The importance of the GTP-binding protein tissue transglutaminase in the regulation of cell cycle progression

Tissue transglutaminase: an enzyme with a split personality

Tissue transglutaminase (tTG) belongs to the family of transglutaminase enzymes that catalyze the posttranslational modification of proteins via Ca(2+)-dependent cross-linking reactions. The catalytic action of tTG results in the formation of an isopeptide bond that is of great physiological significance since it is highly resistant to proteolysis and denaturants. Although tTG-mediated cross-linking reactions have been implicated to play a role in diverse biological processes, the precise physiological function of the enzyme remains unclear. Recent data, however, suggest that the protein polymers resulting from tTG-catalyzed reactions may play a role in commitment of cells to undergo apoptosis. On the same token, tTG-mediated formation of insoluble protein aggregates may underlie the markers of numerous pathological conditions, such as the senile plaques in Alzheimer's disease and the Lewy bodies in Parkinson's disease. In addition to catalyzing Ca(2+)-dependent cross-linking reactions, tTG can also bind and hydrolyze guanosine triphosphate and adenosine triphosphate. By virtue of this ability, tTG has been identified as a novel G-protein that interacts and activates phospholipase C following stimulation of the alpha-adrenergic receptor. The ability of tTG to mediate signal transduction may contribute to its involvement in the regulation of cell cycle progression. The following review summarizes the important features of this multifunctional enzyme that have emerged as a result of recent work from different laboratories.

Interaction of tissue transglutaminase with nuclear transport protein importin-alpha3

Tissue transglutaminase is a multifunctional enzyme which has been involved in the regulation of cell growth, differentiation, and apoptosis. Recently, nuclear localization of tTG has been reported indicating the potential of active nuclear transport. In this study we use the yeast two-hybrid assay and co-immunoprecipitation to show that tTG interacts with the nuclear transport protein importin-alpha3. Using electron microscopy we demonstrate that nuclear expression of tTG in a non-small cell lung cancer cell line is induced by retinoic acid (RA). These data suggest that importin-alpha3 could mediate active nuclear transport of tTG which may be important for the regulation of critical cellular processes.

GTP-dependent conformational changes associated with the functional switch between Galpha and cross-linking activities in brain-derived tissue transglutaminase

GTP and Ca2+, two well-known modulators of intracellular signaling pathways, control a structural/functional switch between two vital and mutually exclusive activities, cross-linking and Galpha activity, in the same enzyme. The enzyme, a brain-derived tissue-type transglutaminase (TGase), was recently cloned by us in two forms, one of which (s-TGN) lacks a C-terminal region that is present in the other (l-TGN). Immunoreaction with antibodies directed against a peptide present in the C-terminus of l-TGN but missing in s-TGN suggested that this site, which is located in the C-terminal fourth domain, undergoes conformational changes as a result of interaction between l-TGN and GTP. Site-directed mutagenesis suggested that the third domain is involved in mediating the inhibition of the cross-linking activity. These results were supported by molecular modeling, which further suggested that domains III and IV both participate in conformational changes leading to the functional switch between the Ca2+-dependent cross-linking activity and the Galpha activity.

Two distinct classes of rat intestinal mucosal enzymes incorporating putrescine into protein

Tissue-transglutaminase (t-TGase) is a family of calcium-dependent enzymes. A Ca2+-independent soluble enzyme, in addition to t-TGase, capable of incorporating polyamines into proteins was demonstrated in rat intestinal mucosa. The Ca2+-independent enzyme was stimulated 2- to 5-fold by Fe2+ and Co2+ ions but inhibited by Cu2+ and Zn2+ ions. The Ca2+-stimulated t-TGase activity was inhibited by divalent ions in the following order: Zn2+, Fe2+ >Co2+ > Cu2+. The opposite effects of EGTA, Fe2+ and Co2+ on these two enzyme activities indicate that they are two distinct classes of enzymes. Competition studies demonstrated differential preferences of the two enzymes for substrates. The Ca2+-dependent enzyme preferred putrescine, monodansylcadaverine > cadaverine, spermidine, spermine > 1,10-diaminodecane > triethylbutylamine. On the other hand, the Ca2+-independent enzyme preferred putrescine > cadaverine > spermine, I,10-diaminodecane > spermidine > monodansylcadaverine > triethylbutylamine. Further studies with divalent ions excluded the possible association of this novel Ca2+-independent enzyme with diamine oxidase. Finally, the Ca2+-independent enzyme had a higher affinity for putrescine (Km = 0.02 mM) than did Ca2+-dependent t-TGase (0.2 mM). As judged by gel filtration on HiPrep Sephacryl 200 column, the Ca2+-independent enzyme had a molecular weight of approximately 48 kDa, the intestinal Ca2+-dependent t-TGase was about 188 kDa while that of testicular t-TGase was about 96 kDa. In conclusion, the Ca2+-independent enzyme is stimulated by cobalt or ferric ions, and selectively incorporates aliphatic diamines or polyamines with symmetric amino groups. The observed Ca2+-independent enzyme activity is not related to diamine oxidase or its products. With a 10 times greater affinity for putrescine, the calcium-independent, 48-kDa intestinal enzyme may mediate polyamine function better than calcium dependent, 188-kDa intestinal tissue transglutaminase in the intestinal mucosa.

Induction of tissue transglutaminase by dexamethasone: its correlation to receptor number and transglutaminase-mediated cell death in a series of malignant hamster fibrosarcomas

Treatment of the hamster fibrosarcoma cell lines (Met B, D and E) and BHK-21 hamster fibroblast cells with the glucocorticoid dexamethasone led to a powerful dose-dependent mRNA-synthesis-dependent increase in transglutaminase activity, which can be correlated with dexamethasone-responsive receptor numbers in each cell line. Increasing the number of dexamethasone-responsive receptors by transfection of cells with the HG1 glucocorticoid receptor protein caused an increase in transglutaminase activity that was proportional to the level of transfected receptor. In all experiments the levels of the tissue transglutaminase-mediated detergent-insoluble bodies was found to be comparable with increases in transglutaminase activity. Despite an increase in detergent-insoluble body formation, an increase in apoptosis as measured by DNA fragmentation was not found. Incubation of cells with the non-toxic competitive transglutaminase substrate fluorescein cadaverine led to the incorporation of this fluorescent amine into cellular proteins when cells were damaged after exposure to trypsin during cell passage. These cross-linked proteins containing fluorescein cadaverine were shown to be present in the detergent-insoluble bodies, indicating that the origin of these bodies is via activation of tissue transglutaminase after cell damage by trypsinization rather than apoptosis per se, since Met B cells expressing the bcl-2 cDNA were not protected from detergent-insoluble body formation. We describe a novel mechanism of cell death related to tissue transglutaminase expression and cell damage.

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.

Tissue transglutaminase is not increased during apoptosis of HT-1080 human fibrosarcoma cells

Tissue transglutaminase (tTGase), a cytosolic enzyme which catalyzes the covalent cross-linking of proteins, is thought to be involved in the apoptosis. Here, we tested whether tTGase is involved during HT-1080 fibrosarcoma cell apoptosis induced by the YIGSR (Tyr-Ile-Gly-Ser-Arg) peptide. This sequence is derived from the laminin alpha1 chain, and its potency is increased by the formation of a 16mer polymerization using a lysine tree structure. Cells were treated with several different concentrations of Ac-Y 16 for 16 hours, and apoptosis was increased in dose-dependent manner. When assayed by incorporation of [14C] putrescine into succinylated casein, total transglutaminase activity was decreased in parallel with the change in the number of attached cells. Western blot analysis using polyclonal antibody against tTGase showed that the tTGase protein level had not been significantly changed when equal amounts of the protein were applied. To confirm this result, we induced apoptosis of these cells by coating the tissue culture plates with non-adhesive poly-hydroxyethyl methacrylate (HEMA). Western blot analysis showed that the tTGase protein level did not change during this process of apoptosis. Although it has been suggested that tTGase is involved in the process of apoptosis of various cells in vitro and in vivo, our data demonstrate that tTGase is not involved in the process of apoptosis of HT-1080 human fibrosarcoma cell induced by either Ac-Y 16 or a non-adhesive culture surface.

Regulated expression of tissue transglutaminase in Swiss 3T3 fibroblasts: effects on the processing of fibronectin, cell attachment, and cell death

Tissue transglutaminase (tTgase) catalyzes the posttranslational modification of proteins by forming Ca2(+)-dependent intermolecular epsilon (gamma-glutamyl) lysine crosslinks; however, its physiological function is unclear despite increasing evidence for its involvement in the extracellular environment. To define where the enzyme is active and characterizes targets of crosslinking we have modulated tTgase expression in stably transformed Swiss 3T3 cell lines, generated by transfecting tTgase cDNA under the control of a tetracycline-regulated inducible promoter. Induced expression of tTgase enabled the detection of two pools of transglutaminase antigen, one intracellular and the other extracellular, which has a cellular distribution comparable to fibronectin. Incubation of cells with the fluorescent tTgase substrate fluorescein cadaverine indicated incorporation only in the extracellular matrix of healthy cells even though the amine was freely permeable to cells. Incorporation paralleled the deposition of fibronectin during fibril assembly when monitored by immunofluorescence. Fibronectin polymerization was confirmed by Western blotting. Cell surface-related tTgase was further demonstrated by preincubation of cells with tTgase antibody which led to inhibition of activity and cell attachment. Activation of the intracellular tTgase by increasing cytosolic Ca2+ using ionomycin resulted in cell death accompanied by extensive crosslinking in the cytoplasm, nucleus, and cell substratum contacts of induced cells. These dead cells were not typical of those undergoing apoptosis or necrosis since they remained adherent, preserved their microtubule network, and showed little DNA fragmentation. Modulation of expression of tTgase has indicated a possible physiological function for the enzyme in cell attachment, the crosslinking of fibronectin during fibril assembly, and the maintenance of cellular integrity in a novel form of cell death.

Identification of the eukaryotic initiation factor 5A as a retinoic acid-stimulated cellular binding partner for tissue transglutaminase II

GTP-binding protein/transglutaminases (tissue transglutaminases or TGases) have been implicated in a variety of cellular processes including retinoic acid (RA)-induced apoptosis. Recently, we have shown that RA activates TGases as reflected by stimulated GTP binding, increased membrane association, and stimulated phosphoinositide lipid turnover. This prompted us to search for cellular proteins that bind TGases in a RA-stimulated manner. In this report, we show that the eukaryotic initiation factor (eIF-5A), a protein that is essential for cell viability, perhaps through effects on protein synthesis and/or RNA export, associates with the TGase in vivo. The interaction between eIF-5A and TGase is specific for the GDP-bound form of the TGase and is not detected when the TGase is pre-loaded with GTP gamma S. The TGase-eIF-5A interaction also is promoted by Ca2+, Mg2+, and RA treatment of HeLa cells. In the presence of retinoic acid, millimolar levels of Ca2+ are no longer required for the TGase-eIF-5A interaction. Nocodazole treatment, which blocks the cell cycle at mitosis (M phase), strongly inhibits the interaction between eIF-5A and cytosolic TGase. The interaction between TGase and eIF-5A and its sensitivity to the nucleotide-occupied state of the TGase provides a potentially interesting connection between RA signaling and protein synthesis and/or RNA trafficking activities.

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.

Expression of tissue transglutaminase in human bladder carcinoma

The expression of cell adhesion proteins is frequently deranged in bladder carcinomas. Since tissue transglutaminase (tTG) can increase cellular adhesiveness, its expression was studied in both superficial and invasive transitional cell carcinomas. No expression of tTG was found in normal urothelium or in grade 1-3 papillary tumours without invasion. Expression of tTG was seen in the invasive processes of five grade 3 tumours with microinvasion and in 49 of 63 solid grade 3 and 4 tumours. In six cases, both primary tumours and biopsies from liver metastases were studied. In all these cases, the liver metastases were tTG-negative, while the primary tumours were tTG-negative in five cases and consisted of both tTG-positive and tTG-negative cells in one case. Analysis of tTG protein in tumour extracts by Western blotting showed expression of the 77 kD tTG, with no evidence for expression of the truncated form found in some cell types. It is suggested that tTG, when expressed, contributes to the deranged adhesive properties of the carcinoma cells and may influence the course of invasion. These results also raise the possibility that lack of tTG expression may increase the risk of developing blood-borne metastases.

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.

Expression of GTP-dependent and GTP-independent tissue-type transglutaminase in cytokine-treated rat brain astrocytes

Tissue-type transglutaminases (TGases) were recently shown to exert dual enzymatic activities; they catalyze the posttranslational modification of proteins by transamidation, and they also act as guanosine triphosphatase (GTPase). Here we show that a tissue-type TGase is expressed in rat brain astrocytes in vitro, and is induced by the inflammation-associated cytokines interleukin-1beta and to a lesser extent by tumor necrosis factor-alpha. Induction is accompanied by overexpression and appearance of an additional shorter clone, which does not contain the long 3'-untranslated region and encodes for a novel TGase enzyme whose C terminus lacks a site that affects the enzyme's interaction with guanosine triphosphate (GTP). Expression of two clones revealed that the long form is inhibited noncompetitively by GTP, but the short form significantly less so. The different affinities for GTP may account for the difference in physiological function between these two enzymes.

Alpha1-adrenergic receptor signaling via Gh is subtype specific and independent of its transglutaminase activity

Tissue transglutaminase (TGase II) is a Ca2+- and thiol-dependent enzyme that catalyzes the post-translational modification of proteins via the formation of epsilon(gamma-glutamyl) lysine bonds. We have shown previously that the GTP-binding protein, Gh, is a TGase II that mediates intracellular signaling by the alpha1B-adrenergic receptor (AR) (Nakaoka, H., Perez, D. M., Baek, K. J., Das, T., Husain, A., Mison, K., Im, M.-J., and Graham, R. M. (1994) Science 264, 1593-1596). Here, we evaluated the ability of Gh as compared with Gq to mediate receptor-stimulated inositol phosphate turnover by the three alpha1-subtypes (alpha1A, alpha1B, and alpha1D). In addition, we questioned if the transglutaminase function of Gh is involved in its receptor signaling activity. A mutant form of a human TGase II cDNA in which the codon for the active site cysteine (Cys277) was replaced by serine was cloned into the mammalian expression vector pMT2'. Compared with wild-type TGase II, no transglutaminase activity was observed with transient transfection of this Cys-->Ser mutant in COS-1 cells. However, like wild-type TGase, the Cys-->Ser mutant mediated receptor-stimulated inositol phosphate turnover when cotransfected with an alpha1B-AR cDNA. Galphaq supported alpha1-AR-mediated inositol phosphate turnover by all three receptor subtypes. By contrast, although both the wild-type and Cys-->Ser construct mediated receptor signaling by the alpha1B AR and alpha1D AR, the alpha1A-AR was unable to interact with Gh. However, a Gh-dependent signaling phenotype could be rescued by a chimeric alpha1A construct in which the third intracellular loop of the alpha1A-AR was replaced by that of the alpha1B-AR. Thus, the signaling function of Gh is independent of its transglutaminase activity and is alpha1-AR subtype specific. This subtype specificity of the interaction between alpha1 ARs and Gh involves important determinants in their third intracellular loops.

C-terminal deletion of human tissue transglutaminase enhances magnesium-dependent GTP/ATPase activity

Tissue transglutaminase (tTG) exhibits a magnesium-dependent GTP/ATPase activity that is involved in the regulation of the cell cycle and cell receptor signaling. The portion of the molecule involved in GTP/ATP hydrolysis is unknown. We expressed and purified a series of C-terminal truncation mutants of human tTG as glutathione S-transferase fusion proteins (DeltaS538, DeltaE447, DeltaP345, DeltaC290, DeltaV228, and DeltaF185) to determine the effect on GTP/ATPase activity. The truncation of the C terminus did not change significantly the apparent Km value for either GTP or ATP. In contrast, the Kcat value for GTP was increased by 4.6- and 3-fold for the DeltaS538 and DeltaE447 mutants, respectively. The DeltaP345 mutant had the highest hydrolysis activity with a 34-fold increase. The hydrolysis activity then declined to 8.1-, 8.7-, and 1. 9-fold for the DeltaC290, DeltaV228, and DeltaF185 mutants, respectively. The Kcat for ATP changed in parallel with the GTPase results. Thin layer chromatography analysis of the hydrolysis reaction products revealed that ATP was rapidly converted to ADP followed by a much slower conversion of ADP to AMP when incubated with wild type tTG or the DeltaP345 mutant. There was a substantial decrease in the calcium-dependent TGase activity when the last 149 amino acid residues were deleted from the C terminus. Less than 5% of the TGase activity was detected for the DeltaS538 and DeltaE447 mutants. In conclusion, we have located the ATP and GTP hydrolytic domain to amino acid residues 1-185. The C terminus functions to inhibit the expression of endogenous GTP/ATPase activity of tTG, and the potential role of the C terminus in modulating this activity is discussed.

G protein gene expression during mouse oocyte growth and maturation, and preimplantation embryo development

Fertilization in mammals initiates "egg activation," a series of events leading to embryo development. The signal transduction events that occur as a result of sperm-egg interactions and that initiate egg activation may be analogous to a ligand-receptor-effector pathway, but the details of this signaling pathway are poorly understood. Several lines of evidence support a role for guanine nucleotide-binding regulatory proteins (G proteins) in mammalian egg activation. Prior to initiating studies to examine further the role of specific G proteins in sperm-induced mouse egg activation, we needed to define the complement of G proteins expressed in the egg. Using a reverse transcription-polymerase chain reaction (RT-PCR) assay, the relative levels of mRNAs encoding specific G protein alpha, beta, and gamma subunits were determined in meiotically incompetent oocytes, fully-grown competent oocytes, metaphase II-arrested eggs, one-, two-, and eight-cell embryos, and blastocysts. mRNA transcripts representing all of the heterotrimeric G protein families were present at all of the stages examined, and all underwent significant changes in their patterns of expression. The following heterotrimeric G protein mRNA transcripts were present in oocytes, eggs, or preimplantation embryos: G alpha q family (q, 11, and 14), G alpha 12 family (12 and 13), G alpha i family (i1, i2, i3, t2, z, and s), beta subunits 1, 2, 4, and 5, and gamma subunits 2, 3, 5, and 7. A recently described large molecular weight G protein, G alpha h (Nakaoka et al., 1994: Science 264:1593-1596), was also present, G alpha 15, G alpha t1, G alpha olf, G alpha oA, G beta 3, G gamma 1, and G gamma 8 mRNA transcripts were not detected using this method. The most common pattern of expression observed was a maturation-associated decrease followed by an increase after the two-cell stage. Some transcripts, however, were expressed at low levels until the eight-cell to blastocyst stages, whereas others were expressed at high levels in the oocyte but following maturation declined and remained at a low level throughout preimplantation development.

Measurement of tissue transglutaminase activity in a permeabilized cell system: its regulation by Ca2+ and nucleotides

Electropermeabilized human endothelial cells (ECV-304) were used to study the regulation of tissue transglutaminase (tTGase) activity in the intracellular environment. An ELSA (enzyme-linked sorbent assay) plate assay was developed for intracellular tTGase activity, using the incorporation of a biotinylated primary amine, 5-{[(N-biotinoylamino)hexanoyl]amino}pentylamine(biotin-x-cadaveri ne; BTC), into endogenous protein substrates of tTGase. This incorporation process was inhibited by competitive inhibitors of tTGase, cystamine and monodansylcadaverine, in a dose-dependent manner. Over a 30 min period tTGase and its protein substrates did not leak out of the cell, and no incorporation of BTC occurred in unpermeabilized cells, indicating the reaction to be intracellular. In the presence of 10 nM or 10 muM CA2+, when nucleotides ATP and GTP were added at concentrations mimicking cytosolic levels, tTGase activity was decreased virtually to zero. Only at 100 muM Ca2+, when nucleotides were low or absent was tTGase activity observed. Under these conditions a variety of proteins was labelled by the enzyme, with the major labelling found in a protein of molecular mass around 51 kDa when analysed by SDS/PAGE/Western blotting.