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

Mass Spectrometric Identification of a Novel Factor XIIIa Cross-Linking Site in Fibrinogen

Transglutaminases are a class of enzymes that catalyze the formation of a protein:protein cross-link between a lysine and a glutamine residue. These cross-links play important roles in diverse biological processes. Analysis of cross-linking sites in target proteins is required to elucidate their molecular action on target protein function and the molecular specificity of different transglutaminase isozymes. Mass-spectrometry using settings designed for linear peptide analysis and software designed for the analysis of disulfide bridges and chemical cross-links have previously been employed to identify transglutaminase cross-linking sites in proteins. As no control peptide with which to assess and improve the mass spectrometric analysis of TG cross-linked proteins was available, we developed a method for the enzymatic synthesis of a well-defined transglutaminase cross-linked peptide pair that mimics a predicted tryptic digestion product of collagen I. We then used this model peptide to determine optimal score thresholds for correct peptide identification from y- and b-ion series of fragments produced by collision-induced dissociation. We employed these settings in an analysis of fibrinogen cross-linked by the transglutaminase Factor XIIIa. This approach resulted in identification of a novel cross-linked peptide in the gamma subunit. We discuss the difference in behavior of ions derived from different cross-linked peptide sequences and the consequent demand for a more tailored mass spectrometry approach for cross-linked peptide identification compared to that routinely used for linear peptide analysis.

A recent update on the use of microbial transglutaminase for the generation of biotherapeutics

The recent scientific progresses on the use of enzyme-mediated reactions in organic, non-aqueous and aqueous media have significantly supported the growing demand of new biotechnological and/or pharmacological products. Today, a plethora of microbial enzymes, used as biocatalysts, are available. Among these, microbial transglutaminases (MTGs) are broadly used for their ability to catalyse the formation of an isopeptide bond between the γ-amide group of glutamines and the ε-amino group of lysine. Due to their promiscuity towards primary amine-containing substrates and the more stringent specificity for glutamine-containing peptide sequences, several combined approaches can be tailored for different settings, making MTGs very attractive catalysts for generating protein-protein and protein small molecule's conjugates. The present review offers a recent update on the modifications attainable by MTG-catalysed bioreactions as reported between 2014 and 2019. In particular, we present a detailed and comparative overview on the MTG-based methods for proteins and antibodies engineering, with a particular outlook on the synthesis of homogeneous antibody-drug conjugates.

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.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

NEW INSIGHTS INTO STRUCTURAL CHANGES OF LENS PROTEINS

X-ray scattering techniques were used to study the effects of heating on whole eye lens and α-crystallin gels. The temperature range used was from 20 to 70°C. The position of single X-ray reflection seen in whole lens was unchanged in the temperature range 20 to 45°C, with a continuous spacing of 152 Å. However, at 50°C the spacing increased from 152 Å to 165 Å. An interpretation of these results is that in eye lens, α-crystallin is protecting other lens proteins from super-aggregation up to 50°C. In α-crystallin gels a moderate increase in both the spacing and intensity of the reflection was observed from 20 to 45°C, followed by a dramatic increase from 45 to 70°C. Over the whole temperature range the spacing changed from 138 Å at 70°C to 195 Å at 70°. After eleven hours of cooling, this effect was found to be irreversible.

Novel MRI and fluorescent probes responsive to the Factor XIII transglutaminase activity

Transglutaminases, including factor XIII and tissue transglutaminase, participate in multiple extracellular processes associated with remodeling of the extracellular matrix during wound repair, blood clotting, tumor progression and fibrosis of ischemic injuries. The aim of this work was to evaluate a novel substrate analog for transglutaminase optimized by molecular modeling calculations (DCCP16), which can serve for molecular imaging of transglutaminase activity by magnetic resonance imaging and by near-infrared imaging. Experimental data showed covalent binding of Gd-DCCP16 and DCCP16-IRIS Blue to human clots, to basement membrane components and to casein in purified systems as well as in three-dimensional multicellular spheroids. In vivo, DCCP16 showed enhancement with a prolonged retention in clots and tumors, demonstrating the ability to detect both factor XIII and tissue transglutaminase mediated covalent binding of the contrast material.

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.

Heparan sulfate and transglutaminase activity are required for the formation of covalently cross-linked hedgehog oligomers

Sonic hedgehog (Shh) signaling plays major roles in embryonic development and has also been associated with the progression of certain cancers. Here, Shh family members act directly as long range morphogens, and their ability to do so has been linked to the formation of freely diffusible multimers from the lipidated, cell-tethered monomer (ShhNp). In this work we demonstrate that the multimeric morphogen secreted from endogenous sources, such as mouse embryos and primary chick chondrocytes, consists of oligomeric substructures that are "undisruptable" by boiling, denaturants, and reducing agents. Undisruptable (UD) morphogen oligomers vary in molecular weight and possess elevated biological activity if compared with recombinant Sonic hedgehog (ShhN). However, ShhN can also undergo UD oligomerization via a heparan sulfate (HS)-dependent mechanism in vitro, and HS isolated from different sources differs in its ability to mediate UD oligomer formation. Moreover, site-directed mutagenesis of conserved ShhN glutamine residues abolishes UD oligomerization, and inhibitors directed against transglutaminase (TG) activity strongly decrease the amount of chondrocyte-secreted UD oligomers. These findings reveal an unsuspected ability of the N-terminal hedgehog (Hh) signaling domain to form biologically active, covalently cross-linked oligomers and a novel HS function in this TG-catalyzed process. We suggest that in hypertrophic chondrocytes, HS-assisted, TG-mediated Hh oligomerization modulates signaling via enhanced protein signaling activity.

Substrate preference of transglutaminase 2 revealed by logistic regression analysis and intrinsic disorder examination

Tissue transglutaminase (TG2) catalyzes the Ca(2+)-dependent posttranslational modification of proteins via formation of isopeptide bonds between their glutamine and lysine residues. Although substrate specificity of TG2 has been studied repeatedly at the sequence level, no clear consensus sequences have been determined so far. With the use of the extensive structural information on TG2 substrate proteins listed in TRANSDAB Wiki database, a slight preference of TG2 for glutamine and lysine residues situated in turns could be observed. When the spatial environment of the favored glutamine and lysine residues was analyzed with logistic regression, the presence of specific amino acid patterns was identified. By using the occurrence of the predictor amino acids as selection criteria, several polypeptides were predicted and later identified as novel in vitro substrates for TG2. By studying the sequence of TG2 substrate proteins lacking available crystal structure, the strong favorable influence on substrate selection of the presence of substrate glutamine and lysine residues in intrinsically disordered regions could also be revealed. The collected structural data have provided novel understanding of how this versatile enzyme selects its substrates in various cell compartments and tissues.

Cloning and characterization of hemolymph clottable proteins of kuruma prawn (Marsupenaeus japonicus) and white shrimp (Litopenaeus vannamei)

The hemolymph clottable protein (CP) of Marsupenaeus japonica (designated as Mj-CP) was purified by a DEAE anion-exchanger and a Sepharose CL-6B gel filtration column. In the presence of Ca(2+), it formed stable clots in vitro upon the addition of the hemocytes lysate containing transglutaminase. Results of gel filtration chromatography and SDS-PAGE indicated that Mj-CP mainly existed as disulfide-linked homodimers of 390 kDa. Specific primers were designed; PCR as well as RACE help to clone and sequence Mj-CP cDNA of 5660 bp. The predicted CP-precursor contains a signal peptide followed by a subunit of 1671 amino acids (isoelectric point 5.6), including two RGD motifs and three potential N-glycosylation sites. Mj-CP is structurally 80% and 38% identical to the CPs of tiger shrimp and crayfish, respectively. Likewise, CP cDNA of white shrimp (Litopenaeus vannamei) was also cloned and sequenced; the predicted CP has 1666 amino acid residues and an isoelectric point of 5.2. Both CPs bear potential transglutaminase cross-linking sites, i.e. seven Ser-Lys-Thr repeats near the N-terminus, a Ser- and Gln-rich region in the middle, and polyGln (n=8-11) near the C-terminus. Phylogenetic analyses of crustacean CPs and vitellogenins revealed divergent evolution of the two protein families. By RT-PCR, the sub-cuticular epidermis was identified as one of the major tissues that express CP in M. japonica.

Facile coupling of synthetic peptides and peptide-polymer conjugates to cartilage via transglutaminase enzyme

Covalent attachment of synthetic and biological molecules to tissue surfaces can be used to enhance local drug delivery, reduce adhesions after surgery, and attach reconstructive biomaterials and tissue-engineered matrices to tissues. We present here a mild approach to coupling polymers to tissue surfaces through an enzyme catalyzed reaction between peptide modified polymer and native protein components of the tissue extracellular matrix (ECM). Tissue transglutaminase (tTG), a Ca2+-dependent enzyme that catalyzes the reaction between lysine and glutamine residues to form a epsilon(gamma-glutaminyl) lysine isopeptide bond, was incubated with cartilage in the presence of lysine (FKG-NH2) and glutamine (GQQQLG-NH2) peptides as well as peptide functionalized poly(ethylene glycol) (PEG). Immunohistochemistry was used to detect the presence of covalently bound PEG polymer at the tissue surface as well as to a depth of as much as 10 microm below the surface. Collagen II, fibronectin, osteopontin and osteonectin were found to react with the peptides and peptide modified PEG in the presence of tTG in solution, suggesting these cartilage ECM components as being substrates in the tissue reaction. The results illustrate the use of tTG as a simple, effective and biologically compatible method of coupling synthetic and biological molecules to cartilage and other tissues containing ECM proteins that are substrates of tTG.

Method for screening and MALDI-TOF MS sequencing of encoded combinatorial libraries

We describe a new method for encoded synthesis, efficient on-resin screening, and rapid unambiguous sequencing of combinatorial peptide libraries. An improved binary tag system for encoding peptide libraries during synthesis was designed to facilitate unequivocal assignment of isobaric residues by MALDI-TOF MS analysis. The improved method for encoded library synthesis was combined with a new versatile on-resin screening strategy that permitted multiple stages and types of screening to be employed successively on one library under mild conditions. The new method facilitated a combinatorial study of transglutaminase (TGase) enzyme substrate peptides, revealing new details of the effect of amino acid composition on TGase substrates. The approach was first demonstrated for an encoded library (130,321 compounds) of lysine pentapeptide substrates of TGase, synthesized using the "split-mix" method. The library was reacted on-resin with TGase enzyme and a soluble desthiobiotin labeled glutamine substrate. Initial screening was performed by adsorbing streptavidin-coated magnetic microparticles onto library beads, followed by magnetic separation. The differential binding affinities of desthiobiotin and biotin for streptavidin were exploited to release the magnetic microparticles and regenerate the desthiobiotin-labeled resin beads for further screening by flow-cytometry-based automated bead sorting, resulting in 345 beads that were sequenced by MALDI-TOF MS analysis. A second library consisted of encoded glutamine hexapeptide substrates, which was reacted on-resin with TGase enzyme and a soluble desthiobiotin-labeled cadaverine. Two-stage screening identified 267 glutamine peptides as TGase-reactive, of which 21 were further analyzed by solution-phase enzyme kinetics. Kinetic results indicated that the peptide PQQQYV from the library has a 68-fold greater substrate specificity than the best known glutamine substrate QQIV. The new encoding and screening strategies described here are expected to be broadly applicable to synthesis and screening of combinatorial peptide libraries in the future.

Mammalian transglutaminases. Identification of substrates as a key to physiological function and physiopathological relevance

Transglutaminases form a large family of intracellular and extracellular enzymes that catalyse the Ca2+-dependent post-translational modification of proteins. Despite significant advances in our understanding of the biological role of most mammalian transglutaminase isoforms, recent findings suggest new scenarios, most notably for the ubiquitous tissue transglutaminase. It is becoming apparent that some transglutaminases, normally expressed at low levels in many tissue types, are activated and/or overexpressed in a variety of diseases, thereby resulting in enhanced concentrations of cross-linked proteins. As applies to all enzymes that exert their metabolic function by modifying the properties of target proteins, the identification and characterization of the modified proteins will cast light on the functions of transglutaminases and their involvement in human diseases. In this paper we review data on the properties of mammalian transglutaminases, particularly as regards their protein substrates and the relevance of transglutaminase-catalysed reactions in physiological and disease conditions.

Rational design of transglutaminase substrate peptides for rapid enzymatic formation of hydrogels

Short peptide substrates with high specificity toward transglutaminase (TGase) enzyme were designed, characterized, and coupled to a biocompatible polymer, allowing for rapid enzymatic cross-linking of peptide-polymer conjugates into hydrogels. Eight acyl acceptor Lys-peptide substrates and three acyl donor Gln-peptide substrates were rationally designed and synthesized. The kinetic constants of these peptides toward tissue transglutaminase were measured by enzyme assay using RP-HPLC analysis with the aid of LC-ESI/MS. Several acyl donor and acyl acceptor peptides with high specificities toward TGase were identified, including a few containing the unusual amino acid l-3,4-dihydroxylphenylalanine (DOPA), which is found in the adhesive proteins secreted by marine and freshwater mussels. Acyl donor and acyl acceptor peptides with high substrate specificities were separately coupled to branched poly(ethylene glycol) (PEG) polymer molecules. Equimolar solutions of these polymer-peptide conjugates rapidly formed hydrogels in less than 2 min in the presence of transglutaminase under physiological conditions. The use of biocompatible building blocks, their rapid solidification from a liquid precursor under physiologic conditions, and the ability to incorporate adhesive amino acid residues using biologically benign enzymatic cross-linking are advantageous properties for the use of such materials for tissue repair, drug delivery, and tissue engineering applications.

Structural characterization of transglutaminase-catalyzed cross-linking between glyceraldehyde 3-phosphate dehydrogenase and polyglutamine repeats

The accumulation of abnormal polyglutamine-containing protein aggregates within the cytosol and nuclei of affected neurons is a hallmark of the progressive neurodegenerative disorders caused by an elongated (CAG)(n) repeat in the genome. The polyglutamine domains are excellent substrates for the enzyme transglutaminase type 2 (tissue), resulting in the formation of cross-links with polypeptides containing lysyl groups. Enzymatic activity toward the Q(n) domains increases greatly upon lengthening of such Q(n) stretches (n > 40). Among the possible amine donors, the glycolytic enzyme glyceraldehyde-3-phosphate-dehydrogenase was shown to tightly bind several proteins involved in polyglutamine expansion diseases. Recently, the authors have shown that K191, K268, and K331, out of the 26 lysines present in glyceraldehyde-3-phosphate-dehydrogenase, are the reactive amine-donor sites forming cross-links with substance P, which bears the simplest Q(n) domain (n = 2). The present study reports that synthetic peptides of both pathological and nonpathological length (n = 43 and 17, respectively) form cross-links with the same K residues located in the C-terminal region of glyceraldehyde-3-phosphate-dehydrogenase. In addition, it is shown that extra K residues present in the C termini of glyceraldehyde-3-phosphate-dehydrogenase are susceptible to cross-linking in the presence of transglutaminase. The present results indicate a possible modulating effect of Q(n) stretches on tissue transglutaminase substrate specificity and mechanism of recognition.

Identification of tissue transglutaminase-reactive lysine residues in glyceraldehyde-3-phosphate dehydrogenase

Polyglutamine domains are excellent substrates for tissue transglutaminase resulting in the formation of cross-links with polypeptides containing lysyl residues. This finding suggests that tissue transglutaminase may play a role in the pathology of neurodegenerative diseases associated with polyglutamine expansion. The glycolytic enzyme GAPDH previously was shown to tightly bind several proteins involved in such diseases. The present study confirms that GAPDH is an in vitro lysyl donor substrate of tissue transglutaminase. A dansylated glutamine-containing peptide was used as probe for labeling the amino-donor sites. SDS gel electrophoresis of a time-course reaction mixture revealed the presence of both fluorescent GAPDH monomers and high molecular weight polymers. Western blot analysis performed using antitransglutaminase antibodies reveals that tissue transglutaminase takes part in the formation of heteropolymers. The reactive amino-donor sites were identified using mass spectrometry. Here, we report that of the 26 lysines present in GAPDH, K191, K268, and K331 were the only amino-donor residues modified by tissue transglutaminase.

Thiols enhance the sensitivity of luminescent assays for non-cross-linked and covalently cross-linked aminophthalhydrazides

Transglutaminases catalyse an acyl-transfer reaction between the gamma-carboxamide of a protein or peptide-bound glutamine (P-CH2-gammaCH2-CO-1NH2) and the primary amino group of mono- or polyamines (R-2NH2), covalently cross-linking the reactants by an isopeptide bond: P-CH2-gammaCH2-CO-1NH2 + R-2NH2 --> P-CH2-gammaCH2-CO-2NH-R + 1NH3. We reported that N-(4-aminobutyl)-N-ethylisoluminol (ABEI) was a chemiluminescent (CL) amine substrate for transglutaminases. We now identified N-(6-aminohexyl)-N-ethylisoluminol (AHEI) as a second, less reactive, transglutaminase substrate. A structure-based explanation is offered for the lower reactivity of AHEI. Optimum CL from non-cross-linked or cross-linked ABEI or AHEI was elicited in the presence of 10 mmol/L dithiothreitol, by oxidizing with a mixture of 20 mmol/L potassium ferricyanide and 10 mmol/L hydrogen peroxide in 100 mmol/L NaOH. The limits of detection and quantitation for non-cross-linked aminophthalhydrazides obtained in this system were: 20 fmol and 60 fmol for ABEI and 10 fmol and 30 fmol for AHEI. These values represented a 500-800-fold improved sensitivity. Delayed peak CL and CL decay in the presence of dithiothreitol contributed to improving the sensitivity. The data could be useful for improving the immunoassays for aminophthalhydrazides and facilitate the development of high throughput assays for transglutaminases.

S100A7, S100A10, and S100A11 are transglutaminase substrates

S100 proteins are a family of 10-14 kDa EF-hand-containing calcium binding proteins that function to transmit calcium-dependent cell regulatory signals. S100 proteins have no intrinsic enzyme activity but bind in a calcium-dependent manner to target proteins to modulate target protein function. Transglutaminases are enzymes that catalyze the formation of covalent epsilon-(gamma-glutamyl)lysine bonds between protein-bound glutamine and lysine residues. In the present study we show that transglutaminase-dependent covalent modification is a property shared by several S100 proteins and that both type I and type II transglutaminases can modify S100 proteins. We further show that the reactive regions are at the solvent-exposed amino- and carboxyl-terminal ends of the protein, regions that specify S100 protein function. We suggest that transglutaminase-dependent modification is a general mechanism designed to regulate S100 protein function.

Molecular cloning and characterization of a hemolymph clottable protein from tiger shrimp (Penaeus monodon)

To investigate the coagulation system in crustacean decapoda, a homodimeric glycoprotein of 380 kDa was purified from the hemolymph of tiger shrimp (Penaeus monodon) by sequential DEAE anion exchange chromatography. The purified protein was coagulated by the shrimp hemocyte transglutaminase in the presence of Ca2+. The clottable protein contains 44% alpha helices and 26% beta sheets as determined by circular dichroism spectra. Its conformation is stable in buffer of pH 4-9. To solve its primary structure, partial sequences of the purified polypeptides from cyanogen bromide cleavage and endopeptidase digestion were also determined. A shrimp cDNA expression library was constructed. By combination with antibody screening, reverse transcriptase PCR using degenerate primers from determined amino acid sequences and cDNA library screening with digoxigenin-labeled DNA probes, the entire cDNA of 6124 bp was obtained. This cDNA encodes a protein of 1670 amino acids, including a 14-amino acid signal peptide. With four potential N-glycosylation sites, the clottable protein was found to contain 3.8% high-mannose glycan; and Man8GlcNAc and Man9GlcNAc were released upon endo-beta-N-acetylglucosaminidase hydrolysis. Upon conducting a protein sequence database survey, the shrimp clottable protein shows 36% identities to the crayfish clotting protein and lower similarities to members of insect vitellogenins, apolipoprotein B and mammalian von Willebrand factor. Notably, a region rich in Gln residues, a polyGln motif and five Ser-Lys-Thr-Ser repeats are present in the shrimp protein, suggesting this protein might be a transglutaminase substrate. Northern blot analysis revealed that the clottable protein is expressed in most of the shrimp tissues but not in the mature hemocytes.

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.

Analysis of factor XIII substrate specificity using recombinant human factor XIII and tissue transglutaminase chimeras

Human factor XIII (FXIII) and tissue transglutaminase (tTG) are homologous proteins. FXIII requires thrombin for activation and cross-links the gamma chains of fibrin(ogen) more efficiently than the Aalpha chains. On the other hand, tTG is thrombin-independent and forms predominantly Aalpha and Aalpha-gamma chain complexes. Previous work from this laboratory demonstrated that amino acid residues within exon 7 of FXIII were important for catalysis (Hettasch, J. M., and Greenberg, C. S. (1994) J. Biol. Chem. 269, 28309-28313). To determine to what extent the primary amino acid sequence within exon 7 defines substrate specificity, exon 7 of FXIII was replaced with the corresponding exon of tTG using gene splicing by overlap extension. Other work from this laboratory (Achyuthan, K. E., Slaughter, T. F., Santiago, M. A., Enghild, J. J., and Greenberg, C. S. (1993) J. Biol. Chem. 268, 21284-21292) using synthetic peptides identified two other domains that might play a role in substrate recognition (located in exons 3 and 5). Therefore, recombinant chimeras of FXIII/tTG were also created in which these two exons were exchanged. FXIII, tTG, and chimeras 3, 5, and 7 were expressed in Escherichia coli, purified, and the nature of the fibrin cross-linking pattern of these five proteins was determined by immunoblot analysis. FXIII preferentially formed the gamma-gamma dimer, whereas tTG formed Aalpha-gamma complexes. Chimera 7 formed Aalpha-gamma complexes that resembled the cross-linking pattern of tTG. This finding demonstrates that the primary amino acid sequence of exon 7 of tTG confers some of the specificity for the Aalpha and Aalpha-gamma cross-link pattern characteristic of tTG. Chimera 5 exhibited reduced cross-linking activity (50% of FXIII activity) but still retained preference for formation of the gamma-gamma dimer, whereas chimera 3 was not active. In conclusion, exchanging the primary amino acid sequence of the active site exon of human FXIII with that of human tTG modifies the enzyme such that the fibrin cross-linking pattern more closely resembles that of tTG (Aalpha and Aalpha-gamma complexes) instead of FXIII (gamma-gamma dimers).

Identification and sequence analysis of two new members of the SKALP/elafin and SPAI-2 gene family. Biochemical properties of the transglutaminase substrate motif and suggestions for a new nomenclature

The human epithelial proteinase inhibitor SKALP/elafin and the porcine sodium-potassium ATPase inhibitor SPAI-2 are two highly homologous proteins that share an NH2-terminal transglutaminase substrate domain and a COOH-terminal whey acidic protein (WAP) domain. Here we describe the bovine and simian orthologs of SKALP/elafin as well as two new bovine family members that are designated Trappin-4 and Trappin-5 on the basis of a new nomenclature that we propose (Trappin = TRansglutaminase substrate and WAP motif-containing ProteIN). Sequence analysis of Trappin-4 and Trappin-5 revealed a domain structure that is very similar to SPAI-2 (Trappin-1) and SKALP/elafin (Trappin-2). The transglutaminase substrate motifs are conserved although the number of repeats varies among species and among family members. The sequence of Trappin-4 and Trappin-5 diverges from Trappin-1 and Trappin-2 at the putative reactive site in the WAP domain. The bovine ortholog of Trappin-2 is expressed in tongue and snout epidermis; Trappin-4 is expressed in trachea, ileum, and tongue; and Trappin-5 is expressed at low levels in trachea, as determined by RNase protection and Northern blot analysis. Based on the analysis of 67 transglutaminase substrate repeats as present in all known Trappin gene family members from four different mammalian species a consensus sequence could be established: Gly-Gln-Asp-Pro-Val-Lys (GQDPVK). Using biotinylated hexapeptide probes we found that the GQDPVK sequence is a very efficient transglutaminase substrate both for guinea pig liver transglutaminase and for epidermal transglutaminase, and it acts as acyl donor as well as acceptor. We propose that the Trappin protein family forms a new group of enzyme inhibitors with various specificities of the WAP domain, which share transglutaminase substrate motifs that can act as an anchoring sequence.

Biochemical, structural, and transglutaminase substrate properties of human loricrin, the major epidermal cornified cell envelope protein

Loricrin is the major protein of the cornified cell envelope of terminally differentiated epidermal keratinocytes which functions as a physical barrier. In order to understand its properties and role in cornified cell envelope, we have expressed human loricrin from a full-length cDNA clone in bacteria and purified it to homogeneity. We have also isolated loricrin from newborn mouse epidermis. By circular dichroism and fluorescence spectroscopy, the in vivo mouse and bacterially expressed human loricrins possess no alpha or beta structure but have some organized structure in solution associated with their multiple tyrosines and can be reversibly denatured by either guanidine hydrochloride or temperature. The transglutaminase (TGase) 1, 2, and 3 enzymes expressed during epidermal differentiation utilized loricrin in vitro as a complete substrate, but the types of cross-linking were different. The TGase 3 reaction favored certain lysines and glutamines by forming mostly intrachain cross-links, whereas TGase 1 formed mostly large oligomeric complexes by interchain cross-links involving different lysines and glutamines. Together, the glutamines and lysines used in vitro are almost identical to those seen in vivo. The data support a hypothesis for the essential and complementary roles of both TGase 1 and TGase 3 in cross-linking of loricrin in vivo. Failure to cross-link loricrin by TGase 1 may explain the phenotype of lamellar ichthyosis, a disease caused by mutations in the TGase 1 gene.

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.

The proteins elafin, filaggrin, keratin intermediate filaments, loricrin, and small proline-rich proteins 1 and 2 are isodipeptide cross-linked components of the human epidermal cornified cell envelope

The cornified cell envelope (CE) is a 15-nm thick layer of insoluble protein deposited on the intracellular side of the cell membrane of terminally differentiated stratified squamous epithelia. The CE is thought to consist of a complex amalgam of proteins cross-linked by isodipeptide bonds formed by the action of transglutaminases, but little is known about how or in which order the several putative proteins are cross-linked together. In this paper, CEs purified from human foreskin epidermis were digested in two steps by proteinase K, which released as soluble peptides about 30% and then another 35% of CE protein mass, corresponding to approximately the outer third (cytoplasmic surface) and middle third, respectively. Following fractionation, 145 unique peptides containing two or more sequences cross-linked by isodipeptide bond(s) were sequenced. Based on these data, most (94% molar mass) of the outer third of CE structure consists of intra- and interchain cross-linked loricrin, admixed with SPR1 and SPR2 proteins as bridging cross-links between loricrin. Likewise, the middle third of CE structure consists largely of cross-linked loricrin and SPR proteins, but is mixed with the novel protein elafin which also forms cross-bridges between loricrin. In addition, cross-links involving loricrin and keratins 1, 2e, and 10 or filaggrin were recovered in both levels. The data establish for the first time that these several proteins are indeed cross-linked protein components of the CE structure. In addition, the data support a model for the intermediate to final stages of CE assembly: the proteins elafin, SPR1 and SPR2, and loricrin begin to be deposited on a preformed scaffold; later, elafin deposition decreases as loricrin and SPR accumulation continues to effect final assembly. The recovery of cross-links involving keratins further suggests that the subjacent cytoplasmic keratin intermediate filament-filaggrin network is anchored to the developing CE during these events.