Transglutaminase involvement in the secretion of insulin from electropermeabilised rat islets of langerhans

Ca²⁺-Induced insulin release from electropermeabilised islets is inhibited by the transglutaminase inhibitors monodansylcadaverine, glycine methylester, methylamine and cystamine but not by the control compounds dimethyl monodansylcadaverine and sarcosine methylester which lack the primary amine group. Neither monodansylcadaverine nor glycine methylester inhibited insulin secretion induced by either cAMP or the phorbol ester PMA at basal levels (10 nM) of Ca²⁺. These data provide further evidence for the involvement of transglutaminase in Ca²⁺ induced insulin secretion, they also suggest that insulin secretion induced by either cAMP or PMA may act in part by a mechanism independent of that induced by Ca²⁺.     

Transglutaminase and cellular motile events: Retardation of proinsulin conversion by glycine methylester

Glycine methylester, an inhibitor of transglutaminase, decreased glucose-stimulated insulin release and delayed proinsulin conversion in rat pancreatic islets pulselabelle with L-[4-[³H]phenylalanine. Sarcosine methylester, which does not inhibit transglutaminase activity, failed to affect insulin release and proinsulin conversion. The incorporation of L-[4-³H]phenylalanine into islet peptides, the ratio of hormonal to total tritiated peptides and the insulin content of the islets failed to be affected by either of these methylesters. It is proposed that transglutaminase participates in the control of motile events involved in both the transfer of proinsulin from its site of synthesis to its site of conversion, and the translocation of insulin from its site of storage to its site of release.     

Transglutaminase and neuronal differentiation

Summary During mouse brain maturation cellular transglutaminase specific activity increases 2.5 fold from day 3 to adulthood. A more pronounced increase is seen during morphological differentiation of mouse neuroblastoma cells, where serum withdrawal induces neurite outgrowth concomitant with a 10 fold increase in transglutaminase specific activity. In contrast, non-dividing neuroblastoma cells lacking neurites show only a 1.5 fold increase in enzyme specific activity. Transglutaminase activity does not reach maximal levels until extensive neurite formation has occurred. More than 80% of the transglutaminase activity is found in the soluble component of brain and neuroblastoma homogenates. Using [³H]-putrescine as the acyl acceptor, endogenous acyl donor substrates in the neuroblastoma cells included proteins that comigrated on SDS-PAGE with tubulin and actin; however, very high molecular weight crosslinked material is the major reaction product in vitro. When purified brain tubulin, microtubule associated proteins and microtubules were compared as exogenous substrates, only the polymeric microtubules were a good acyl donor substrate. Furthermore, preincubation of purified tubulin with transglutaminase and putrescine stimulated both the rate and extent of microtubule assembly. These findings suggest that transglutaminase may mediate covalent cross-linking of microtubules to other cellular components, or the post-translational modification of tubulin by the formation of -glutamylamines.     

Metabolic and secretory effects of methylamine in pancreatic islets

The metabolic and secretory effects of methylamine in rat pancreatic islets were investigated. Methylamine accumulated in islet cells, was incorporated into endogenous islet proteins, and inhibited the incorporation of [2,5-³H] histamine into either N,N-dimethylcasein or endogenous islet proteins. Methylamine (2 mM ) did not affect the oxidation of glucose or endogenous nutrients or the intracellular pH in islet cells. Glucose did not affect the activity of transglutaminase in islet homogenates, the uptake of ¹⁴C-methylamine by intact islets or its incorporation into endogenous islet proteins. Methylamine inhibited insulin release evoked by glucose, other nutrient secretagogues, and non-nutrient insulinotropic agents such as L -arginine or gliclazide. The inhibitory effect of methylamine upon insulin release was diminished in the presence of cytochalasin B or at low extracellular pH. Methylamine retarded the conversion of proinsulin to insulin. Trimethylamine (0.7 mM ) was more efficiently taken up by islet cells than methylamine (2.0 mM ), and yet caused only a modest inhibition of insulin release. These findings suggest that methylamine interferes with a late step in the secretory sequence, possibly by inhibiting the access of secretory granules to their exocytotic site.     

Enhancing the incorporation of lysine derivatives into proteins with methylester forms of unnatural amino acids

We have improved the incorporation of l- and d-forms of unnatural amino acid (UAA) N^ε-thiaprolyl-l-lysine (ThzK) into ubiquitin (UB) and green fluorescent protein (GFP) by 2–6 folds with the use of the methylester forms of the UAAs in E coli cell culture. We also improved the yields of UAA-incorporated UB and GFP with the methylester forms of N^ε-Boc-l-Lysine (BocK) and N^ε-propargyl-l-Lysine (PrK) by 2–5 folds compared to their free acid forms. Our work demonstrated that using methylester-capped UAAs for protein expression is a useful strategy to enhance the yields of UAA-incorporated proteins.     

Gelation of Casein and Soybean Globulins by Transglutaminase

Guinea pig liver transglutaminase is a Ca²⁺ dependent enzyme which catalyzes the formation of inter- and intramolecular ε-(γ-glutamyl)lysyl cross-links between protein molecules. We have found that solutions of several proteins (α_s1-casein, and soybean 11S and 7S globulins) were gelatinized firmly by transglutaminase. The gel formation depended on the protein concentration. In the case of α_s1-casein, a reaction mixture containing below 2% was incapable of gelation. However, above 3%, a firm gel was formed by transglutaminase. As to soybean 11S and 7S globulins, reaction mixtures containing below 5% did not form gels, while, above 8%, firm gels were formed. The protein solutions in the presence of EDTA, an inhibitor of transglutaminase, were not gelatinized on treatment with transglutaminase. Thus, transglutaminase and a higher concentration of a substrate protein are indispensable for firm gel formation. It is supposed that the protein gels are formed through covalent bonds with transglutaminase.     

Involvement of transglutaminase in the formation of covalent cross-links in the cell wall ofCandida albicans

Activity of the enzyme glutaminyl-peptide-γ-glutamylyl-transferase (EC 2.3.2.13; transglutaminase), which forms the interpeptidic cross-link N^∈-(γ-glutamic)-lysine, was demonstrated in cell-free extracts obtained from both the yeast like and mycelial forms ofCandida albicans. Higher levels of enzymatic activity were observed in the cell wall fraction, whereas the cytosol contained only trace amounts of activity. Cystamine, a highly specific inhibitor of the enzyme, was used to analyze a possible role of transglutaminase in the organization of the cell wall structure of the fungus. Cystamine delayed protoplast regeneration and inhibited the yeast-to-mycelium transition and the incorporation of proteins into the cell wall. The incorporation of covalently bound high-molecular-weight proteins into the wall was sensitive to cystamine. Proteic epitopes recognized by two monoclonal antibodies, one of which is specific for the mycelial walls of the fungus, were also sensitive to cystamine. These data suggest that transglutaminase may be involved in the formation of covalent bonds between different cell wall proteins during the final assembly of the mature cell wall.     

Involvement of a Novel Class C Beta-Lactamase in the Transglutaminase Mediated Cross-Linking Cascade of Streptomyces mobaraensis DSM 40847

Streptomyces mobaraensis DSM 40847 secretes transglutaminase that cross-links proteins via γ-glutamyl-ε-lysine isopeptide bonds. Characterized substrates are inhibitory proteins acting against various serine, cysteine and metalloproteases. In the present study, the bacterial secretome was examined to uncover additional transglutaminase substrates. Fractional ethanol precipitation of the exported proteins at various times of culture growth, electrophoresis of the precipitated proteins, and sequencing of a 39 kDa protein by mass spectrometry revealed the novel beta-lactamase Sml-1. As indicated by biotinylated probes, Sml-1, produced in E. coli, exhibits glutamine and lysine residues accessible for transglutaminase. The chromogenic cephalosporin analogue, nitrocefin, was hydrolyzed by Sml-1 with low velocity. The obtained K_m and k_cat values of the recombinant enzyme were 94.3±1.8 μM and 0.39±0.03 s^-1, respectively. Penicillin G and ampicillin proved to be weak inhibitors of nitrocefin hydrolysis (K_i of 0.1 mM and 0.18 mM). Negligible influence of metals on β-lactamase activity ruled out that Sml-1 is a Zn²⁺-dependent class B beta-lactamase. Rather, sequence motifs such as SITK, YSN, and HDG forming the active core in a hypothetical structure may be typical for class C beta-lactamases. Based on the results, we assume that the novel transglutaminase substrate ensures undisturbed growth of aerial hyphae in Streptomyces mobaraensis by trapping and inactivating hostile beta-lactam antibiotics.     

Transglutaminase-catalyzed modification of cytoskeletal proteins by polyamines during the germination of Malus domestica pollen

 We investigated polyamine linkage to different structural proteins in pollen of Malus domestica Borkh. cv Red Chief at different phases of germination. This linkage has the characteristics of covalent linkages, indeed, it could be catalyzed by transglutaminase (TGase; EC 2.3.2.13). This assumption is supported by: (1) formation of a labelled TCA pellet and selective labelling of endogenous proteins by covalent binding of radioactive polyamines and (2) cross-reactivity of two different polyclonal antibodies against mammalian TGases; western blot analysis allowed us to detect a protein of about 80 kDa in both rehydrated ungerminated and germinated pollen. TGase activity was high at 90 min after germination and was influenced by Ca²⁺ supply only in the rehydrated ungerminated pollen. Extraction by Triton X-100 suggests that pollen TGase was at least partially membrane-bound. The enzyme catalyzed the incorporation of polyamines mainly into proteins having a molecular mass of 43 kDa and 52–58 kDa in both ungerminated and germinated pollen. These bands matched immunolabelled spots identified by mouse monoclonal anti-actin and anti-α-tubulin antibodies. Supplying exogenous actin and tubulin in a cell-free extract of rehydrated ungerminated and germinated pollen enhanced the activity. Autoradiography of the SDS-PAGE of these samples clearly showed that both actin and tubulin were substrates of TGase. Thus, the pollen TGase may be involved in the rapid cytoskeletal rearrangement which takes place during rehydration of ungerminated pollen and organization and growth of pollen tubes. Received: 9 August 1996 / Revision accepted: 26 October 1996     

On coupling bovine fibrinogen to the surface of malignant murine plasma cells by means of transglutaminase.

It was found that native, as well as¹²⁵I labelled fibrinogen, may be coupled by means of transglutaminase to surface proteins of malignant plasma cells of the mouse. Binding of fibrinogen, although greatly affecting the [¹⁴C] putrescine labelling of several surface proteins, leaves the cells viable and malignant.     

Factors affecting transglutaminase activity catalysing polyamine conjugation to endogenous substrates in the entire chloroplast

The chemical, physical and biological factors, which can affect the activity of transglutaminase (R-glutaminyl-peptide:amine γ-glutamyl-transferase, EC 2.3.2.13) in chloroplasts (ChlTGase) when photosynthesis is active, were assayed in chloroplasts isolated from the leaves of Helianthus tuberosus. Chloroplasts were incubated with putrescine (PU) in the presence of light to monitor the transglutaminase-catalysed incorporation of this polyamine into endogenous proteins. The enzyme was identified using a monoclonal antibody raised against the active site sequence of TGase K and was found to contain a thiol group, which can be slightly activated by Ca²⁺ and severely inhibited by EGTA. Mg²⁺ had a slight inhibitory effect. The enzymic activity, monitored by the isolation of glutamyl-putrescine, while already detectable above pH 7 was found to increase sharply from pH 8.0 to 9.5, with an optimal temperature of 45 °C. A hyperbolic curve was observed when the activity was measured as a function of the putrescine concentration, the apparent K_m being 1 mM. A biphasic relationship was obtained between the TGase activity and the concentration of the substrate (endogenous proteins) as well as the time of assay. The reaction products of the TGase assay, carried out at three pH values, were analysed for the presence of γ-glutamyl-putrescine; mono- and bis-derivatives were detected, showing that most of the modifications of Chl proteins are catalysed by the enzyme. Due to the stimulatory effect that proteases have on some animal TGases, protease inhibitors were also tested and found to reduce the post-translational modification of the substrates.     

A role for polyamines in stimulus-secretion coupling in the pancreatic β-cell

Measurement of the content of polyamines in pancreatic islets indicated that no significant change in their concentration took place during glucose-stimulated insulin release. The finding, together with the absence of any effect of -difluoromethylornithine on glucosestimulated insulin release suggested that rapid synthesis of polyamines is not involved in stimulus-secretion coupling in the -cell. The concentration of polyamines found in islets were high enough for them to act as substrates for the Ca²⁺-dependent islet transglutaminase during insulin release. This was further demonstrated by the ability of islet transglutaminase to incorporate [¹⁴C]putrescine into proteins from islet homogenates and by the demonstration of an increase in the covalent incorporation of [¹⁴C]putrescine into the proteins of intact islets following their challenge with glucose. Unlike monoamine substrates of transglutaminase, putrescine failed to effectively inhibit insulin release when its intracellular concentration was increased. A role for polyamines in the secretory process through their incorporation into islet proteins is suggested.     

Human mononuclear leukocyte transglutaminase activity is enhanced by streptococcal erythrogenic toxin and a staphylococcal mitogenic factor associated with toxic shock syndrome

Transglutaminase activity in human peripheral lymphocytes is enhanced after incubation of the cells with concanavalin A. Streptococcal proliferative factor toxin (erythrogenic toxin) from Streptococcus pyogenes and Toxic shock syndrome toxin from Staphylococcus aureus were purified and tested for their ability to enhance transglutaminase activity. Mononuclear leukocyte transglutaminase activity was enhanced 3–5-fold 30 min after incubation with either toxin. Enhancement occurred only when toxin was incubated with intact cells; addition of toxin to cell lysates was without effect. Transglutaminase was not measurable extracellularly. Histamine and dansyl cadaverine, competitive substrates for transglutaminase, inhibited [³H]putrescine incoporation into casein and [³H]thymidine incorporation into DNA. Incubation of lymphocytes with cycloheximide and either toxin or concanavalin A did not inhibit enzyme activity. These bacterial toxins, like phytomitogens, may perturb the cellular membrane and mediate their effect by transglutaminase-mediated cross-linking of membrane proteins.     

In vivo acylation of Dictyostelium actin with palmitic acid

Cells of Dictyostelium discoideum were incubated with [³H]palmitic acid during development, and recovery of the fatty acid label in soluble and membrane-associated proteins was investigated. One of the major labeled proteins was found exclusively in the soluble fraction. This protein, with an apparent mol. wt. of 44 kd, was identified as actin based on its labeling with a monoclonal anti-actin antibody, its coincidence with the major [³⁵S]methionine-labeled protein after two-dimensional electrophoresis and its binding to a DNase I affinity column. The ³H-label was resistant to chloroform-methanol extraction and boiling in SDS-containing buffer. After partial purification by preparative SDS-polyacrylamide gel electrophoresis, the 44-kd protein was treated with KOH, the fatty acids released were derivatized to methyl esters and palmitic acid methylester was identified by gas-liquid chromatography.     

Isolation of a New Exopolygalacturonase Producing Digalacturonic Acid from Pectic Acid

Transglutaminases catalyze the cross-linking and amine incorporation of proteins, and are implicated in various biological phenomena such as blood clotting, wound healing, apoptosis, and cell differentiation. Streptomyces lavendulae Y-200, isolated from soil, produced a substance that inhibited transglutaminases. The inhibitory substance was purified from the cultured medium by procedures of acid precipitation, deoxyribonuclease treatment, and gel filtration chromatography. The partially purified sample was dark brown. The inhibitory activity was stable under acidic, alkaline, and high temperature conditions, and resistant to the treatment with proteinases such as trypsin and Pronase. The molecular weight of the inhibitory substance was estimated to be between 10⁴ and 10⁵ from its permeability through ultrafilter membranes. The acid hydrolysate of the inhibitory substance contained amino acids and sugars. The inhibitory substance inhibited both calcium-dependent and calcium-independent transglutaminases in a competitive manner with a glutamine substrate. The extent of inhibition caused by the calcium-dependent transglutaminase increased with increasing calcium concentration. The results obtained here may help identify a novel regulatory substance of transglutaminase in biological systems.     

Isopeptide bond formation in epidermis

Summary Proteins which are major substrates of epidermal transglutaminases can be identified in cultured keratinocytes of human, cow, and new-born rat.Cow and human keratinocytes both contain substrate proteins which are 30000 to 50000 daltons in size but dissociable in SDS to 12000 daltons or less. In both species these proteins correspond to in vivo synthesized proteins which are probable precursors of cornified envelope. Human keratinocytes synthesize a 125000 dalton protein which is also a precursor of cornified envelope both in cells and tissue. By SDS electrophoresis two 100000 dalton substrate proteins are seen in cow keratinocyte extracts and a 23000 dalton substrate protein is seen in rat keratinocyte extracts. Minor substrates of transglutaminase are seen in human keratinocytes, and one has been isolated by preparative electrophoresis. Major structural proteins of epidermis which are in vitro substrates of epidermal transglutaminase include the keratins and the stratum corneum basic protein.     

Transglutaminase reactions associated with the rat semen clotting system: modulation by macromolecular polyanions.

The coagulation of rodent semen after ejaculation involves the establishment of ?-(γ-glutamyl)lysyl cross linkages between seminal vesicle secretion proteins as catalyzed by Ca⁺⁺-dependent transglutaminases secreted by the coagulating (anterior prostate) gland. During enzymic clotting of rat vesicular secretion proteins, low molecular weight amines such as putrescine are incorporated into covalent linkage with proteins of both the coagulum and the clot liquor. Bulbourethral gland secretions and certain macromolecular polyanions (notably poly-L-glutamate) enhance the enzymic coagulation of rat vesicular secretion proteins and putrescine incorporation therein. The stimulatory macromolecular polyanions appear to exert their effects by facilitating the ability of vesicular secretion proteins to serve as transglutaminase amine acceptor substrates.     

Increased Susceptibility to Transglutaminase of Eye Lens Proteins Exposed to Activated Oxygen Species Produced in the Glucose-Glucose Oxidase Reaction

In order to test whether a mild oxidative stress could promote the transglutaminase damaging effect on eye lens proteins, total lens soluble proteins and purified β_L-crystallin have been exposed to H₂O₂: slowly produced by the glucose-glucose oxidase reaction. Soon after the pretreatment, the substrate capacity of the lens proteins for an exogenous transglutaminase has been evaluated. Exposure to the oxidative stress increased the susceptibility of the lens proteins to transglutaminase. When ferrous ions were added to the preincubation medium, in order to convert the H₂O₂ into the hydroxyl radical, the increase was more evident.     

Formation of Mutagens by Heating Creatine or Amino Acids with Addition or Fatty Acids

α_s1-Casein was deamidated specifically in the moiety of the glutaminyl side chains by transglutaminase. The deamidation was achieved by reversibly blocking the amino groups in substrate protein. The deamidated product was found out to have 80% (11 residues) of the total deamidated glutaminyl residues. No significant conformational changes were observed even after the modifications. Consequently, the solubility in acidic pH regions, especially at pH 5, was increased. On the other hand, the Ca²⁺-sensitivity largely decreased to the extent that nearly all the deamidated product was soluble at 20 mm CaCl₂. Deamidation by transglutaminase may be useful as a new method to solubilize insoluble proteins without destroying protein structures.     

Semisynthesis of Arg15, Glu15, Met15, and Nle15-aprotinin involving enzymatic peptide bond resynthesis

The semisynthesis of homologues of aprotinin, the bovine pancreatic trypsin inhibitor, is described. The P₁ lysine¹⁵ residue was replaced by two methods. The first procedure, which consisted of two enzymatic steps for the incorporation of other amino acids has previously been described. The second approach consisted of six steps of both enzymatic and chemical nature. The modified inhibitor, in which the lysine¹⁵-alanine¹⁶ peptide bond is hydrolyzed, was used as the starting material. All carboxyl groups of the modified inhibitor were esterified with methanol; the lysine¹⁵ methylester group was then selectively hydrolyzed. Afterward, lysine¹⁵ itself was split off. Arginine, glutamic acid, methionine, andl-2-aminohexanoic acid (norleucine, Nle) were incorporated using water-soluble carbodiimide combined with an acylation catalyst. The methylester group was used to prevent polymerization. The reactive-site peptide bonds were resynthesized using either chymotrypsin or trypsin.