulin (CRT). When the regulatory role of CRT in the GTPase activity of TGII was examined, CRT inhibited GTP (GTPgammaS) binding and hydrolysis in a concentration-dependent manner. Moreover, CRT interacted only with GDP-bound TGII. These results demonstrate that CRT down-regulates the GTPase activity of TGII by associating with GDP-bound TGII. Studies on the modulation of the TGase activity of TGII revealed that CRT also inhibited TGase activity. The inhibition showed the two characteristics depend on guanine nucleotides occupying the GTPase active site. The inhibition of the "empty" form of the GTPase active site increased the Ca2+ requirement without changing the Vmax. On the other hand, the inhibition of the GDP-bound form decreased Vmax, but did not alter the Ca2+ requirement. Moreover, the GTPgammaS-bound TGII was virtually resistant to Ca2+-mediated stimulation of the TGase activity, indicating that the GTP-bound TGII does not function as a TGase. We concluded that CRT is the regulatory protein of TGII that down-regulates both GTPase and TGase activities, opposing the activators of TGII function.

Alpha 1B-adrenoceptor interacts with multiple sites of transglutaminase II: characteristics of the interaction in binding and activation

We previously reported that a novel GTP binding protein (G alpha h) is tissue type transglutaminase (TGII) and transmits the alpha 1B-adrenoceptor (AR) signal to phospholipase C (PLC) through its GTPase function. We have also shown that PLC-delta 1 is the effector in TGII-mediated signaling. In this study, interaction sites on TGII for the alpha 1B-AR were identified using a peptide approach and site-directed mutagenesis, including in vivo reconstitution of TGIIs with the alpha 1B-AR and PLC-delta 1. To identify the interaction sites, 11 synthetic peptides covering approximately 132 amino acid residues of the C-terminal domain of TGII were tested. The studies with the peptides revealed that three peptides, L547-I561, R564-D581, and Q633-E646, disrupted formation of an alpha 1-agonist-alpha 1B-AR-TGII complex and blocked alpha 1B-AR-mediated TGase inhibition in a dose-dependent manner, indicating that these peptide regions are involved in recognition and activation of TGII by the alpha 1B-AR. These three regions were further evaluated with full-length TGIIs by constructing and coexpressing each site-directed mutant with the alpha 1B-AR and PLC-delta 1 in COS-1 cells. Supporting the findings with these peptides, these TGII mutants lost 56-82% the receptor binding ability and reduced by 29-68% the level of alpha 1B-AR-mediated IP3 production via PLC-delta 1 as compared to those with wild-type TGII. The results also revealed that the regions of R564-D581 and Q633-E646 were the high-affinity binding sites of TGII for the receptor and critical for the activation of TGII by the receptor. Taken together, the studies demonstrate that multiple regions of TGII interact with the alpha 1B-AR and that the alpha 1B-AR stimulates PLC-delta 1 via TGII.

Identification of a distinct molecular mass G alpha(h) (transglutaminase II) coupled to alpha1-adrenoceptor in mouse heart

Our previous studies on alpha1-adrenoceptor signaling suggested that G alpha(h) family is a signal mediator in different species. To elucidate the species-specificity of G alpha(h) family in molecular mass, we used the solubilized membranes from mouse heart and the ternary complex preparations containing alpha1-agonist/receptor/G-protein. Binding of [35S]GTPgammaS and the intensity of the [alpha-32P]GTP photoaffinity labeled protein resulting from activation of the alpha1-adrenoceptor were significantly attenuated by the antagonist, phentolamine. The molecular mass of the specific GTP-binding protein was approximately 72-kDa; homologous with G alpha(h) (transglutaminase II) family. Furthermore, immunological cross-reactivity of ternary complex from mouse heart and purified G alpha(h) from rat, guinea pig, and bovine using anti-G alpha(h7) antibody showed that their molecular masses were distinctly different and approximately 72-kDa G alpha(h) from mouse heart was the lowest molecular mass. Consistent with these observations, in co-immunoprecipitation and co-immunoadsorption of the alpha1-adrenoceptor in the ternary complex preparation by anti-G alpha(h7) antibody, the G alpha(h) family protein tightly coupled to alpha1-adrenoceptor. These results demonstrate the species-specificity of G alpha(h) family in molecular mass, especially the lowest molecular mass in mouse.

Altered distribution of Galphah/type 2 transglutaminase following catecholamine deprivation is associated with depression of adrenoreceptor signal transduction in cultured ventricular zone germinal cells

Type 2 transglutaminase (Tg), which catalyzes the covalent cross-linking of cytoplasmic proteins during apoptosis, also functions as the alpha subunit of a heterodimeric G-protein (Gh) which can activate phospholipase C-delta1 during the signal transduction pathway linked to alpha1-adrenoreceptors. Continued stimulation of rat forebrain ventricular zone (VZ) germinal cells with the alpha1-agonist phenylephrine during development in vitro suppresses apoptosis and promotes DNA synthesis [Pabbathi et al., Brain Res., 760, 1997, 22-33]. Immunocytochemistry with a monoclonal antibody to Galphah/Tg reveals that alpha1-agonist deprivation during culture of VZ cells in the presence of a protein synthesis inhibitor results after 20 h in a loss of peripheral distribution of the protein and an increase in the reaction product of Tg in the cytoplasm of cells undergoing apoptosis. Using photoaffinity labelling, we observed reduced GTP binding to Galphah/Tg in phenylephrine-deprived cultures. Formation of inositol triphosphate (IP3) and intracellular Ca2+ transients occurred in the presence of phenylephrine. In cultures grown in phenylephrine-deprived conditions in the presence of protein synthesis inhibitor, both the IP3 response and the amplitude and duration of Ca2+ transients were reduced. These results show that loss of signal transduction coincides with the onset of transglutaminase activity in VZ cells during a period when cell survival is reduced following withdrawal of alpha1-agonist, and support the hypothesis that Tg/Galphah could be implicated in both signal transduction and programmed cell death.

Modulation of the in situ activity of tissue transglutaminase by calcium and GTP

Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the posttranslational modification of proteins by transamidation of specific polypeptide-bound glutamine residues. Previous in vitro studies have demonstrated that the transamidating activity of tTG requires calcium and is inhibited by GTP. To investigate the endogenous regulation of tTG, a quantitative in situ transglutaminase (TG) activity assay was developed. Treatment of human neuroblastoma SH-SY5Y cells with retinoic acid (RA) resulted in a significant increase in tTG levels and in vitro TG activity. In contrast, basal in situ TG activity did not increase concurrently with RA-induced increased tTG levels. However, stimulation of cells with the calcium-mobilizing drug maitotoxin (MTX) resulted in increases in in situ TG activity that correlated (r2 = 0.76) with increased tTG levels. To examine the effects of GTP on in situ TG activity, tiazofurin, a drug that selectively decreases GTP levels, was used. Depletion of GTP resulted in a significant increase in in situ TG activity; however, treatment of SH-SY5Y cells with a combination of MTX and tiazofurin resulted in significantly less in situ TG activity compared with treatment with MTX alone. This raised the possibility of calcium-dependent proteolysis due to the effects of tiazofurin, because in vitro GTP protects tTG against proteolysis by trypsin. Studies with a selective membrane permeable calpain inhibitor indicated that tTG is likely to be an endogenous substrate of calpain, and that depletion of GTP increases tTG degradation after elevation of intracellular calcium levels. TG activity was also increased in response to activation of muscarinic cholinergic receptors, which increases intracellular calcium through inositol 1,4,5-trisphosphate generation. The results of these experiments demonstrate that selective changes in calcium and GTP regulate the activity and levels of tTG in situ.

Regulation of human tissue transglutaminase function by magnesium-nucleotide complexes. Identification of distinct binding sites for Mg-GTP and Mg-ATP

Tissue transglutaminase (tTG) catalyzes a Ca(2+)-dependent transglutaminase (TGase) activity that stabilizes tissues and a GTP hydrolysis activity that regulates cell receptor signaling. The purpose of this study was to examine the true substrates for nucleotide hydrolysis and the effects of these substrates on modulating the dual enzymatic activities of tTG. We found that Mg-GTP and Mg-ATP are the true substrates of the hydrolysis reaction. tTG hydrolyzed Mg-GTP and Mg-ATP at similar rates and interacted with Mg-ATP (Km = 38 +/- 10 microM) at a 3-fold greater steady-state affinity than with Mg-GTP (Km = 130 +/- 35 microM). In addition, Mg-ATP inhibited GTP hydrolysis (IC50 = 24 microM), whereas 1 mM Mg-GTP reduced ATP hydrolysis by only 20%. Furthermore, the TGase activity of tTG was inhibited by Mg-GTP, Mg-GDP, and Mg-GMP, with IC50 values of 9, 9, and 400 microM, respectively, whereas the Mg-adenine nucleotides were ineffective. Kinetic analysis of the hydrolysis reaction demonstrates the presence of separate binding sites for Mg-GTP and Mg-ATP. Finally, we found that Mg-GTP protected tTG from proteolytic degradation by trypsin, whereas Mg-ATP was ineffective. In conclusion, we report that Mg-GTP and Mg-ATP can bind to distinct sites and serve as substrates for nucleotide hydrolysis. Furthermore, binding of Mg-GTP causes a conformational change and the inhibition of TGase activity, whereas Mg-ATP is ineffective. The implication of these findings in regulating the intracellular and extracellular function of tTG is discussed.

tTGase/G alpha h protein expression inhibits adenylate cyclase activity in Balb-C 3T3 fibroblasts membranes

Stably transfected Balb-C 3T3 fibroblasts (clone 5), overexpressing a catalytically active tissue transglutaminase, showed a basal adenylate cyclase activity lower than control cells (clone 1). Several modulators of the adenylate cyclase activity (forskolin, Mn2+ and pertussis toxin) showed the existence of a marked negative control on the adenylate cyclase activity present in clone 5 cells. Very interestingly, this same marked negative control was also found in a Balb-C 3T3 fibroblast clone stably transfected with a mutagenized human tissue transglutaminase (mut277 cys > ser) virtually devoid of transglutaminase catalytic activity (clone Ser). Conversely, a significant increase of the adenylate cyclase activity was observed in bovine aortic endothelial cells after the lowering of tissue transglutaminase expression levels by the transfection of an eukaryotic expression vector containing the gene for tissue transglutaminase in antisense orientation. All these findings suggest a possible role for type II tissue transglutaminase as a negative modulator of the adenylate cyclase activity in different cell types, beside its transglutaminase enzyme activity.

Evidence that phospholipase delta1 is the effector in the Gh (transglutaminase II)-mediated signaling

A new class of GTP-binding protein transglutaminase II (Gh) couples to a 69-kDa phospholipase C (PLC). An 8-amino acid region (Leu665-Lys672) of the alpha-subunit of Gh (Galphah) is involved in interaction and activation of PLC, an observation that has now been used to characterize the 69-kDa PLC further. A 20-amino acid peptide corresponding to Leu654-Leu673 of Galphah was used to prepare an affinity resin. On incubation with a partially purified PLC preparation from rat liver membranes, the affinity resin-bound approximately69- and 85-kDa proteins were recognized by an antibody to the 69-kDa PLC. Both purified 69-kDa PLC and PLC-delta1 bound to the affinity resin; moreover, antibodies to PLC-delta1 recognized the 69-kDa PLC, and antibodies to the 69-kDa PLC recognized PLC-delta1. A synthetic peptide corresponding to Leu661-Lys672 of Galphah inhibited the binding of PLC-delta1 to the affinity resin and also stimulated PLC-delta1. Reconstitution of PLC-delta1 with GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)-activated Gh resulted in activation of PLC-delta1. Antibodies to Galphah also coimmunoprecipitated PLC-delta1 upon activation of Gh. These findings indicate that PLC-delta1 is the effector of Gh-mediated signaling.