Isolation of transglutaminase-reactive sequences from complex biological systems: a prominent lysine donor sequence in bovine lens

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

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

Signature Ions in MS/MS Spectra for Dansyl-Aminohexyl-QQIV Adducts on Lysine

Bacterial transglutaminase was used to label human plasma proteins with fluorescent tags. Protein lysines were modified with dansyl-epsilon-aminohexyl-Gln-Gln-Ile-Val-OH (dansylQQIV), while protein glutamines were modified with dansyl cadaverine. Labeled proteins included human butyrylcholinesterase, apolipoprotein A-1, haptoglobin, haptoglobin-related protein, immunoglobulin heavy chain, and hemopexin. Tryptic peptides were analyzed by LC-MS/MS on an Orbitrap Fusion Lumos mass spectrometer. Modified residues were identified in Protein Prospector and Proteome Discoverer searches of mass spectrometry data. The MS/MS fragmentation spectra from dansylQQIV-modified peptides gave intense peaks at 475.2015, 364.1691, 347.1426, 234.0585, and 170.0965 m/z. These signature ions are useful markers for identifying modified peptides. Human butyrylcholinesterase retained full activity following modification by dansylQQIV or dansyl cadaverine.

Acyl transfer mechanisms of tissue transglutaminase

Tissue transglutaminase (TG2) is a calcium-dependent enzyme that catalyses several acyl transfer reactions. The most biologically relevant of these involve protein-bound Gln residues as an acyl-donor substrate, and either water or a primary amine as an acyl-acceptor substrate. The former leads to deamidation of Gln to Glu, whereas the latter leads to transamidation, typically resulting in protein cross-linking when the amine substrate is a protein-bound Lys residue. In this review, we present an overview of over fifty years of mechanistic studies that have led to our current understanding of TG2-mediated hydrolysis and transamidation.

Characterization of amine donor and acceptor sites for tissue type transglutaminase using a sequence from the C-terminus of human fibrillin-1 and the N-terminus of osteonectin

Transglutaminase (TGase)-modified proteins are commonly observed in a wide range of biological systems. Therefore, the identification of TGase substrates and respective consensus sites may contribute to a better understanding of the physiological role of TGase. In this study, we identified enzyme-specific properties of two peptide sequences, EDGFFKI, derived from human fibrillin-1, and the previously characterized APQQEA, derived from human osteonectin. EDGFFKI was identified in a previous publication as an amine donor substrate for tissue TGase; APQ(3)Q(4)EA is an amine acceptor for this enzyme. A widely-used lysine donor mimic, monodansylcadaverine (MDC), was used as a control. EDGFFKI crosslinked specifically only to Q(3) of the acceptor probe. The EDGFFKI sequence also showed enzyme specificity for tissue TGase while no reaction was observed with plasma TGase (Factor XIIIa), consistent with its natural occurrence in vivo. Using this substrate in biotinylated form we demonstrate its value as a tracer probe to detect endogenous TGase activity in human tissues as well as to target potential amine acceptor substrates via an enzyme-directed site-specific labeling. The results of this study show natively derived EDGFFKI and APQQEA are better and more specific indicators of endogenous tissue TGase activity as compared to a small molecule probe; this may be important in diagnostic applications. The specificity with which matrix sequences APQQEA and EDGFFKI interact with tissue TGase but not plasma TGase may also be crucial for understanding and controlling the function of these TGases in vivo and in tissue engineering.

Age-dependent deamidation of lifelong proteins in the human lens

PURPOSE

Deamidation is a common posttranslational modification in human lens crystallins and may be a key factor in the age-related denaturation of such lifelong proteins. The aim of this study was to identify the sites of deamidation in older lenses.

METHODS

High-performance liquid chromatography/mass spectrometry of tryptic digests was used to identify sites of deamidation in the major human lens crystallins. Older normal and age-matched cataractous lenses were compared with fetal lenses.

RESULTS

Approximately equal numbers of glutamine and asparagine residues were deamidated in older lenses; however, the extent of deamidation of Asn was three times greater than that of Gln (Asn, 22.6% +/- 3.6%; Gln, 6.6% +/- 1.3%). Individual crystallins differed markedly in their extent of deamidation, and deamidated residues were typically localized within discrete regions of the polypeptides. A large percentage (42%) of the sites of deamidation were characterized by the presence of a basic amino acid one residue removed from the original Gln or Asn. At nine such sites, the extent of Asn deamidation averaged 50% in aged lenses. There were few differences in deamidation between crystallins of aged normal and nuclear cataractous lenses.

CONCLUSIONS

Equal numbers of Asn and Gln residues are deamidated in crystallins from aged normal and cataractous lenses. Deamidation of Asn/Gln in lifelong proteins, such as those in the lens, may be governed to a significant degree by base-catalyzed processes.

Selectivity in the post-translational, transglutaminase-dependent acylation of lysine residues

Transglutaminases (TGs) are known to exhibit remarkable specificities not only for the Q (or Gln) sites but also for the K (or Lys) sites of proteins with which they react. To gain further insight into K-site specificity, we examined the reactions of dansyl-epsilon-aminocaproyl-GlnGlnIleVal with three chemically and structurally well-characterized proteins (bovine pancreatic ribonuclease A, bovine pancreatic trypsin inhibitor, and chicken egg white lysozyme), as catalyzed by TG2, a biologically important post-translational enzyme. The substrates represent a total of 20 potential surface sites for acylation by the fluorescent Gln probe, yet only two of the lysine side chains reacted with TG2. While the K1 site of ribonuclease and the K15 site of the trypsin inhibitor could be readily acylated by the enzyme, none of the lysines in lysozyme were modified. The findings lead us to suggest that the selection of lysine residues by TG2 is not encoded in the primary amino acid sequence surrounding the target side chain but depends primarily on its being positioned in an accessible segment of the protein structure.

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.

Purification and characterization of zebrafish hatching enzyme - an evolutionary aspect of the mechanism of egg envelope digestion

There are two hatching enzyme homologues in the zebrafish genome: zebrafish hatching enzyme ZHE1 and ZHE2. Northern blot and RT-PCR analysis revealed that ZHE1 was mainly expressed in pre-hatching embryos, whereas ZHE2 was rarely expressed. This was consistent with the results obtained in an experiment conducted at the protein level, which demonstrated that one kind of hatching enzyme, ZHE1, was able to be purified from the hatching liquid. Therefore, the hatching of zebrafish embryo is performed by a single enzyme, different from the finding that the medaka hatching enzyme is an enzyme system composed of two enzymes, medaka high choriolytic enzyme (MHCE) and medaka low choriolytic enzyme (MLCE), which cooperatively digest the egg envelope. The six ZHE1-cleaving sites were located in the N-terminal regions of egg envelope subunit proteins, ZP2 and ZP3, but not in the internal regions, such as the ZP domains. The digestion manner of ZHE1 appears to be highly analogous to that of MHCE, which partially digests the egg envelope and swells the envelope. The cross-species digestion using enzymes and substrates of zebrafish and medaka revealed that both ZHE1 and MHCE cleaved the same sites of the egg envelope proteins of two species, suggesting that the substrate specificity of ZHE1 is quite similar to that of MHCE. However, MLCE did not show such similarity. Because HCE and LCE are the result of gene duplication in the evolutionary pathway of Teleostei, the present study suggests that ZHE1 and MHCE maintain the character of an ancestral hatching enzyme, and that MLCE acquires a new function, such as promoting the complete digestion of the egg envelope swollen by MHCE.

Enzymatically crosslinked collagen-mimetic dendrimers that promote integrin-targeted cell adhesion

Collagen is made up of a diverse family of the extracellular matrices, most of which are generally found crosslinked in vivo. To more closely mimic the biological function of collagen, this work focuses on establishing a molecular strategy to engineer a functional biomimetic collagen that exhibits stable collagen-like triple-helical conformation with cell-binding activity, in addition to an enzyme-mediated crosslinking by tissue transglutaminase (tTGase). A novel sequence spanning residues 2800-2807 of human fibrillin-1 (EDGFFKI) was first identified as an amine donor substrate for tTGase, using a previously characterized APQQEA derived from human osteonectin as an amine acceptor probe. Subsequently, collagen-mimetic peptides (CMPs) supplemented with a cell-binding sequence (GFOGER) and the identified EDGFFKI and APQQEA substrate sequences were conjugated onto a generation 2 poly(amidoamine) dendrimer, resulting in a crosslinkable collagen-mimetic dendrimer, denoted as CMD-K and CMD-Q, respectively. Both CMD-K and CMD-Q exhibited enhanced triple-helical stability and supported cell adhesion in an integrin-specific manner. Finally, tTGase-mediated crosslinking between CMD-K and CMD-Q resulted in a supramolecular structure that exhibited stable collagen-like triple-helical conformation and improved cellular recognition. The results show that the triple-helical structure is important in preserving the GFOGER cell-binding site while the tTGase-mediated protein crosslinking may also be crucial for the recognition by cell surface integrin receptors.

Tissue transglutaminase catalyzes the deamidation of glutamines in lens betaB(2)- and betaB(3)-crystallins

Tissue transglutaminase (tTG) is a Ca(2+)-dependent enzyme catalyzing the formation of covalent crosslinks between peptide-bound glutamine and lysine residues. Lens crystallins, including alphaB-crystallin and several beta-crystallins, are in vitro substrates for tTG. In both human and bovine fetal lens extracts treated with commercially available guinea pig liver tTG we detected the formation of high molecular weight (HMW) aggregates containing crosslinked betaB(2)- and betaA(3)-crystallin. More interestingly, 2D-gel electrophoresis combined with mass spectrometry analysis revealed that glutamines present in the N-terminal arms of betaB(2)- and betaB(3)-crystallins deamidate readily in the presence of tTG. We found that both tTG-catalyzed crosslinking and deamidation disrupt the beta-crystallin complex, suggesting that these tTG-catalyzed modifications can influence the macromolecular assembly of lens crystallins. These data together suggest that tTG can contribute to the age-related deamidation of glutamine residues of lens crystallins.

Site-specific transamidation and deamidation of the small heat-shock protein Hsp20 by tissue transglutaminase

Crosslinking of small heat-shock proteins (sHsps) by tissue transglutaminase (tTG) is enhanced by stress and under pathological conditions. We here used hexapeptide probes to determine the amine donor (K) and acceptor (Q) sites for tTG in Hsp20. Mass spectrometric peptide mass fingerprinting and peptide fragmentation established that Q31 and the C-terminal K162 are involved in inter- and intramolecular crosslinking (transamidation). Q31 is a conserved glutamine in sHsps where the neighboring residue determines its reactivity. Moreover, we detected highly efficient simultaneous deamidation of Q66, which suggests that tTG-catalyzed transamidation and deamidation is specific for different glutamine residues.

High-throughput scintillation proximity assay for transglutaminase activity measurement

Members of the transglutaminase enzyme family are involved in a broad range of biological phenomena, including haemostasis, apoptosis, semen coagulation, skin formation, and wound healing. A new and rapid method for measurement of transglutaminase activity is described in this article. The enzyme links tritium-labeled putrescine to biotinylated oligoglutamine, and the tritiated peptide is bound to a streptavidin-coated microtiter plate permanently covered by a thin layer of scintillant. Only the radioisotope incorporated into the peptide substrate is close enough to the scintillant molecules for photons to be produced. The signal generation depends on the transglutaminase activity, and it can be detected by appropriate light-measuring instrumentation without separation steps. The assay is sensitive, specific, linear at concentrations of tissue transglutaminase between 0.05 and 1.6m U/ml, and suitable for high-throughput measurements.

Transglutaminase catalyzes differential crosslinking of small heat shock proteins and amyloid-beta

Crosslinking of proteins by tissue transglutaminase (tTG) is enhanced in amyloid (Abeta) deposits characteristic of Alzheimer's disease and sporadic inclusion body myositis. Small heat shock proteins (sHsps) also occur in amyloid deposits. We here report the substrate characteristics for tTG of six sHsps. Hsp27, Hsp20 and HspB8 are both lysine- and glutamine-donors, alphaB-crystallin only is a lysine-donor, HspB2 a glutamine-donor, and HspB3 no substrate at all. Close interaction of proteins stimulates crosslinking efficiency as crosslinking between different sHsps only takes place within the same heteromeric complex. We also observed that alphaB-crystallin, Hsp27 and Hsp20 associate with Abeta in vitro, and can be readily crosslinked by tTG.

Amine donor protein substrates for transglutaminase activity in Caenorhabditis elegans

Transglutaminase dependent cross-linking of proteins has been implicated in a wide range of biological phenomena occurring in both extracellular and intracellular compartments. Clarification of the physiological role of transglutaminases requires identification of substrate molecules. Here we report the detection, purification, and identification by mass spectrometry of proteins, the glutamate dehydrogenase, a protein disulfide isomerase, and aldehyde dehydrogenase as amine donor substrates for the transglutaminase activity of the nematode Caenorhabditis elegans utilizing a novel biotinylated oligoglutamine peptide as a substrate. We also purified and identified streptavidin-binding proteins of the worm.

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.

Enhanced expression of transglutaminase 2 in anterior polar cataracts and its induction by TGF-beta in vitro

BACKGROUND/AIMS

Transglutaminase activity has long been implicated in the cataract formation. However, the precise mechanism of how it is produced and involved in this process remains unclear. Here the authors sought to examine whether transglutaminase 2 (TGase 2) is expressed in lens epithelial cells from patients with anterior polar cataracts, to determine whether TGase 2 expression is induced by transforming growth factor (TGF-beta) in cultured lens epithelial cells, and to determine whether TGase 2 participates in the crosslinking of fibronectin in lens epithelial cells in vitro.

METHODS

Lens epithelial cells from anterior polar cataracts, nuclear cataracts, and non-cataractous clear lenses were examined for the expression of TGase 2 using reverse transcription-polymerase chain reaction, western blot analysis, and immunohistochemical analysis. The modulation of extracellular TGase 2 activity by TGF-beta was measured by the formation of fibronectin polymers and the incorporation of fluorescein cadaverine into extracellular matrix proteins. The effect of TGase 2 overexpression was analysed by immunofluorescence staining and western blot analysis of human lens epithelial (HLE) B-3 cells transiently transfected with TGase 2 gene.

RESULTS

The expression of TGase 2 mRNA and its protein was markedly enhanced in lens epithelial cells from patients with anterior polar cataracts. Treatment of HLE B-3 cells with TGF-beta caused an increase in TGase 2 protein, its extracellular activity, and the crosslinking of fibronectin. Transient transfection of HLE B-3 cells with the TGase 2 gene led to the increased production of fibronectin monomers and polymers.

CONCLUSIONS

This study shows that TGase 2 is overexpressed in lens epithelial cells from anterior polar cataracts and that TGF-beta may be a causative factor in the induction of TGase 2. The enhanced expression of TGase 2 might cause the accumulation and crosslinking of the extracellular matrix proteins and might play a part in anterior polar cataract development.

Transglutaminase-mediated cross-linking of alpha-crystallin: structural and functional consequences

Aggregation and covalent cross-linking of the crystallins, the major structural proteins of the eye lens, increase light scattering by the lens leading to opacification and cataract. Disturbance of calcium homeostasis in the tissue is one of the factors implicated in cataractogenesis. Calcium-activated transglutaminase (TG)-catalyzed cross-linking of some lens proteins has been reported earlier. We show here that alpha-crystallin, a major structural protein in the lens and a member of the small heat shock protein family, is also a substrate for TG-mediated cross-linking, indicating the presence of donor Lys and acceptor Gln residues in the protein. Upon TG-catalyzed dimerization, the secondary and tertiary structures of the protein are altered, and its surface hydrophobicity reduced. The chaperone-like property of the protein, suspected to be one of its functions in situ, is substantially reduced upon such cross-linking. These results, taken together with earlier ones on lens beta-crystallins and vimentin, suggest that TG-mediated events might compromise lens function. Also, since alpha-crystallin occurs not only in the lens but in other tissues as well, such TG-catalyzed cross-linking and the associated alterations in its structure and activity would be of general pathological interest.

Identification of Protein Substrates for Transglutaminase in Caenorhabditis elegans

Transglutaminase-dependent cross-linking of proteins leads to protein polymerisation that confers stability as well as resistance to mechanical disruption and chemical attack. Various transglutaminases have been implicated in a wide range of biological phenomena occurring in both extracellular and intracellular compartments, but further clarification of the physiological role of these enzymes requires identification of possible substrate molecules. Here we report the detection, purification, and identification of two proteins, enolase and ATP synthase alpha subunit as glutamine donor protein substrates for the transglutaminase of the nematode Caenorhabditis elegans.

An atypical form of alphaB-crystallin is present in high concentration in some human cataractous lenses. Identification and characterization of aberrant N- and C-terminal processing

Two unique polypeptides, 22.4 and 16.4 kDa, were prominent in some human cataracts. Both proteins were identified as modified forms of the small heat shock protein, alphaB-crystallin. The concentration of total alphaB-crystallin in most of these cataracts was significantly increased. The 22.4-kDa protein was subsequently designated as alphaB(g). Mass spectrometric analyses of tryptic and Asp-N digests showed alphaB(g) is alphaB-crystallin minus the C-terminal lysine. alphaB(g) constituted 10-90% of the total alphaB-crystallin in these cataracts and was preferentially phosphorylated over the typical form of alphaB-crystallin. Human alphaB(g) and alphaB-crystallin were cloned and expressed in Escherichia coli. The differences in electrophoretic mobility and the large difference in native pI values suggest some structural differences exist. The chaperone-like activity of recombinant human alphaB(g) was comparable to that of recombinant human alphaB-crystallin in preventing the aggregation of lactalbumin induced by dithiothreitol. The mechanism involved in generating alphaB(g) is not known, but a premature termination of the alphaB-crystallin gene was ruled out by sequencing the polymerase chain reaction products of the last exon for the alphaB-crystallin gene from lenses containing alphaB(g). The 16.4-kDa protein was an N-terminally truncated fragment of alphaB(g). The high concentration of alphaB-crystallin in these cataracts is the first observation of this kind in human lenses.

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.

Properties of purified lens transglutaminase and regulation of its transamidase/crosslinking activity by GTP

On account of its protein crosslinking activity, the Ca2+-dependent transglutaminase of the lens is likely to be involved in the formation of cataracts. We have now purified the rabbit lens enzyme to near homogeneity as judged by SDS-PAGE (Mr approximately 78 kDa), and a key feature of the procedure was the use of a highly selective affinity chromatographic step with a fibronectin fragment as ligand. The catalytic activity of the lens transglutaminase, measured by the incorporation of dansylcadaverine into dimethylcasein, was compared with those of two similar enzymes isolated from human red cells and from guinea pig liver, respectively. All three enzymes were inhibited by GTP, but the lens enzyme was most sensitive to inhibition by the nucleotide. Moreover, GTP was also shown to inhibit the formation of the approximately 55 kDa betacrystallin dimers in the Ca2+-treated rabbit lens homogenate, proving that the nucleotide is a negative regulator for the crosslinking activity of transglutaminase in this tissue.

Novel inhibitors against the transglutaminase-catalysed crosslinking of lens proteins

Post-translational modifications by transglutaminase may contribute to the remodeling of cellular architecture in the development of lens fiber cells, and there is evidence that the enzyme may also play a role in cataract formation. It catalyses hydrolytic deamidations as well as amide exchanges on select glutamine side chains at endo positions in a small subset of proteins of the lens. N epsilon(gamma-glutamyl)lysine crosslinks, the characteristic hallmarks of transglutaminase activity, were identified in polymers isolated from human cataract. Following up on our earlier studies relating to the inhibition of protein crosslinking by the Ca(2+)-activated transglutaminase in the lens, we have now examined the effects of 2-[(2-oxopropyl)thio]-imidazolium derivatives, recently described as active site-directed inhibitors for this family of enzymes. First, we have shown that the compounds at concentrations of 1-2 microM were effective in blocking the transamidating activities of partially purified lens transglutaminase. Then we focused on their efficacy in preventing the formation of the ca. 55 kDa beta crystallin dimers in the whole lens tissue. The production of these dimers, crosslinked by N epsilon(gamma-glutamyl)lysine isopeptide bridges, is an early sign of transglutaminase action in rabbit lens, and it can be readily documented by the SDS-PAGE analysis of proteins remaining in the soluble phase after brief exposure of the homogenate to Ca2+. The new compounds proved to be potent inhibitors of transglutaminase also in this preparation, preventing the crosslinking event at ca. 1 microM concentration. Moreover, even when applied at a 1,000-fold greater concentration (2 mM), they did not interfere with the action of calpain which, similarly to the activation of the transglutaminase system, is triggered by the addition of Ca2+. The high selectivity of the new compounds for differentially blocking only the transglutaminase and not the calpain of the lens, is all the more remarkable because these two enzymes share several mechanistic and structural similarities.

The intermediate filament protein, vimentin, in the lens is a target for cross-linking by transglutaminase

Mere addition of Ca2+ to a lens cortical homogenate (bovine) generates a series of products composed of a variety of high molecular weight vimentin species. The Ca2+-induced cross-linking of this cytoskeletal element seems to be mediated by the intrinsic transglutaminase of lens, because the reaction could be blocked at the monomeric state of vimentin by the inclusion of small synthetic substrates of the enzyme dansylcadaverine or dansyl-epsilon-aminocaproyl-Gln-Gln-Ile-Val. These compounds are known to compete against the Gln or Lys functionalities of proteins that would participate in forming the Nepsilon(gamma-glutamyl)lysine protein-to-protein cross-links. The cytosolic transglutaminase-catalyzed reactions could be reproduced with purified bovine lens vimentin and also with recombinant human vimentin preparations. Employing the latter system, we have titrated the transglutaminase-reactive sites of vimentin and, by sequencing the dansyl-tracer-labeled segments of the protein, we have shown that residues Gln453 and Gln460 served as acceptor functionalities and Lys97, Lys104, Lys294, and Lys439 as electron donor functionalities in vimentin. The transglutaminase-dependent reaction of this intermediate filament protein might influence the shape and plasticity of the fiber cells, and the enzyme-catalyzed cross-linking of vimentin, in conjunction with other lens constituents, may contribute to the process of cataract formation.

A fluorescent substrate of transglutaminase for detection and characterization of glutamine acceptor compounds

A fluorescent dipeptide was designed to discover glutamine acceptor proteins of transglutaminase. Starting materials for synthesis were the commercially available compounds carbobenzoxy-L-glutaminylglycine (CBZ-Gln-Gly) and monodansylcadaverine (C-DNS) which were coupled to obtain CBZ-Gln-Gly-C-DNS 1 [1-N-(carbobenzoxy-L-glutaminylglycyl)-5-N- (5'-N', N'-dimethylamino-1'-naphthalenesulfonyl)- diamidopentane]. The glutamine peptide is a substrate of bacterial transglutaminase from Streptoverticillium mobaraense as well as of the guinea pig liver enzyme. This could be shown by incorporating 1 into alpha s1-casein resulting in a significant increase in fluorescence intensity and a concomitant inhibition of cross-linking reaction. Additionally, dipeptide 1 is a useful tool to characterize the specificity of transglutaminase toward small primary amines. We established a sensitive HPLC assay and determined the kinetic parameters of several alkylamines. Hydrolysis of 1 is suppressed in the presence of the nucleophiles as it could be demonstrated with different concentrations of butylamine in semiquantitative studies. Together with labeled primary amines, reagent 1 seems to be a particularly suitable tool for examining acceptor-donor relationships of transglutaminase substrates.

Hydrolysis of gamma:epsilon isopeptides by cytosolic transglutaminases and by coagulation factor XIIIa

Nepsilon-(gamma-glutamyl)lysine cross-links, connecting various peptide chain segments, are frequently the major products in transglutaminase-catalyzed reactions. We have now investigated the effectiveness of these enzymes for hydrolyzing the gamma:epsilon linkage. Branched compounds were synthesized, in which the backbone on the gamma-side of the cross-bridge was labeled with a fluorophor (5-(dimethylamino)-1-naphthalenesulfonyl or 2-aminobenzoyl) attached through an epsilon-aminocaproyl linker in the N-terminal position, and the other branch of the bridge was constructed with Lys methylamide or diaminopentane blocked by 2,4-dinitrophenyl at the Nalpha position. Hydrolysis of the cross-link could be followed in these internally quenched substrates by an increase in fluorescence. In addition to the thrombin and Ca2+-activated human coagulation Factor XIIIa, cytosolic transglutaminases from human red cells and from guinea pig liver were tested. All three enzymes were found to display good isopeptidase activities, with Km values of 10(-4) to 10(-5) M. Inhibitors of transamidation were effective in blocking the hydrolysis by the enzymes, indicating that expression of isopeptidase activity did not require unusual protein conformations. We suggest that transglutaminases may play a dynamic role in biology not only by promoting the formation but also the breaking of Nepsilon-(gamma-glutamyl)lysine isopeptides.

Novel transglutaminase inhibitors reduce the cornified cell envelope formation

Transglutaminase (TGase) is a calcium-dependent enzyme which catalyzes the iso-peptide cross-link between peptide-bound glutamine and lysine in vivo. Though the cross-link is developed as a barrier function in the skin system, overexpression of this could invoke skin hyperkeratosis in psoriasis and roughness in aged skin. In former research, many strong irreversible TGase inhibitors failed application because of high cytotoxicity. We selected one peptide after primary screening of six synthetic peptides designed from domains of known TGase substrates. Then we attempted to reduce the size and finally obtained two tetrameric peptides. When we treated keratinocyte with these TGase inhibitors under calcium-induced differentiation, the formation of a cornified cell envelope (CE) was decreased to the same level of CE under proliferating conditions without cytotoxic effect. Therefore, we propose that these TGase inhibitors may be useful for solving the physiological hypercross-linking problems for pharmaceutical or cosmetic purposes.

Identification of the alpha chain lysine donor sites involved in factor XIIIa fibrin cross-linking

Biochemical studies of fibrin cross-linking were conducted to identify the specific Aalpha chain lysine residues that potentially serve as Factor XIIIa amine donor substrates during alpha polymer formation. A previously characterized Factor XIIIa fibrin lysine labeling system was employed to localize sites of donor activity based on their covalent incorporation of a synthetic peptide acceptor substrate analog modelled after the NH2-terminal cross-linking domain of alpha2 antiplasmin. Peptide-decorated fibrin was prepared using purified fibrinogen as the starting material. Cyanogen bromide digestion, immunoaffinity chromatography, high pressure liquid chromatography (HPLC), and enzyme-linked immunosorbent assay (anti-peptide) methodologies were employed to isolate purified CNBr fibrin fragments whose structures included the acceptor probe in cross-linked form and, therefore, represented regions of (amine) donor activity. Five alpha chain CNBr fragments (within Aalpha 208-610) and one gamma chain CNBr fragment (gamma 385-411) were the only portions of fibrin found associated with the acceptor peptide, based on collective sequencing, mass, and compositional data. Trypsin digestion, HPLC, and enzyme-linked immunosorbent assay (anti-peptide) methodologies were used to isolate smaller derivatives whose structures included an alpha chain tryptic cleavage product (the donor arm) cross-linked to the trypsin-resistant synthetic peptide (the acceptor arm). Biochemical characterization and quantitative peptide recovery data revealed that 12 of the 23 potential lysine donor residues within alpha 208-610 had incorporated the peptide probe, whereas gamma chain donor activity was due solely to peptide cross-linking at (gamma) Lys406; the alpha chain lysines, Lys556 and Lys580, accounted for 50% of the total alpha chain donor cross-linking activity observed, with Lys539, Lys508, Lys418, and Lys448 contributing an additional 28% and Lys601, Lys606, Lys427, Lys429, Lys208, Lys224, and/or Lys219 responsible for the remaining proportion (2-5%, each). The collective findings extend current models proposed for the mechanism of alpha polymer formation, raise questions concerning the physiological role of multiple alpha chain donor sites, and, most importantly, provide specific information that should facilitate future efforts to identify the respective lysine and glutamine partners involved in native fibrin alpha chain cross-linking.

Substrate requirements for transglutaminases. Influence of the amino acid residue preceding the amine donor lysine in a native protein

Thirteen recombinant alpha A-crystallin mutants were constructed that differed in the type of amino acid residue directly preceding the sole amine donor lysine for transglutaminases in this protein. The capacity of these mutants to be cross-linked to amine acceptor substrates by tissue transglutaminase and factor XIII was assessed. Two different biotinylated glutamine-containing oligopeptides were used as amine acceptor probes. It appears that the type of residue preceding the amine donor lysine has a considerable influence on the substrate potential of alpha A-crystallin for transglutaminases. This influence shows qualitatively similar trends for tissue transglutaminase and factor XIII and is irrespective of the amine acceptor probe. In general, glycine or aspartic acid before the amine donor lysine has the strongest adverse effects on substrate reactivity, and proline, histidine, and tryptophan are less favorable. Valine, arginine, and phenylalanine, and to a more variable or somewhat lesser extent also serine, alanine, leucine, tyrosine, and asparagine, have an enhancing effect. This pattern of preference is largely in agreement with that observed for the limited number of characterized amine donor lysines in protein substrates for transglutaminases. It can be concluded that tissue transglutaminase and factor XIII have a rather broad yet clearly differentiated tolerance with respect to the residue preceding the amine donor lysine substrate in native proteins.

Association of a transglutaminase-related antigen with intermediate filaments

A mouse monoclonal antibody, G92.1.2, raised against guinea pig liver transglutaminase (TGase) recognizes an antigen present in primary mouse dermal fibroblasts. A filamentous pattern, bearing remarkable similarity to the vimentin intermediate filament (IF) network, is seen when these cells are fixed and processed for indirect immunofluorescence with the antibody. Double-label immunofluorescence reveals that the antigen reacting with the antibody colocalizes precisely with vimentin IF and that this colocalization is retained after the treatment of fibroblasts with colchicine, which induces a redistribution of the majority of IFs into perinuclear aggregates. These morphological observations are further supported by the finding that the protein reacting with G92.1.2 is retained in IF-enriched cytoskeletal preparations made by using nonionic detergent-containing high ionic strength solutions. Western blots of the IF fraction show that G92.1.2 recognizes a major band of approximately 280 kDa and does not cross react with vimentin. Furthermore, when the antibody is microinjected into live dermal fibroblasts, it causes a collapse of the vimentin IF network in the majority of injected cells. The results suggest that a form of TGase, or a TGase-related antigen, is closely associated with the vimentin IF network of primary cultures of mouse dermal fibroblasts.

The amine-donor substrate specificity of tissue-type transglutaminase. Influence of amino acid residues flanking the amine-donor lysine residue

The amine-donor substrate specificity of tissue-type transglutaminase has been studied in a series of recombinant alpha A-crystallin mutants. These mutant proteins have been provided with a potential substrate lysine residue, flanked by different amino acid residues, in the C-terminal extended arm of alpha A-crystallin. A biotinylated amine-acceptor hexapeptide was used as a probe for labelling the amine-donor sites. Wild-type bovine alpha A-crystallin does not function as an amine-donor substrate for tissue-type transglutaminase. Yet, upon introduction of a lysine residue at the C-terminal or penultimate position, all mutant alpha A-crystallins act as amine-donor substrates, although to different extents. This shows that accessibility is the primary requirement for a lysine residue to function as an amine-donor substrate for transglutaminase and that the enzyme has a broad tolerance towards the neighbouring residues. However, the nature of the flanking amino acid residues does clearly affect the reactivity of the substrate lysine residue. Notably, we found that a proline or glycine residue in front of the substrate lysine has a strong adverse effect on the substrate reactivity as compared to a preceding leucine, serine, alanine or arginine residue.

Exposure of beta H-crystallin to hydroxyl radicals enhances the transglutaminase-susceptibility of its existing amine-donor and amine-acceptor sites

beta H-crystallin was exposed to radiolytically generated hydroxyl radicals at defined radical concentrations, and its capacity to act as an amine-acceptor substrate and as an amine-donor substrate for transglutaminase were investigated. [14C]Methylamine was used as a probe for labelling amine-acceptor sites; a novel biotinylated hexapeptide was used to label amine-donor sites. The results demonstrate that both primary amine incorporation and hexapeptide incorporation by transglutaminase are considerably increased after oxidative attack on the crystallin. The identity of the labelled subunits was established, and it is shown that, in both cases, this increased incorporation is not due to the production of new substrates, but that the existing incorporation sites become more susceptible. Moreover, using the newly developed probe, we could identify, for the first time, the major crystallin subunits active as amine-donor substrates (both before and after treatment) to be beta B1-, beta A3- and beta A4-crystallin. These data support the proposal that oxidative stress and transglutaminase activity may be jointly involved in the changes found in lens crystallins with age and in the development of cataract.