Do-it-yourself histidine-tagged bovine enterokinase: a handy member of the protein engineer's toolbox

Enterokinase, a two-chain duodenal serine protease, activates trypsinogen by removing its N-terminal propeptide. Due to a clean cut after the non-primed site recognition sequence, the enterokinase light chain is frequently employed in biotechnology to separate N-terminal affinity tags from target proteins with authentic N-termini. In order to obtain large quantities of this protease, we adapted an in vitro folding protocol for a pentahistidine-tagged triple mutant of the bovine enterokinase light chain. The purified, highly active enzyme successfully processed recombinant target proteins, while the pentahistidine-tag facilitated post-cleavage removal. Hence, we conclude that producing enterokinase in one's own laboratory is an efficient alternative to the commercial enzyme.

Fusion expression and purification of four disulfide-rich peptides reveals enterokinase secondary cleavage sites in animal toxins

Animal toxins are powerful tools for testing the pharmacological, physiological, and structural characteristics of ion channels, proteases, and other receptors. However, most animal toxins are disulfide-rich peptides that are difficult to produce functionally. Here, a glutathione S-transferase (GST) fusion expression strategy was used to produce four recombinant animal toxin peptides, ChTX, StKTx23, BmP01, and ImKTx1, with different isoelectric points from 4.7 to 9.2. GST tags were removed by enterokinase, a widely used and effective commercial protease that cleaves after lysine at the cleavage site DDDDK. Using this strategy, two disulfide-rich animal toxins ChTX and StKTx23 were obtained successfully with a yield of approximately 1-2 mg/l culture. Electrophysiological experiments further showed that these two recombinant toxins showed good bioactivities, indicating that our method was effective in producing large amounts of functional disulfide-rich animal toxins. Interestingly, by analyzing the separated fractions of BmP01, StKTx23, and ImKTx1 using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, four new enterokinase secondary cleavage sites were found, consisting of the sequences "WEYR," "EDK," "QNAR," and "DNDK." To our knowledge, this is the first report of the presence of secondary cleavage sites for commercial enterokinase in animal toxins. These findings will help us use commercial enterokinase appropriately as a cleavage tool in the production of animal toxins.

[Designing of hybrid human interferon alfa-2 strain-producers and the use of enteropeptidase for obtaining N-terminal methionine-free interferons]

A system for production of human interferon-alpha2a (IFN-alpha2a) and IFN-alpha2b lacking N-terminal methionine has been developed. Plasmids containing genes of hybrid IFN-alpha2 under the control of different promoters were constructed; a sequence encoding the enteropeptidase hydrolysis site being introduced in proximal part of the genes. As the result, 4 strains of Escherichia coli producing hybrid IFN-alpha2 have been obtained. The methodology for IFN-alpha2 renaturation, hydrolysis of its N-terminal part, chromatographic purification of N-terminal methionine-free IFN-alpha2 has been developed.

Design and efficient production of bovine enterokinase light chain with higher specificity in E. coli

Enterokinase light chain (EKL) is a serine protease that recognizes Asp-Asp-Asp-Asp-Lys (D(4)K) sequence and cleaves the C-terminal peptide bond of the lysine residue. The utility of EKL as a site-specific cleavage enzyme is hampered by sporadic cleavage at other sites than the canonical D(4)K recognition sequence. In order to produce more site-specific EKL, we have generated several EKL mutants in E. coli with substitutions at Tyr174 and Lys99 using PDI (protein disulfide isomerase) fusion system. Substitution of Tyr174 by basic residues confers higher specificity on EKL. The production of EKL with higher specificity could widen the utility of EKL as a site-specific cleavage enzyme to produce various recombinant proteins with therapeutic or industrial values.

[Refolding of the fusion protein of recombinant enterokinase light chain rEKL]

The fusion protein of enterokinase light chain, DsbA-rEKL, was expressed mainly in inclusion body in E. coli. The recombinant bacteria was fermented to high density, with high expression of the fusion protein. After being washed with 0.5% Triton X-100 and 4mol/L urea, the inclusion body was dissolved in 6mol/L guanidine and 100mmol/L DTP, derivatized by cystine and refolded by pulse refolding. The strategy of pulse refolding involved the addition of 0.03mg/mL of fusion protein until its final concentration reached 0.3mg/mL. The refolded protein was autocleaved and the active EKL molecule was released after adding 2mmol/L CaCl2. Using the two-step purification processes of IDA-Sepharose chromatography and Q-Sepharose chromatography, the purity of rEKL was found to be above 95%, with a high activity to cleave the recombinant reteplase fusion protein Trx-rPA. The yield of purified rEKL was more than 60mg/L of cultures. As a result, the therapeutic proteins like rPA could be produced on a large-scale in a way such as expressed in the form of fusion proteins.

The trail of my studies on glycoproteins from enterokinase to tumor markers

This review describes the results of the author's studies on glycoproteins which have been carried out for more than 50 years. Starting from the elucidation of basic structures of glycoproteins, i.e. the structure of the linkage between an amino acid and a sugar and the occurrence of the beta-mannosidic linkage as the common structure of glycoproteins, the author became interested in the cell membrane glycoproteins focused on the comparison of cancer cells versus normal cells. These studies were then extended to the establishment of sugar-directed and cancer-associated monoclonal antibodies. Some of the monoclonal antibodies are useful for cancer diagnosis.

[Preparation of rhIL-11 from fusion protein by using enterokinase]

The study was aimed to investigate the technological parameters of rhIL-11 preparation from fusion protein by using enterokinase. Fusion expression vector pET32a/IL-11 was transferred into E.coli BL21 (DE3) pLysS and its expression was induced by IPTG, the lysis supernatant of the engineering strain was purified by Ni-NTA resin and then the target rhIL-11 was digested by auto-cleavaged DsbA-EKL-(His)(8). The results showed that after affinity chromatography, Trx-IL-11 was obtained with the yield of 11.25mg/g, the purity of 89.2% and the recovery of 91.8%. Finally the target rhIL-11 was digested and purified to over 95%. In conclusion, the preparation method of rhIL-11 from fusion protein by using enterokinase is simple and feasible with good separation, which can meet industrial requirements.

Hydrogen peroxide enhances enterokinase-catalysed proteolytic cleavage of fusion protein

The effects of hydrogen peroxide on enterokinase catalysis were studied using several fusion proteins recombinantly produced from E. coli. It was demonstrated that hydrogen peroxide enhanced the rate of enterokinase cleavage reaction, leading to a faster release of the target peptide as discussed in patent WO07149053. Among the conditions tested, we observed that hydrogen peroxide could exert its effect on the cleavage of fusion proteins over a wide range of pH and temperature. This finding might provide a simple solution for the accelerated enterokinase cleavage of thermolabile fusion proteins at low temperature.

Trypsin IV or Mesotrypsin and p23 Cleave Protease-activated Receptors 1 and 2 to Induce Inflammation and Hyperalgesia

Although principally produced by the pancreas to degrade dietary proteins in the intestine, trypsins are also expressed in the nervous system and in epithelial tissues, where they have diverse actions that could be mediated by protease-activated receptors (PARs). We examined the biological actions of human trypsin IV (or mesotrypsin) and rat p23, inhibitor-resistant forms of trypsin. The zymogens trypsinogen IV and pro-p23 were expressed in Escherichia coli and purified to apparent homogeneity. Enteropeptidase cleaved both zymogens, liberating active trypsin IV and p23, which were resistant to soybean trypsin inhibitor and aprotinin. Trypsin IV cleaved N-terminal fragments of PAR(1), PAR(2), and PAR(4) at sites that would expose the tethered ligand (PAR(1) = PAR(4) > PAR(2)). Trypsin IV increased [Ca(2+)](i) in transfected cells expressing human PAR(1) and PAR(2) with similar potencies (PAR(1), 0.5 microm; PAR(2), 0.6 microm). p23 also cleaved fragments of PAR(1) and PAR(2) and signaled to cells expressing these receptors. Trypsin IV and p23 increased [Ca(2+)](i) in rat dorsal root ganglion neurons that responded to capsaicin and which thus mediate neurogenic inflammation and nociception. Intraplantar injection of trypsin IV and p23 in mice induced edema and granulocyte infiltration, which were not observed in PAR (-/-)(1)(trypsin IV) and PAR (-/-)(2) (trypsin IV and p23) mice. Trypsin IV and p23 caused thermal hyperalgesia and mechanical allodynia and hyperalgesia in mice, and these effects were absent in PAR (-/-)(2) mice but maintained in PAR (-/-)(1) mice. Thus, trypsin IV and p23 are inhibitor-resistant trypsins that can cleave and activate PARs, causing PAR(1)- and PAR(2)-dependent inflammation and PAR(2)-dependent hyperalgesia.

Purification and refolding optimization of recombinant bovine enterokinase light chain overexpressed in Escherichia coli

The nucleotide sequence encoding bovine enterokinase light chain (EK) from Chinese northern yellow bovine was isolated. Two single-nucleotide mutations, namely, C245G and A528T were identified. The gene encoding the Pro82Arg/Glu176Asp variant of known bovine EK was fused with glutathione S-transferase and overexpressed mainly as an inclusion body in Escherichia coli BL21 (DE3), upon induction with IPTG and glucose. Effective fusion protein purification, refolding, auto-catalytic cleavage and mature EK recovery were described. The specific activity of the purified EK was determined as 110+/- 10 U/mg, which was comparable to a specific activity of > or =20 U/mg of the E. coli expressed EK sample provided by Sigma (Cat. No. E4906). This procedure produced approximately 53 mg of EK per 500 mL of cell culture, which was much higher than previous reports, thus providing a basis for large-scale production of EK and for further applications in biotechnology.

The Ways of Realization of High Specificity and Efficiency of Enteropeptidase

Comparative substrate analysis of full-length bovine enteropeptidase and trypsin, bovine and human enteropeptidase light chains was performed using model N-terminal dodecapeptides corresponding to wild-type human trypsinogen and pancreatitis-associated mutant trypsinogens K23R and D22G. The substitution of Lys residue by Arg at P1 leads to 2-fold increase in the efficiency of enteropeptidase hydrolysis; the absence of the negatively charged residue at P2 reduces the efficiency of such hydrolysis by two orders of magnitude. The difference in efficiency of peptide chain hydrolysis after Lys/Arg residues by enteropeptidase compared to trypsin is equal to the difference in hydrolysis by serine proteases of different primary specificity of their specific substrates.

An SRLLR motif downstream of the scissile bond enhances enterokinase cleavage efficiency

In a previous paper, we reported more efficient enterokinase cleavage at a C-terminal non-target LKGDR(201) site compared with an internally sited canonical recognition site, DDDDK(156). When this non-target site was placed internally to replace DDDDK(156) between the thioredoxin moiety and mouse NT-proCNP(1-50), this site was poorly processed leading us to conclude that efficient processing at LKGDR(201) in the first instance was due to its accessibility at the C-terminus of the fusion protein. Subsequently, we reasoned that treatment of thioredoxin-fused NT-proCNP(1-81) would allow us to retrieve full-length NT-proCNP(1-81) without undue processing at the LKGDR(201) site since this non-target site would now be located internally about 36 residues away from the C-terminus and hence not be hydrolyzed efficiently. Surprisingly, ESI-MS data showed that the LKGDR site in thioredoxin-fused human NT-proCNP(1-81) was still very efficiently cleaved and revealed a new but slow hydrolysis site with the sequence RVDTK/SRAAW to yield a peptide consistent with NT-proCNP(58-81). The evidence obtained from these experiments led us to postulate that efficient cleavage at the non-target LKGDR(201) site was not merely influenced by steric constraints but also by the sequence context downstream of the scissile bond. Hence, we constructed variants of thioredoxin-mouse NT-proCNP(1-50) where SRLLR residues (i.e. those immediately downstream from the LKGDR(201) site in NT-proCNP(1-50)) were systematically added one at a time downstream of the internal DDDDK(156) site. To evaluate the relative effects of site accessibility and downstream sequence context on the efficiency of enterokinase cleavage, we have also replaced the native LKGDR(201) sequence with DDDDK(201). Our results showed that incremental addition of SRLLR residues led to a steady increase in the rate of hydrolysis at DDDDK(156). Further variants comprising DDDDK(156)SS, DDDDK(156)SD and DDDDK(156)RR showed that the minimal critical determinants for enhanced enterokinase cleavage are serine in the P1' position followed by a serine or a basic residue, lysine or arginine, in the P2' position. Our data provided conclusive evidence that the influence of downstream sequences on recombinant light chain enterokinase activity was greater than accessibility of the target site at the terminus region of the protein. We further showed that the catalytic efficiency of the native holoenzyme was influenced primarily by residues on the N-terminal side of the scissile bond while being neutral to residues on the C-terminal side. Finally, we found that cleavage of all nine fusion proteins reflects accurate hydrolysis at the DDDDK(156) and DDDDK(201) sites when recombinant light chain enterokinase was used while non-specific processing at secondary sites were observed when these fusion proteins were treated with the native holoenzyme.

Compatible solutes as protectants for zymogens against proteolysis

Compatible solutes are small organic osmoprotectants that have the capability to stabilize proteins. In coupled assays, the effect of the solutes ectoine, hydroxyectoine and betaine on the activation of the zymogens trypsinogen and chymotrypsinogen, catalyzed by enteropeptidase and trypsin, respectively, was studied. To different extents, all solutes protected the zymogens against activation. Ectoine (800 mM) was the most potent solute in reducing the formation of trypsin to 4% of the control value and of chymotrypsin to 23%. In separate experiments, the ability of the solutes to preserve proteolytic activity during incubation was investigated. After 4 h, trypsin and chymotrypsin completely lost their activity, but in the presence of ectoine, approximately 50% residual activity was maintained. It is proposed that a conformational shift of the protein towards folded, native-like states induced by preferential exclusion of the solute is responsible for the stabilizing and chaperone-like effects.

Biochemical characterization of human enteropeptidase light chain

The synthetic gene encoding human enteropeptidase light chain (L-HEP) was cloned into plasmid pET-32a downstream from the gene of fusion partner thioredoxin immediately after the DNA sequence encoding the enteropeptidase recognition site. The fusion protein thioredoxin (Trx)/L-HEP was expressed in Escherichia coli BL21(DE3). Autocatalytic cleavage of the fusion protein and activation of recombinant L-HEP were achieved by solubilization of inclusion bodies and refolding of Trx/L-HEP fusion protein. The kinetic parameters of human and bovine enteropeptidases in the presence of different concentrations of Ca2+ and Na+ for cleavage of the specific substrate GD4K-na and nonspecific substrates such as small ester Z-Lys-SBzl and chromogenic substrates Z-Ala-X-Arg-pNA have been comparatively analyzed. It is demonstrated that positively charged ions increased the Michaelis constant (Km) for cleavage of specific substrate GD4K-na, while the catalytic constant (k(cat)) remained practically unchanged. L-HEP demonstrated secondary specificity to the chromogenic substrate Z-Ala-Phe-Arg-pNA with k(cat)/Km 260 mM(-1) x sec(-1). Enzymatic activity of L-HEP was suppressed by inhibitors of trypsin-like and cysteine (E-64), but not metallo-, amino-, or chymotrypsin-like proteinases. L-HEP was active over a broad range of pH (6-9) with optimum activity at pH 7.5, and it demonstrated high stability to different denaturing agents.

Tyrosine-based “Activatable Pro-Tag”: Enzyme-catalyzed protein capture and release

Protein recovery is often achieved by a series of capture and release steps that often involve chromatographic binding and elution. We report an alternative, non-chromatographic, capture and release approach that employs enzymes and the stimuli-responsive polysaccharide chitosan. We capture our protein using the enzyme tyrosinase that oxidizes accessible tyrosine residues of the protein and "activates" these residues for covalent capture (i.e., conjugation) onto chitosan. Using fusions of green fluorescent protein (GFP) we observed that: (i) enzymatic activation is required for protein capture to chitosan; and (ii) capture is enhanced (approximately five-fold) by engineering the protein to have a penta-tyrosine fusion tag that provides additional accessible tyrosine residues for enzymatic activation. Because the fusion tag appears to be the primary site for capture, and capture requires activation, we designate penta-tyrosine as a "pro-tag." The captured GFP-chitosan conjugate possesses the pH-responsive solubility that is characteristic of chitosan. We exploit this pH-responsive solubility to facilitate purification of the captured protein. Two enzymatic methods were explored to release the captured GFP from the chitosan conjugate. The first method employs enterokinase (EK) to cleave the protein at an engineered EK-cleavage site. The second method employs chitosanase to hydrolyze the chitosan backbone. Using GFP as a model protein, we demonstrated that enzymatic capture and release provides a simple, non-chromatographic means to recover proteins directly from cell lysates.

Preparation of recombinant thioredoxin fused N-terminal proCNP: Analysis of enterokinase cleavage products reveals new enterokinase cleavage sites

C-type natriuretic peptide (CNP) acts as a paracrine hormone to dilate blood vessels and is also required for the growth of long bones. In vivo, CNP is produced by cleavage from the C-terminal end of a larger proCNP peptide. The remaining N-terminal proCNP fragment (NT-proCNP) escapes into the circulation where its concentration is much higher than that of CNP due presumably to a lower clearance rate. Our strategy to obtain large quantities of pure NT-proCNP for further physiological investigations was to express it as a fusion protein with His(6)-tagged thioredoxin followed by cleavage using enterokinase to yield NT-proCNP alone. We have successfully designed and artificially synthesized the coding sequence specifying both mouse and human NT-proCNP with built-in codon bias towards Escherichia coli codon preference. An enterokinase recognition sequence was incorporated immediately upstream of the NT-proCNP coding sequence to allow the fusion protein to be cleaved without leaving any extra residues on the NT-proCNP peptide. High levels of fusion proteins were obtained, constituting 50-58% of total bacterial proteins. Greater than 90% of recombinant thioredoxin/NT-proCNP was expressed in the soluble form and purified to near homogeneity in a single chromatographic step using nickel as the metal ion in IMAC. A time course analysis of the products released from enterokinase cleavage of the recombinant proteins by ESI-MS revealed three sensitive secondary cleavage sites: two were located on vector-associated sequences linking the thioredoxin moiety and NT-proCNP, and one at the C-terminal end of NT-proCNP. Clearly, substrate specificity of both the native and recombinant forms of enterokinase for the recognition sequence DDDDK was by no means exclusive. Hydrolysis at the unexpected LKGDR site located towards the carboxyl end on NT-proCNP was significantly more efficient than at the internally sited DDDDK target sequence. However, when this same sequence was sited internally replacing the DDDDK in another construct of thioredoxin/mouse NT-proCNP, it was found to be poorly processed by enterokinase. Our results showed that non-target sequences can be preferentially recognized over the canonical DDDDK sequence when located accessibly at the ends of proteins.

Effect of Calcium Ions on Enteropeptidase Catalysis

The effects of calcium ions on hydrolysis of low molecular weight substrates catalyzed by different forms of enteropeptidase were studied. A method for determining activity of truncated enteropeptidase preparations lacking a secondary trypsinogen binding site and displaying low activity towards trypsinogen was developed using N-alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester (Z-Lys-S-Bzl). The kinetic constants for hydrolysis of this substrate at pH 8.0 and 25 degrees C were determined for natural enteropeptidase (K(m) 59.6 microM, k(cat) 6660 min(-1), k(cat)/K(m) 111 microM(-1) x min(-1)), as well as for enteropeptidase preparation with deleted 118-783 fragment of the heavy chain (K(m) 176.9 microM, k(cat) 6694 min(-1), k(cat)/K(m) 37.84 microM(-1) x min(-1)) and trypsin (K(m) 56.0 microM, k(cat) 8280 min(-1), k(cat)/K(m) 147.86 microM(-1) x min(-1)). It was shown that the enzymes with trypsin-like primary active site display similar hydrolysis efficiency towards Z-Lys-S-Bzl. Calcium ions cause 3-fold activation of hydrolysis of the substrates of general type GD(4)K-X by the natural full-length enteropeptidase. In contrast, the hydrolysis of substrates with one or two Asp/Glu residues at P2-P3 positions is slightly inhibited by Ca2+. In the case of enteropeptidase light chain as well as the enzyme containing the truncated heavy chain (466-800 fragment), the activating effect of calcium ions was not detected for all the studied substrates. The results of hydrolysis experiments with synthetic enteropeptidase substrates GD(4)K-F(NO(2))G, G(5)DK-F(NO(2))G (where F(NO(2)) is p-nitrophenyl-L-phenylalanine residue), and GD(4)K-Nfa (where Nfa is beta-naphthylamide) demonstrate the possibility of regulation of undesired side hydrolysis using natural full-length enteropeptidase for processing chimeric proteins by means of calcium ions.

Expression and preparation of recombinant hepcidin in Escherichia coli

Hepcidin is a low-molecular-weight, highly disulfide bonded peptide relevant to small intestine iron absorption and body iron homeostasis. In this work, hepcidin was expressed in Escherichia coli as a 10.5 kDa fusion protein (His-hepcidin) with a N-terminal hexahistidine tag. The expressed His-hepcidin existed in the form of inclusion bodies and was purified by IMAC under denaturation condition. Since the fusion partner for hepcidin did not contain other cysteine residues, the formation of disulfide bonds was performed before the His-tag was removed. Then, the oxidized His-hepcidin monomer was separated from protein multimers through gel filtration. Following monomer refolding, hepcidin was cleaved from fusion protein by enterokinase and purified with reverse-phase chromatography. The recombinant hepcidin exhibited obvious antibacterial activity against Bacillus subtilis.

Expression and characterization of the carboxyl esterase Rv3487c from Mycobacterium tuberculosis

Rv3487c (lipF), a member of the lipase family of Mycobacterium tuberculosis, is related to virulence of this pathogen. Real-time RT-PCR analysis indicated that Rv3487c was induced at low pH in M. tuberculosis cultured in vitro. The gene of Rv3487c was cloned and expressed as fusion protein in Escherichia coli. After removal of the N-terminal domain of the fusion partner by enterokinase treatment, the effect of pH, temperature, and detergents on the purified enzyme activity and stability was characterized. Rv3487c could efficiently hydrolyze short chain esters. The catalytic triad of Rv3487c consists of residues Ser90, Glu189, and His219 as demonstrated by amino acid sequence alignment, three-dimensional modeling, and site-directed mutagenesis.

Expression of recombinant chinese bovine enterokinase catalytic subunit in P. pastoris and its purification and characterization

Enterokinase is a tool protease widely utilized in the cleavage of recombinant fusion proteins. cDNA encoding the catalytic subunit of Chinese bovine enterokinase (EKL) was amplified by PCR and then fused to the 3' end of prepro secretion signal peptide gene of alpha-mating factor from Saccharomyces cerevisiae to get the alpha-MF signal-EKL-His6 encoding gene by PCR. Then the whole coding sequence was cloned into the integrative plasmid pAO815 under the control of a methanol-inducible promoter and transformed GS115 methylotrophic strain of Pichia pastoris. Secreted expression of recombinant EKL-His6 was attained by methanol induction and its molecular weight is 43 kD. Because of the existence of His6-tag, EKL-His6 was easily purified from P. pastoris fermentation supernatant by using Ni2+ affinity chromatography and the yield is 5.4 mg per liter of fermentation culture. This purified EKL-His6 demonstrates excellent cleavage activity towards fusion protein containing EK cleavage site.

High-level secretory production of recombinant bovine enterokinase light chain by Pichia pastoris

Enterokinase (EC 3.4.21.9) is a serine proteinase with a specific digest sequence (Asp)4-Lys in the duodenum. Its high specificity for the recognition site makes enterokinase (EK) a useful tool for an in vitro cleavage of fusion proteins. In this work, an active bovine enterokinase light chain (EK(L)) was produced in secretory form by a recombinant strain of the methylotrophic yeast Pichia pastoris. The influences of methanol utilization phenotype of the host strain, induction pH, and carbon source on the recombinant production were studied. The production of recombinant EK(L) by Mut(s) strain was much higher than that by Mut+ strain. When inducted at pH 6.0, on a glycerol/methanol medium, the concentration of recombinant EK(L) (rEK(L)) reached 350 mg l(-1), which was 20-fold higher than that reported previously. The recombinant EK(L) was purified in a simple procedure on the anion exchange chromatography and 15 mg pure active EK(L) were obtained from 100 ml culture broth supernatant. The specific activity of purified rEK(L) was approximately 9000 u mg(-1). To facilitate purification and removal of rEKL after cleavage of fusion protein, the C-terminal His-tagged EK(L) (EK(L)/His) was also expressed in P. pastoris, and this His-tagged EK(L) exhibited a similar enzymatic activity to the untagged EK(L).

Evolution of trypsinogen activation peptides

The activation peptide of mammalian trypsinogens contains a highly conserved tetra-aspartate sequence (D19-D20-D21-D22) preceding the K23-I24 scissile peptide bond, which is hydrolyzed as the first step in the activation process. Here, we examined the evolution and function of trypsinogen activation peptides through integrating functional characterization of disease-associated mutations with comparative genomic analysis. Activation properties of three chronic pancreatitis-associated activation peptide mutants (the novel D19A and the previously reported D22G and K23R) were simultaneously analyzed, for the first time, in the context of recombinant human cationic trypsinogen. A dramatic increase in autoactivation of cationic trypsinogen was observed in all three mutants, with D22G and K23R exhibiting the most marked increases. The physiological activator enteropeptidase activated the D19A mutant normally, activated the D22G mutant very poorly, and stimulated activation of the K23R mutant. The biochemical and structural data, taken together with a comprehensive sequence comparison, indicates that the tetra-aspartate sequence in mammalian trypsinogen activation peptides has evolved not only for optimal enteropeptidase recognition in the duodenum but also for efficient inhibition of trypsinogen autoactivation within the pancreas. Moreover, the use of lysine instead of arginine at the P1 position of activation peptides also has an advantageous effect against trypsinogen autoactivation. Finally, fixed substitutions in the key residues of the trypsinogen activation peptide may suggest the evolution of new functions unrelated to digestion, as found in the group III trypsinogens of cold-adapted fishes.

[Inhibition by IFN-(Asp)4-Lys-HIV chimeric protein of hydrolysis of the low molecular substrate by the enteropeptidase light chain]

The full length enteropeptidase or it's light chain have often used for the limited proteolysis of recombinant chimeric proteins incorporating the linker-(Asp)4Lys- to obtain the target protein. Any chimeric proteins were not cleaved by the full length enteropeptidase efficiently. The resistant to the hydrolysis chimeric protein IFN-(Asp)4Lys-HIV earlier was shown to be the competitive inhibitor (Ki = 3,4 x 10(-6) M) in relation to the low molecular substrate. In present study we were determined this chimeric protein competitive inhibited the same substrate hydrolysis by enteropeptidase light chain (Ki = 2,7 x 10(-5) M). Comparison the Ki values for the substrate hydrolysis by full length enzyme and its light chain suggests that the enteropeptidase heavy chain may participate in chimeric protein binding.

[Hydrolysis by enteropeptidase of nonspecific (model) peptide sequences and possible physiological role of this phenomenon]

Enteropeptidase (enterokinase) (EC 3.4.21.9), a highly specific processing protease, initiating a cascade of reactions activating the digestion enzymes. Catalyzing trypsinogen activation enteropeptidase exhibits unique properties for high efficiency hydrolysis of the polypeptide chain after lysine-15 residue in the -DDDDK15- sequence. In 1998 we found an unusual calcium-dependent autolysis of the enteropeptidase heavy chain leading to the drastic loss of its activity towards trypsinogen: after lysine-360 (-NNYEK360-INCN-), -), arginine-384 (-NEWER384-TQGS-), arginine-422 (-GRRER422-VGLL-) and lysine-465 (-QNMEK465-TIFQ-) residues. We used hepta-nona-peptides as the model substrates for autolysys: human angiotensin II--DRVYIHPF and cattle hemoglobin b-chain fragments: LTAEEKA and MLTAEEKAA. Kinetic parameters of enteropeptidase hydrolysis for these substrates were determined. Recent study demonstrates the ability of enteropeptidase to hydrolyze peptide bonds formed by carboxyl groups of Lys or Arg residues if less than four but at least one negative charged amino acid residue is in any of substrate P2-P5 positions. Ca(2+)-dependent autolysis of enteropeptidase heavy chain and of trypsin were compared; the second one serves as the natural defense mechanism against the undesirable premature proenzymes activation in pancreas leading to pancreatitis. The corresponding enteropeptidase inactivation in low Ca2+ environment ought to be the component of the same protective mechanism.

Expression, purification, and characterization of a biologically active bovine enterokinase catalytic subunit in Escherichia coli

Enterokinase (EC 3.4.21.9) is a serine proteinase in the duodenum that exhibits specificity for the sequence (Asp)(4)-Lys. It converts trypsinogen to trypsin. Its high specificity for the recognition site makes enterokinase (EK) a useful tool for in vitro cleavage of fusion proteins. cDNA encoding the catalytic chain of Chinese bovine enterokinase was cloned and its encoding amino acid sequence is identical to the previously reported sequence although there are two one-base mutations which do not change the encoded amino acid. The EK catalytic subunit cDNA was cloned into plasmid pET32a, and fused downstream to the fusion partner thioredoxin (Trx) and the following DDDDK enterokinase recognition sequence. The recombinant bovine enterokinase catalytic subunit was expressed in Escherichia coli BL21(DE3), and most products existed in soluble form. After an in vivo autocatalytic cleavage of the recombinant Trx-EK catalytic domain fusion protein, intact, biologically active EK catalytic subunit was released from the fusion protein. The recombinant intact EK catalytic subunit was purified to homogeneity with a specific activity of 720 AUs/mg protein through ammonium sulfate precipitation, DEAE chromatography, and gel filtration. The purified intact EK catalytic subunit has a K(m) of 0.17 mM, and K(cat) is 20.8s(-1). From 100 ml flask culture, 4.3 mg pure active EK catalytic subunits were obtained.

Engineered recombinant enteropeptidase catalytic subunit: effect of N-terminal modification

Enteropeptidase (enterokinase) is a serine protease highly specific for recognition and cleavage of the target sequence of Asp-Asp-Asp-Asp-Lys (D4K). The three-dimensional structure of the enteropeptidase shows that the N-terminal amino acid is buried inside the protein providing molecular interactions necessary to maintain the conformation of the active site. To determine the influence of the N-terminal amino acid of enteropeptidase light chain (EK(L)) on the enzymatic activity, we constructed various mutants including 17 different single amino acid substitutions and three different extensions at the N-terminal end. The mutants of recombinant enteropeptidase (rEK(L)) were expressed in Saccharomyces cerevisiae and secreted into culture medium. Among 20 different mutants tested, the only mutant with the Ile --> Val substitution exhibited significant activity. The kinetic properties of the mutant protein were very similar to those of the wild-type rEK(L). Based on the three-dimensional structure where the N-terminal Ile is oriented into hydrophobic pocket, the results suggest that Val could substitute Ile without affecting the active conformation of the enzyme. The results also explain why all trypsin-like serine proteases carry either Ile or Val at the N-termini and none other amino acid residues are found. Moreover, this finding provides a mental framework for expressing the N-terminally engineered enteropeptidase in Escherichia coli, utilizing the known property of the methionine aminopeptidase that exhibits poor activity toward the N-terminal Met-Ile bond, but offers efficient cleavage of the Met-Val bond.

Mutations in the proenteropeptidase gene are the molecular cause of congenital enteropeptidase deficiency

Enteropeptidase (enterokinase [E.C.3.4.21.9]) is a serine protease of the intestinal brush border in the proximal small intestine. It activates the pancreatic proenzyme trypsinogen, which, in turn, releases active digestive enzymes from their inactive pancreatic precursors. Congenital enteropeptidase deficiency is a rare recessively inherited disorder leading, in affected infants, to severe failure to thrive. The genomic structure of the proenteropeptidase gene (25 exons, total gene size 88 kb) was characterized in order to perform DNA sequencing in three clinically and biochemically proved patients with congenital enteropeptidase deficiency who were from two families. We found compound heterozygosity for nonsense mutations (S712X/R857X) in two affected siblings and found compound heterozygosity for a nonsense mutation (Q261X) and a frameshift mutation (FsQ902) in the third patient. In accordance with the biochemical findings, all four defective alleles identified are predicted null alleles leading to a gene product not containing the active site of the enzyme. These data provide first evidence that proenteropeptidase-gene mutations are the primary cause of congenital enteropeptidase deficiency.

[Hydrolysis of chimeric proteins by enteropeptidase at the specific linker (Asp)4Lys depending on refolding conditions]

Refolding from inclusion bodies of chimeric proteins containing the enteropeptidase-specific linker (Asp)4Lys was carried out. It was shown that, depending on the refolding conditions, chimeric proteins function as substrates or inhibitors of the enteropeptidase. The efficiency of the enteropeptidase hydrolysis of chimeric proteins containing the (Asp)4Lys linker may depend not only on the amino acid sequence of the protein binding site for the enzyme but also on the site conformation.

Activation of recombinant proenteropeptidase by duodenase

Duodenase, a serine proteinase from bovine Brunner's (duodenal) glands that was predicted to be a natural activator of enteropeptidase zymogen, cleaves and activates recombinant single-chain bovine proenteropeptidase (kcat/Km = 2700 M(-1) s(-1)). The measured rate of proenteropeptidase cleavage by duodenase was about 70-fold lower compared with the rate of trypsin-mediated cleavage of the zymogen. The role of duodenase is supposed to be the primary activator of proenteropeptidase maintaining a certain level of active enteropeptidase in the duodenum. A new scheme of proteolytic activation cascade of digestive proteases is discussed.

Expression of a recombinant Toxoplasma gondii ROP2 fragment as a fusion protein in bacteria circumvents insolubility and proteolytic degradation

A 268-amino-acid-residue carboxy-terminal antigenic fragment of the Toxoplasma gondii rhoptry protein ROP2 (recROP2(t), residues 196-464) was expressed in Escherichia coli. This recombinant fragment was produced at low concentration and in a highly insoluble form. By contrast, the level of recROP2(t) production was drastically greater when the same coding sequence was fused to the C-terminus of thioredoxin (TRX) or to the maltose-binding protein (MBP) gene. While both fusion proteins were found to be mainly insoluble, solubilization could be achieved without significant degradation. MBP was more efficient than TRX in increasing the recovery of soluble protein with more than 10% of total MBP-recROP2(t) being readily expressed in a soluble form. Moreover, the insoluble form of MBP-recROP2(t) could be correctly refolded with a recovery of more than 80%. Both forms of MBP-recROP2(t) were purified to homogeneity by amylose chromatography. In contrast, the refolding of TRX-recROP2(t) promoted aggregation of the protein, which was prevented by the use of zwitterionic detergent during the one-step purification by gel filtration. Subsequent proteolytic cleavages of purified TRX-recROP2(t) and of MBP-recROP2(t) led respectively to the complete degradation or to the truncation of the recROP2(t) moiety. However, recROP2(t), despite the presence of the fusion partners, adopted a suitable conformation recognized by human serum-derived antibodies from T. gondii-seropositive individuals. Finally, both fusion proteins were able to induce specific humoral and cell-mediated immune response to the ROP2 fragment. Such fusions could represent an alternative to study the immunogenicity of T. gondii proteins which are difficult to produce because of insolubility and degradation.

Crystal structure of enteropeptidase light chain complexed with an analog of the trypsinogen activation peptide

Enteropeptidase is a membrane-bound serine protease that initiates the activation of pancreatic hydrolases by cleaving and activating trypsinogen. The enzyme is remarkably specific and cleaves after lysine residues of peptidyl substrates that resemble trypsinogen activation peptides such as Val-(Asp)4-Lys. To characterize the determinants of substrate specificity, we solved the crystal structure of the bovine enteropeptidase catalytic domain to 2.3 A resolution in complex with the inhibitor Val-(Asp)4-Lys-chloromethane. The catalytic mechanism and contacts with lysine at substrate position P1 are conserved with other trypsin-like serine proteases. However, the aspartyl residues at positions P2-P4 of the inhibitor interact with the enzyme surface mainly through salt bridges with the Nzeta atom of Lys99. Mutation of Lys99 to Ala, or acetylation with acetic anhydride, specifically prevented the cleavage of trypsinogen or Gly-(Asp)4-Lys-beta-naphthylamide and reduced the rate of inhibition by Val-(Asp)4-Lys-chloromethane 22 to 90-fold. For these reactions, Lys99 was calculated to account for 1.8 to 2.5 kcal mol(-1) of the free energy of transition state binding. Thus, a unique basic exosite on the enteropeptidase surface has evolved to facilitate the cleavage of its physiological substrate, trypsinogen.

Purification and further characterization of enteropeptidase from porcine duodenum

Enteropeptidase [EC 3.4.21.9] is a membrane-bound serine endopeptidase present in the duodenum that converts trypsinogen to trypsin. We previously cloned the cDNA of the porcine enzyme and deduced its entire amino acid sequence [M. Matsushima et al. (1994) J. Biol. Chem. 269, 19976-19982]. In the present study, we purified the porcine enzyme approximately 2,200-fold in a 12% yield from a duodenal mucosal extract to apparent homogeneity by an improved procedure comprising four steps of chromatography including benzamidine-Sepharose affinity chromatography. Lectin blotting analysis suggested that the enzyme is glycosylated mainly with N-linked carbohydrate chains of the tri- and/or tetraantennary complex type. The H and L chains of the enzyme were separated into two major bands upon SDS-PAGE under reducing conditions, suggesting that the enzyme mainly comprises two isoforms, a higher molecular weight form and a lower molecular weight form. The enzyme was also separated by lectin affinity chromatography into two major fractions, named isoforms I and II, which corresponded to the higher and lower molecular weight forms, respectively. These two isoforms appeared to be different only in the carbohydrate moiety, having essentially the same enzymatic properties. The enzyme was optimally active at pH 8.0 toward Gly-Asp-Asp-Asp-Asp-Lys-beta-naphthylamide, and was inhibited strongly by various serine proteinase inhibitors. Furthermore, it was also strongly inhibited by E-64 [L-trans-epoxysuccinyl-leucylamide-(4-guanido)-butane], a cysteine proteinase inhibitor. Substrate specificity studies involving various synthetic peptides indicated that acidic residues at the P2, P3, and/or P4 positions are especially favorable for maximal activity, but are not absolutely necessary, at least in the cases of peptide substrates.

Apical sorting of bovine enteropeptidase does not involve detergent-resistant association with sphingolipid-cholesterol rafts

Enteropeptidase is a heterodimeric type II membrane protein of the brush border of duodenal enterocytes. In this location, enteropeptidase cleaves and activates trypsinogen, thereby initiating the activation of other intestinal digestive enzymes. Recombinant bovine enteropeptidase was sorted directly to the apical surface of polarized Madin-Darby canine kidney cells. Replacement of the cytoplasmic and signal anchor domains with a cleavable signal peptide (mutant proenteropeptidase lacking the amino-terminal signal anchor domain (dSA-BEK)) caused apical secretion. The additional amino-terminal deletion of a mucin-like domain (HL-BEK) resulted in secretion both apically and basolaterally. Further deletion of the noncatalytic heavy chain (L-BEK) resulted in apical secretion. Thus enteropeptidase appears to have at least three distinct sorting signals as follows: the light chain (L-BEK) directs apical sorting, addition of most of the heavy chain (HL-BEK) inhibits apical sorting, and addition of the mucin-like domain (dSA-BEK) restores apical sorting. Inhibition of N-linked glycosylation with tunicamycin or disruption of microtubules with colchicine caused L-BEK to be secreted equally into apical and basolateral compartments, whereas brefeldin A caused basolateral secretion of L-BEK. Full-length BEK was not found in detergent-resistant raft domains of Madin-Darby canine kidney cells or baby hamster kidney cells. These results suggest apical sorting of enteropeptidase depends on N-linked glycosylation of the serine protease domain and an amino-terminal segment that includes an O-glycosylated mucin-like domain and three potential N-glycosylation sites. In contrast to many apically targeted proteins, enteropeptidase does not form detergent-resistant associations with sphingolipid-cholesterol rafts.

[Structural characteristics providing for high specificity of enteropeptidase]

The effects of structural modification upon the specificity of enteropeptidase were studied. A variation in the unique specificity of the enzyme was shown to be the result of an autolysis caused by the enzyme's loss of calcium ions. The cleavage sites of the autolysis were determined. A truncated enzyme containing the C-terminal fragment of its heavy chain (466-800 residues) and the intact light chain were shown to be the products of autolysis. The kinetic parameters of the hydrolysis of trypsinogen, a recombinant protein, and a peptide substrate with both forms of enteropeptidase were determined. Conditions were found that can help regulate the transition of the native enzyme into the truncated form. A hypothesis was proposed concerning the autoactivational character of proenteropeptidase processing.

Structure of murine enterokinase (enteropeptidase) and expression in small intestine during development

Enterokinase (enteropeptidase) is expressed only in proximal small intestine, where it initiates digestive enzyme activation by converting trypsinogen into trypsin. To investigate this restricted expression pattern, mouse enterokinase cDNA was cloned, and the distribution of enterokinase mRNA and enzymatic activity were determined in adult mice and during gestation. Analysis of enterokinase sequences showed that a mucinlike domain near the NH2 terminus is composed of repeated approximately 15-amino acid Ser/Thr-rich motifs. By Northern blotting and trypsinogen activation assays, enterokinase mRNA and enzymatic activity were undetectable in stomach, abundant in duodenum, and decreased distally until they were undetectable in midjejunum, ileum, and colon. By in situ mRNA hybridization, enterokinase mRNA was localized to the enterocytes throughout the villus. Expression was not observed in goblet cells, Paneth cells, or Brunner's glands. Enterokinase mRNA and enzymatic activity were not detected in the duodenum of fetal mice but were easily detected in the duodenum on postnatal days 2-6. Both enterokinase mRNA and enzymatic activity decreased to very low levels after day 7 but increased after weaning and reached a high level characteristic of adult life by day 60. Therefore, in mice, duodenal enterocytes are the major type of cells expressing enterokinase, which appears to be regulated at the level of mRNA abundance.

Bovine proenteropeptidase is activated by trypsin, and the specificity of enteropeptidase depends on the heavy chain

Enteropeptidase, also known as enterokinase, initiates the activation of pancreatic hydrolases by cleaving and activating trypsinogen. Enteropeptidase is synthesized as a single-chain protein, whereas purified enteropeptidase contains a approximately 47-kDa serine protease domain (light chain) and a disulfide-linked approximately 120-kDa heavy chain. The heavy chain contains an amino-terminal membrane-spanning segment and several repeated structural motifs of unknown function. To study the role of heavy chain motifs in substrate recognition, secreted variants of recombinant bovine proenteropeptidase were constructed by replacing the transmembrane domain with a signal peptide. Secreted variants containing both the heavy chain (minus the transmembrane domain) and the catalytic light chain (pro-HL-BEK (where BEK is bovine enteropeptidase)) or only the catalytic domain (pro-L-BEK) were expressed in baby hamster kidney cells and purified. Single-chain pro-HL-BEK and pro-L-BEK were zymogens with extremely low catalytic activity, and both were activated readily by trypsin cleavage. Trypsinogen was activated efficiently by purified enteropeptidase from bovine intestine (Km = 5.6 microM and kcat = 4.0 s-1) and by HL-BEK (Km = 5.6 microM and kcat = 2.2 s-1), but not by L-BEK (Km = 133 microM and kcat = 0.1 s-1); HL-BEK cleaved trypsinogen at pH 5.6 with 520-fold greater catalytic efficiency than did L-BEK. Qualitatively similar results were obtained at pH 8.4. In contrast to this striking difference in trypsinogen recognition, the small synthetic substrate Gly-Asp-Asp-Asp-Asp-Lys-beta-naphthylamide was cleaved with similar kinetic parameters by both HL-BEK (Km = 0.27 mM and kcat = 0.07 s-1) and L-BEK (Km = 0.60 mM and kcat = 0.06 s-1). The presence of the heavy chain also influenced the rate of reaction with protease inhibitors. Bovine pancreatic trypsin inhibitor preferred HL-BEK (initial Ki = 99 nM and final Ki* = 1.8 nM) over L-BEK (Ki = 698 nM and Ki* = 6.2 nM). Soybean trypsin inhibitor exhibited a reciprocal pattern, inhibiting L-BEK (Ki* = 1.6 nM), but not HL-BEK. These kinetic data indicate that the enteropeptidase heavy chain has little influence on the recognition of small peptides, but strongly influences macromolecular substrate recognition and inhibitor specificity.

[Recombinant proteins containing amino acid sequences of two ectatomin chains]

Artificial genes for chains A and B of ectatomin, an Ectatomma tuberculatum ant toxin, were obtained by chemical and enzymic synthesis and cloned into new plasmid vectors. Expression plasmids with the genes of hybrid proteins were constructed containing human interleukin-3 or its terminal 63-mer fragment as well as chains A and B of ectatomin, which are linked via a region containing the cleavage site of specific protease, enterokinase (hybrid proteins IL3ETOXA, IL3ETOXB, ILETOXA, and ILETOXB). Escherichia coli producer strains providing a high yield of IL3ETOXA and IL3ETOXB proteins as inclusion bodies were obtained.

Refolding of recombinant Pasteurella haemolytica A1 glycoprotease expressed in an Escherichia coli thioredoxin gene fusion system

Pasteurella haemolytica A1 secretes an O-sialoglycoprotein endopeptidase (EC. 3.4.24.57) (glycoprotease: Gcp) which is specific for O-linked sialoglycoproteins. When the cloned gene is expressed in Escherichia coli, the recombinant glycoprotease (rGcp) is secreted to the periplasm where it is present as a disulfide-linked aggregate which lacks enzymatic activity. In vitro refolding and activation of rGcp by mammalian protein disulfide isomerase (PDI) or by the E. coli chaperones (DnaK, DnaJ and GrpE) indicate that the redox environment of rGcp is critical in restoring biological activity. A fusion protein, rTrx-Gcp, was constructed to investigate the role of thioredoxin (E. coli TrxA) in the production of enzymatically active rGcp. This 47 kDa protein was expressed at a high level, in a soluble, monomeric form, in the cytoplasm of E. coli. Cleavage of the fusion protein by enterokinase released the rGcp fragment (35 kDa) with glycoprotease activity. A higher recombinant glycoprotease activity was recovered after anion exchange chromatography of lysates of E. coli expressing rTrx-Gcp. Thus when E. coli TrxA is combined in a recombinant fusion protein with P. haemolytica A1 Gcp, productive folding of the glycoprotease can occur as a result of the chaperone action of the protein disulfide reductase coupled with its ability to retain the fusion gene product in the E. coli cytoplasm.

Complementary DNA cloning and sequencing of rat enteropeptidase and tissue distribution of its mRNA

A cDNA clone encoding enteropeptidase (EC 3.4.21.9), a key enzyme for the conversion of trypsinogen to trypsin, was isolated from a rat duodenal mucosa cDNA library. Sequences of the 3585 base pair clone predicted that enteropeptidase is synthesized as a single-chain precursor form, proenteropeptidase, consisting of 1058 amino acid residues with an internal signal sequence (51 residues) and is then processed into the mature enzyme consisting of three different peptide chains, i.e., mini, light and heavy chains, not the previously reported two-chain enzyme. The structure of enteropeptidase is relatively conserved among different species and the rat enteropeptidase is 24 and 39 amino acids longer than the porcine and human ones, respectively. Northern blot analysis of rNAs from normal rat tissues revealed that the enteropeptidase mRNA of around 4.4 kb in size was expressed only in the duodenal mucosa, and high proteolytic activity of the enzyme was detected in the proximal small intestine. Additional analysis of the RNAs by RT-PCR revealed that a low level of the mRNA was also expressed in the other parts of the small intestine, i.e., jejunum and ileum. These results indicate that the biosynthesis of enteropeptidase takes place mainly in the proximal small intestine, the duodenum, and the importance of the region in the physiology of intestinal protein digestion regulated by the enzyme is suggested. Furthermore a faint signal of the mRNA was also detected in the stomach, colon and brain in which the existence of trypsin-like serine proteases were reported. The significance of the low level expression of the gene is unclear, but the potential peptide-processing function of the enzyme in these tissues is also suggested.

Production of active recombinant human chymase from a construct containing the enterokinase cleavage site of trypsinogen in place of the native propeptide sequence

Human chymase, a chymotrypsin-like proteinase found in mast cells, was produced in an enzymatically active recombinant form. The protein was expressed in Escherichia coli as part of an insoluble fusion protein which was solubilized and renatured. The structure of the fusion protein was NH2-ubiquitin-enterokinase cleavage site-chymase-COOH. The enterokinase cleavage site of trypsinogen replaced the native propeptide sequence of chymase, allowing for activation by a readily available proteinase (enterokinase) of known specificity. Characterization of refolded-activated recombinant chymase with substrates and inhibitors demonstrated properties identical to that of the native proteinase isolated from skin.

Production of recombinant bovine enterokinase catalytic subunit in Escherichia coli using the novel secretory fusion partner DsbA

Enterokinase (EK) is a heterodimeric serine protease which plays a key role in initiating the proteolytic digestion cascade in the mammalian duodenum. The enzyme acts by converting trypsinogen to trypsin via a highly specific cleavage following the pentapeptide recognition sequence (Asp)4-Lys. This stringent site specificity gives EK great potential as a fusion protein cleavage reagent. Recently, a cDNA encoding the catalytic (light) chain of bovine enterokinase (EKL) was identified, characterized, and transiently expressed in mammalian COS cells. We report here the production of EKL in Escherichia coli by a novel secretory expression system that utilizes E. coli DsbA protein as an N-terminal fusion partner. The EKL cDNA was fused in-frame to the 3'-end of the coding sequence for DsbA, with the two domains of the fusion protein separated by a linker sequence encoding an enterokinase recognition site. Active, processed recombinant EKL (rEKL) was generated from this fusion protein via an autocatalytic cleavage reaction. The enzymatic properties of the bacterially produced rEKL were indistinguishable from the previously described COS-derived enzyme. Both forms of rEKL were capable of cleaving peptides, polypeptides and trypsinogen with the same specificity exhibited by the native heterodimeric enzyme purified from bovine duodena. Interestingly, rEKL activated trypsinogen poorly relative to the native heterodimeric enzyme, but was superior in its ability to cleave artificial fusion proteins containing the (Asp)4-Lys recognition sequence.

The SEA module: a new extracellular domain associated with O-glycosylation

Using a variety of homology search methods and multiple alignments, a new extracellular module was identified in (1) agrin, (2) enterokinase, (3) a 63-kDa sea urchin sperm protein, (4) perlecan, (5) the breast cancer marker MUCI (episialin), (6) the cell surface antigen 114/A10, and (7/8) two functionally uncharacterized, probably extracellular, Caenorhabditis elegans proteins. Despite the functional diversity of these adhesive proteins, a common denominator seems to be their existence in heavily glycosylated environments. In addition, the better characterized proteins mentioned above contain all O-glycosidic-linked carbohydrates such as heparan sulfate that contribute considerably to their molecular masses. The common module might regulate or assist binding to neighboring carbohydrate moieties.

Structural characterization of porcine enteropeptidase

Enteropeptidase (EC 3.4.21.9) is a key enzyme in the intestinal digestion cascade responsible for the conversion of trypsinogen to trypsin, which then activates various pancreatic zymogens. In order to structurally characterize the enzyme, we purified the enzyme from porcine duodenal mucosa and showed that it consists of three polypeptide chains, which we named "mini" chain (M chain), light chain (L chain), and heavy chain (H chain) in order of increasing molecular size. Based on their NH2-terminal sequences, a cDNA clone for porcine enteropeptidase was isolated and analyzed. The clone was 3597 base pairs long, which encoded 1034 amino acid residues of a single-chain precursor form of enteropeptidase. The precursor contained an additional NH2-terminal 51-residue sequence including a putative internal signal sequence, followed by the M chain (66 residues), the H chain (682 residues), and the L chain (235 residues) in that order. The H chain had regions partially homologous in sequence with low density lipoprotein receptor and complement components. On the other hand, the L chain was highly homologous with the catalytic domains of trypsin-like serine proteinases. The structural model of the L chain suggests that the sequence, Arg885-Arg-Arg-Lys888, is probably involved in the unique substrate specificity of the enzyme, preferring acidic amino acid residues at the P2-P5 sites.

The amino-terminal sequence of the catalytic subunit of bovine enterokinase

Bovine enterokinase (enteropeptidase) is a serine protease and functions as the physiological activator of trypsinogen. The enzyme has a heavy chain (115 kD) covalently linked to a light or catalytic subunit (35 kD). The amino acid composition showed that the light chain has nine half-cystine residues (four as intramolecular disulfides) and that one half-cystine was in a disulfide link between the light and heavy subunits. The amino-terminal 27 residues of the S-vinylpyridyl derivative of the light chain were determined by gas-phase Edman degradation. The sequence has homologies with other serine proteases containing one or two chains. The homologies suggest that the catalytic subunit has the same three-dimensional structure and, therefore, the same mechanism of enzymatic action as pancreatic chymotrypsin, trypsin, and elastase. The presence of the conserved amino-terminal activation peptide sequence (IVGG) shows that enterokinase must have a zymogen precursor and that the two-chain enzyme arises from limited proteolysis during posttranslational processing.