Functional properties of the recombinant kringle-2 domain of tissue plasminogen activator produced in Escherichia coli

The kringle-2 domain (residues 176-262) of tissue-type plasminogen activator (t-PA) was cloned and expressed in Escherichia coli. The recombinant peptide, which concentrated in cytoplasmic inclusion bodies, was isolated, solubilized, chemically refolded, and purified by affinity chromatography on lysine-Sepharose to apparent homogeneity. [35S]Cysteine-methionine-labeled polypeptide was used to study the interactions of kringle-2 with lysine, fibrin, and plasminogen activator inhibitor-1. The kringle-2 domain bound to lysine-Sepharose and to preformed fibrin with a Kd = 104 +/- 6.2 microM (0.86 +/- 0.012 binding site) and a Kd = 4.2 +/- 1.05 microM (0.80 +/- 0.081 binding site), respectively. Competition experiments and direct binding studies showed that the kringle-2 domain is required for the formation of the ternary t-PA-plasminogen-intact fibrin complex and that the association between the t-PA kringle-2 domain and fibrin does not require plasmin degradation of fibrin and exposure of new COOH-terminal lysine residues. We also observed that kringle-2 forms a complex with highly purified guanidine-activated plasminogen activator inhibitor-1, dissociable by 0.2 M epsilon-aminocaproic acid. The kringle-2 polypeptide significantly inhibited tissue plasminogen activator/plasminogen activator inhibitor-1 interaction. The kringle-2 domain bound to plasminogen activator inhibitor-1 in a specific and saturable manner with a Kd = 0.51 +/- 0.055 microM (0.35 +/- 0.026 binding site). Therefore, the t-PA kringle-2 domain is important for the interaction of t-PA not only with fibrin, but also with plasminogen activator inhibitor-1 and thus represents a key structure in the regulation of fibrinolysis.     

Purification and characterization of tissue plasminogen activator kringle-2 domain expressed in Escherichia coli

We have expressed the 174-263 fragment (kringle-2 domain) of human tissue-type plasminogen activator (t-PA) in Escherichia coli by secretion into the periplasmic space using the alkaline phosphatase promoter and stII enterotoxin signal sequence. A large portion of the secreted protein is associated with an insoluble cellular fraction. This material can be solubilized by extraction with denaturant and reducing agent and then recovered in active form by refolding in the presence of reduced and oxidized glutathione. Kringle-2 is then easily purified by affinity chromatography on lysine-Sepharose followed by cation-exchange chromatography. The isolated protein has an amino acid composition and N-terminal sequence as expected for the 174-263 fragment of t-PA, indicating that the signal peptide has been properly removed. Circular dichroic spectra suggest that the protein is folded similar to the kringle-4 domain of plasminogen [Castellino et al. (1986) Arch. Biochem. Biophys. 247, 312-320]. Equilibrium dialysis experiments indicate a single binding site on kringle-2 for L-lysine having a KD of 100 microM. Using a method based on elution of kringle from lysine-Separose with omega-aminocarboxylic acids [Winn et al. (1980) Eur. J. Biochem. 104, 579-586], we have shown the lysine binding site of t-PA kringle-2 to have a preference for a ligand with 8.8-A separation between amine and carboxylate functions. Charge interactions with the epsilon-amino group of L-lysine are important in binding since the affinities for N epsilon-acetyl-L-lysine, L-arginine, and gamma-guanidinobutyric acid are decreased greater than 2000-fold, 200-fold, and 12-fold, respectively, relative to the affinity for L-lysine.(ABSTRACT TRUNCATED AT 250 WORDS)     

The plasminogen activator inhibitor-1 binding site in the kringle-2 domain of tissue-type plasminogen activator

We have shown that synthetic peptides containing the amino acid sequence Asn-Arg-Arg-Leu, derived from the amino acid sequence of the inner loop of the kringle-2 domain of tissue-type plasminogen activator (tPA), inhibited complex formation between two chain tPA and plasminogen activator inhibitor-1 (PAI-1) by binding to PAI-1. This binding was reversible and was inhibited by not only tPA but also by enzymatically inactive tPA. Quantitative analyses of the interaction of PAI-1 with the peptide containing the Asn-Arg-Arg-Leu sequence indicated that the PAI-1 binding site residues in the inner loop of the kringle-2 domain and is preferentially expressed in two chain tPA.     

Secretion of active recombinant human tissue plasminogen activator derivatives in Escherichia coli

The DNA fragment coding for kringle 2 plus serine protease domains (K2S) of tissue plasminogen activator (tPA) was inserted into a phagemid vector, pComb3HSS. In the recombinant vector, pComb3H-K2S, the K2S gene was fused to gpIII of PhiM13 and linked to the OmpA signal sequence. The resulting gene, rK2S-gpIII, was inducibly expressed in Escherichia coli XL-1 Blue. The protein was presented on the phage particle. To stop the expression of gpIII, a stop codon between K2S and the gpIII gene was inserted by site-directed mutagenesis. This mutated vector, MpComb3H-K2S, was transformed in XL-1 Blue. After induction with IPTG (isopropyl-beta-D-thiogalactopyranoside), rK2S was found both in the periplasm as an inactive form of approximately 32% and in the culture supernatant as an active form of approximately 68%. The secreted form of rK2S was partially purified by ammonium sulfate (55%) precipitation. The periplasmic form was isolated from whole cells by chloroform extraction. The fibrin binding site of kringle 2 was demonstrated in all expressed versions (phage-bound, periplasmic, and secreted forms) using the monoclonal anti-kringle 2 antibody (16/B). Only the secreted form of rK2S revealed a fibrinogen-dependent amidolytic activity with the specific activity of 236 IU/microg. No amidolytic activity of rK2S was observed in either the periplasmic or the phage-bound form. The secretion of rK2S as an active enzyme offers a novel approach for the production of the active-domain deletion mutant tPA, rK2S, without any requirements for bacterial compartment preparation and in vitro refolding processes. This finding is an important technological advance in the development of large-scale, bacterium-based tPA production systems.     

Effect of residue 65 substitutions on thermal stability of tissue plasminogen activator kringle-2 domain

We have used differential scanning calorimetry to measure the effect of replacements of valine 65 on thermal stability of the isolated kringle-2 domain of tissue plasminogen activator (t-PA). The role of this site in stability was examined because a human t-PA variant having this valine (residue 245 in t-PA numbering) replaced with a methionine has been described [Johnston, M.D., & Berger, H. (1987) U.K. Patent Application GB 2176702A]. Mutants of kringle-2 having valine 65 replaced with Met, Leu, Ile, Thr, Ala, or Ser were constructed by using site-directed mutagenesis in conjunction with a restricted site selection strategy. Isolated kringle-2 domains were expressed in Escherichia coli and purified as previously described for the wild-type domain [Cleary, S., Mulkerrin, M.G., & Kelley, R.F. (1989) Biochemistry 28, 1884-1891]. None of these substitutions results in a significant perturbation of the native conformation of kringle-2 as judged by far-UV circular dichroism and equilibrium dialysis measurements of L-lysine affinity. A two-state analysis of the heat capacity profile observed for heating a solution of wild-type (w-t) kringle-2 containing 100 mM citrate, pH 4.5, provides values of 64.3 +/- 0.8 degrees C for Tg (melting temperature), 81 +/- 5 kcal/mol for delta H g, and 1.2 +/- 0.9 kcal/(mol-deg) for delta C p. Thermal denaturation of w-t kringle-2 is reversible in the pH range 3-6 as indicated by the observation of similar heat capacity profiles for consecutive heating cycles and also recovery of spectroscopic and lysine binding properties upon cooling the heat-denatured protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of a modified human tissue plasminogen activator comprising a kringle-2 and a protease domain

To study structure/function relationships of tissue plasminogen activator (t-PA) activity, one of the simplest modified t-PA structures to activate plasminogen in a fibrin-dependent manner was obtained by constructing an expression vector that deleted amino acid residues 4-175 from the full-length sequence of t-PA. The expression plasmid was introduced into a Syrian hamster cell line, and stable recombinant transformants, producing high levels of the modified plasminogen activator, were isolated. The resulting molecule, mt-PA-6, comprising the second kringle and serine protease domains of t-PA, produced a doublet of plasminogen activator activity having molecular masses of 40 and 42 kDa. The one-chain mt-PA-6 produced by cultured Syrian hamster cells was purified in high yield by affinity and size exclusion chromatography. The purified mt-PA-6 displayed the same two types of microheterogeneity observed for t-PA. NH2-terminal amino acid sequencing demonstrated that one-chain mt-PA-6 existed in both a GAR and a des-GAR form. Purified mt-PA-6 also existed in two glycosylation forms that accounted for the 40- and 42-kDa doublet of activity produced by the cultured Syrian hamster cells. Separation of these two forms by hydrophobic interaction chromatography and subsequent tryptic peptide mapping demonstrated that both forms contained N-linked glycosylation at Asn448; in addition, some mt-PA-6 molecules were also glycosylated at Asn184. Plasmin treatment of one-chain mt-PA-6 converted it to a two-chain molecule by cleavage of the Arg275-Ile276 bond. This two-chain mt-PA-6, like t-PA, had increased amidolytic activity. The fibrinolytic specific activities of the one- and two-chain forms of mt-PA-6 were similar and twice that of t-PA. The plasminogen activator activity of one-chain mt-PA-6 was enhanced greater than 80-fold by CNBr fragments of fibrinogen, and the one-chain enzyme lysed human clots in vitro in a dose-dependent manner. The ability to produce and purify a structurally simple plasminogen activator with desirable fibrinolytic properties may aid in the development of a superior thrombolytic agent for the treatment of acute myocardial infarction.     

Thermodynamics of ligand binding and denaturation for His64 mutants of tissue plasminogen activator kringle-2 domain

The contribution of His64 to the function and stability of tissue plasminogen activator (t-PA) kringle-2 domain (His244 in t-PA numbering) has been studied by using microcalorimetric methods to compare the ligand binding and thermal denaturation behavior of wild-type kringle-2 and mutants having His64 replaced with Tyr or Phe. This site was examined because modeling studies suggested that the His64 side chain could play an important role in ligand binding by forming an ion-pair with the carboxylate of the ligand, L-lysine. Kringle-2 domains were expressed by secretion of the 174-263 portion of t-PA in E. coli and purified as previously described for the wild-type domain. Both mutant proteins retain affinity for L-lysine, although reduced three- to four-fold relative to wild-type, demonstrating that His64 does not interact with the ligand carboxylate through an ion-pair interaction or by hydrogen bonding. The H64Y substitution does result in an altered specificity of the lysine binding site with the mutant domain having greatest affinity for a ligand of 6.8 A chain length, whereas the wild-type domain prefers an 8.8 A long ligand. For both wild-type and mutant, the binding of the optimal chain length ligand is dominated by enthalpic effects (delta H = -6,000 to -7,000 cal/mol) and T delta S accounts for less than 15% of delta G. In addition, the H64Y mutant differs from wild-type in the effect of ligand alpha-amino group modification on binding affinity. Based on examination of the x-ray structure recently determined for wild-type kringle-2, the specificity changes accompanying the H64Y substitution probably result from changes in side chain interactions in the lysine binding site. Thermal denaturation experiments show that the H64Y mutant is also more stable than the wild-type protein with the difference in stabilization free energy (delta delta G) equal to 2.7 kcal/mol at 25 degrees C and pH 3. The increased stability of the mutant appears to be related to the difference in hydrophobicity between His and Tyr.     

Interactions between the finger and kringle-2 domains of tissue-type plasminogen activator and plasminogen activator inhibitor-1.

We have shown that plasminogen activator inhibitor-1 (PAI-1) inhibits the fibrin binding of both the single chain and two chain forms of tissue-type plasminogen activator (tPA) through two different mechanisms. PAI-1 inhibits the finger domain-dependent fibrin binding of diisopropylfluorophosphate-inactivated single chain tPA and the kringle-2 domain-dependent fibrin binding of diisopropylfluorophosphate-inactivated two chain tPA. In accordance with the data, preformed complexes of single chain tPA/PAI-1 and of two chain tPA/PAI-1 lost the fibrin binding abilities mediated by the finger and kringle-2 domains, respectively. These effects of PAI-1 appear to be mediated by steric hindrance of the fibrin binding sites after PAI-1 binding to adjacent regions in the functional domains of tPA. We thus propose a model in which a PAI-1 binding site resides in the finger domain of a single chain, and plays a role in the reversible association of single chain tPA and PAI-1. Conformational changes may take place during the conversion of single chain tPA to two chain tPA, resulting in burying of the original PAI-1 binding site and exposure of an alternate PAI-1 binding site on the surface of the kringle-2 domain.     

Functional and structural consequences of aromatic residue substitutions within the kringle-2 domain of tissue-type plasminogen activator.

Aromatic amino acid residues within kringle domains play important roles in the structural stability and ligand-binding properties of these protein modules. In previous investigations, it has been demonstrated that the rigidly conserved Trp25 is primarily involved in stabilizing the conformation of the kringle-2 domain of tissue-type plasminogen activator (K2tpA), whereas Trp63, Trp74, and Tyr76 function in omega-amino acid ligand binding, and, to varying extents, in stabilizing the native folding of this kringle module. In the current study, the remaining aromatic residues of K2tPA, viz., Tyr2, Phe3, Tyr9, Tyr35, Tyr52, have been subjected to structure-function analysis via site-directed mutagenesis studies. Ligand binding was not significantly influenced by conservative amino acid mutations at these residues, but a radical mutation at Tyr35 destabilized the interaction of the ligand with the variant kringle. In addition, as reflected in the values of the melting temperatures, changes at Tyr9 and Tyr52 generally destabilized the native structure of K2tPA to a greater extent than changes at Tyr2, Phe3, and Tyr35. Taken together, results to date show that, in concert with predictions from the crystal structure of K2tpA, ligand binding appears to rely most on the integrity of Trp63 and Trp74, and aromaticity at Tyr76. With regard to aromatic amino acids, kringle folding is most dependent on Tyr9, Trp25, Tyr52, Trp63, and Tyr76. As yet, no obvious major roles have been uncovered for Tyr2, Phe3, or Tyr35 in K2tpA.     

Purification and characterization of active and latent forms of recombinant plasminogen activator inhibitor 1 produced in Escherichia coli.

Plasminogen activator inhibitor 1 (PAI-1), the principal physiological inhibitor of tissue plasminogen activator (tPA), is a protein of 379 amino acids and belongs to the SERPIN family of serine protease inhibitors. We have previously described methods to express [Sisk et al. (1990) Gene 96, 305-309] and purify [Reilly et al. (1990) J. Biol. Chem. 265, 9570-9574] a highly active form of the protein in substantial amounts, from Escherichia coli. Further analyses of this material showed the presence of small but significant amounts of latent rPAI-1. The present paper describes for the first time purification of latent and active forms of rPAI-1 from a single preparation, as well as the functional and structural characteristics of the two forms. Latent rPAI-1, which has properties similar to the latent forms described by other groups, was separated from active rPAI-1 by high-resolution ion-exchange chromatography or by affinity chromatography using immobilized anhydrotrypsin. It had low intrinsic activity (< 5% of active rPAI-1) and was partially reactivated by guanidine hydrochloride treatment or by incubation with vitronectin. Conversion of the active rPAI-1 to the latent form was influenced by temperature and additives including sucrose, EDTA, and arginine. Active and latent rPAI-1 did not show any obvious differences in their primary structures and displayed remarkably similar secondary structures as determined by circular dichroism spectral analyses. However, they did exhibit differences in tryptophan fluorescence, suggesting tertiary structural differences between the two forms.     

Twin-arginine translocation of active human tissue plasminogen activator in Escherichia coli

When eukaryotic proteins with multiple disulfide bonds are expressed at high levels in Escherichia coli, the efficiency of thiol oxidation and isomerization is typically not sufficient to yield soluble products with native structures. Even when such proteins are secreted into the oxidizing periplasm or expressed in the cytoplasm of cells carrying mutations in the major intracellular disulfide bond reduction systems (e.g., trxB gor mutants), correct folding can be problematic unless a folding modulator is simultaneously coexpressed. In the present study we explored whether the bacterial twin-arginine translocation (Tat) pathway could serve as an alternative expression system for obtaining appreciable levels of recombinant proteins which exhibit complex patterns of disulfide bond formation, such as full-length human tissue plasminogen activator (tPA) (17 disulfides) and a truncated but enzymatically active version of tPA containing nine disulfides (vtPA). Remarkably, targeting of both tPA and vtPA to the Tat pathway resulted in active protein in the periplasmic space. We show here that export by the Tat translocator is dependent upon oxidative protein folding in the cytoplasm of trxB gor cells prior to transport. Whereas previous efforts to produce high levels of active tPA or vtPA in E. coli required coexpression of the disulfide bond isomerase DsbC, we observed that Tat-targeted vtPA and tPA reach a native conformation without thiol-disulfide oxidoreductase coexpression. These results demonstrate that the Tat system may have inherent and unexpected benefits compared with existing expression strategies, making it a viable alternative for biotechnology applications that hinge on protein expression and secretion.     

Purification and characterization of recombinant plasminogen activator inhibitor-1 from Escherichia coli

A recombinant form of plasminogen activator inhibitor-1 (rPAI-1) has been purified from lysates of pCE1200, a bacterial expression vector containing the full length PAI-1 gene, by utilizing sequential anion exchange and cation exchange chromatography on Q-Sepharose and S-Sepharose columns. Approximately 140 mg of rPAI-1, estimated at 98% purity on the basis of analytical high performance liquid chromatography, could be obtained from 200 g wet weight of cells. The purified protein exhibited a single Coomassie Blue-stainable band at the region of Mr = 42,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and an NH2-terminal amino acid sequence consistent with the expected translation product of the pCE1200 PAI-1 insert. The rPAI-1 rapidly inhibited single- and two-chain tissue plasminogen activators, as well as urokinase, with apparent second order rate constants in the range of 2-5 x 10(7) M-1 s-1. A specific activity measurement of 250,000 units/mg was calculated for the rPAI-1 based on its ability to inhibit the enzymatic activity of a single-chain tissue plasminogen activator. Stability studies showed that the activity of the rPAI-1 was very stable when stored at temperatures of 25 degrees C or lower, but decayed within hours when stored at 37 degrees C. Sodium dodecyl sulfate treatment, which partially activates the latent form of natural PAI-1, inactivated rPAI-1. These results show that the purified rPAI-1 produced from pCE1200 displays many of the properties associated with the biologically active form of natural PAI-1.     

On the interaction of the finger and the kringle-2 domain of tissue-type plasminogen activator with fibrin. Inhibition of kringle-2 binding to fibrin by epsilon-amino caproic acid

The binding of tissue-type plasminogen activator (t-PA) to fibrin is mediated both by its finger domain and by its kringle-2 domain. In this report, we investigate the relative affinities of these domains for lysine. Human recombinant t-PA deletion-mutant proteins were prepared and their ability to bind to lysine-Sepharose was investigated. Mutants containing the kringle-2 domain bound to lysine-Sepharose, whereas mutants lacking this domain but containing the finger domain, the epidermal growth factor domain or the kringle-1 domain did not bind to lysine-Sepharose. Mutant proteins containing the kringle-2 domain could be specifically eluted from lysine-Sepharose with epsilon-amino caproic acid. This lysine derivative also abolished fibrin binding by the kringle-2 domain but had no effect on the fibrin-binding property of the finger domain. Thus, a lysine-binding site is involved in the interaction of the kringle-2 domain with fibrin but not in the interaction of the finger domain with fibrin. The implications of the nature of these two distinct interactions of t-PA with fibrin on plasminogen activation by t-PA will be discussed.     

High expression vectors for the production of recombinant single-chain urinary plasminogen activator from Escherichia coli

Summary An expression cassette containing a synonymous gene for human single-chain urokinase-type plasminogen activator (Rscu-PA) 5'-flanked by a trp promoter and the Shine-Dalgarno sequence of the xyl A operon of Bacillus subtilis and terminated by the terminators trp A and Tn10 was constructed and inserted into a pBR322 derivative to yield pBF160. When compared to pUK54 trp 207-1 containing the natural scu-PA gene without the Shine-Dalgarno sequence and terminator, the expression efficiency of pBF160 in Escherichia coli strains was improved by one order of magnitude. Replacement of the trp by the tac promoter (pBF171) did not affect expression. Inserting the Shine-Dalgarno sequence and Tn10 terminator into pUK54 trp 207-1 (pWH1320) slightly increased the expression level, whereas elimination of the Shine-Dalgarno sequence and the terminators from pBF160 with almost complete conservation of the synonymous structural gene (pBF191) significantly reduced the expression. Variation of the distance between the Shine-Dalgarno sequence and the start codon between 8 and 10 bp (pBF163) proved irrelevant. In conclusion, poor expression of mammalian genes in E. coli may result from both improperly designed regulatory elements and structural features of the coding region and therefore de-novo synthesis of the gene may be required to obtain satisfactory expression.     

Biosynthesis of sulfated glycoprotein-N-glycans present in recombinant human tissue plasminogen activator.

Recombinant human tissue plasminogen activator expressed in murine epithelial cells carries, in part, sulfated N-glycans, which are characterized by the presence of a NeuAc alpha 3[SO4-6]Gal unit. In order to study the biosynthesis of this novel structural element, corresponding sulfated asialooligosaccharide alditols were resialylated in vitro using a crude sialyltransferase preparation from murine liver which was shown to contain Gal beta 1,3(4)GlcNAc alpha 2,3-sialyltransferase activity. Products were analyzed for transfer of sialic acid residues by anion-exchange HPLC. The results demonstrated that resialylation of SO4-6Gal-residues did not occur. Therefore, it may be concluded that transfer of the sulfate group is the final step in the biosynthesis of this structural epitope.     

DNA-binding domain of human c-Myc produced in Escherichia coli.

We have identified the domain of the human c-myc protein (c-Myc) produced in Escherichia coli that is responsible for the ability of the protein to bind sequence-nonspecific DNA. Using analysis of binding of DNA by proteins transferred to nitrocellulose, DNA-cellulose chromatography, and a nitrocellulose filter binding assay, we examined the binding properties of c-Myc peptides generated by cyanogen bromide cleavage, of mutant c-Myc, and of proteins that fuse portions of c-Myc to staphylococcal protein A. The results of these analyses indicated that c-Myc amino acids 265 to 318 were responsible for DNA binding and that other regions of the protein (including a highly conserved basic region and a region containing the leucine zipper motif) were not required. Some mutant c-Mycs that did not bind DNA maintained rat embryo cell-cotransforming activity, which indicated that the c-Myc property of in vitro DNA binding was not essential for this activity. These mutants, however, were unable to transform established rat fibroblasts (Rat-1a cells) that were susceptible to transformation by wild-type c-Myc, although this lack of activity may not have been due to their inability to bind DNA.     

Crystal structure of the kringle 2 domain of tissue plasminogen activator at 2.4-A resolution.

The crystal structure of the kringle 2 domain of tissue plasminogen activator was determined and refined at a resolution of 2.43 A. The overall fold of the molecule is similar to that of prothrombin kringle 1 and plasminogen kringle 4; however, there are differences in the lysine binding pocket, and two looping regions, which include insertions in kringle 2, take on very different conformations. Based on a comparison of the overall structural homology between kringle 2 and kringle 4, a new sequence alignment for kringle domains is proposed that results in a division of kringle domains into two groups, consistent with their proposed evolutionary relation. The crystal structure shows a strong interaction between a lysine residue of one molecule and the lysine/fibrin binding pocket of a noncrystallographically related neighbor. This interaction represents a good model of a bound protein ligand and is the first such ligand that has been observed in a kringle binding pocket. The structure shows an intricate network of interactions both among the binding pocket residues and between binding pocket residues and the lysine ligand. A lysine side chain is identified as the positively charged group positioned to interact with the carboxylate of lysine and lysine analogue ligands. In addition, a chloride ion is located in the kringle-kringle interface and contributes to the observed interaction between kringle molecules.     

Existence of beta-methylnorleucine in recombinant hirudin produced by Escherichia coli.

A gene encoding for hirudin, a potent thrombin inhibitor, was expressed in Escherichia coli, which is the most widely used host. When the recombinant hirudin analog, CX-397, was overproduced by E. coli (600 mg l(-1)) in the absence of nutrient amino acids in the culture medium, the presence of two derivatives in the final product was observed with extremely increased retention times on reverse-phase high-performance liquid chromatography. Each derivative was due to methylation of an isoleucine residue at Ile29 or Ile59 in the CX-397. The structure was deducible as beta-methylnorleucine (beta MeNle; (2S,3S)-2-amino-3-methylhexanoic acid). The modification pathway of beta MeNle is not thought to be a post-translational modification of the protein because Ile has no functional group in its side-chain. Additionally, beta MeNle is synthesized by mutants of Serratia marcescens that belong to the same family, Enterobacteriaceae, as E. coli (J. Antibiot. 34 (1981a) 1278). These findings suggest that the lack of nutrient amino acids in the culture medium leads to the synthesis of beta MeNle in E. coli, which is then activated by E. coli isoleucyl-tRNA synthetase and incorporated into the overproduced recombinant protein.     

Domain structure and domain-domain interactions of recombinant tissue plasminogen activator

The melting of recombinant tissue plasminogen activator (rtPA) has been investigated by differential scanning calorimetry and fluorescence spectroscopy. At neutral pH, rtPA melts with only partial reversibility in a single sharp peak that can be deconvoluted into four transitions. By contrast, at acidic pH the melting process is spread over a broad range of temperature and is highly reversible. Under these conditions five transitions are resolved by deconvolution analysis. Additional measurements in 6 M guanidinium chloride reveal a sixth transition representing an extremely stable domain. Comparison of the melting curves of several fragments with those of the parent protein allowed all of the transitions to be assigned. The results indicate that rtPA is comprised of six independently folded domains. Two of these domains correspond to the two kringle modules whose thermodynamic properties are similar to those of the kringles in plasminogen. Two additional domains are formed by the epidermal growth factor (EGF)-like and finger modules, the latter of which is extremely stable, requiring the presence of a chemical denaturant for its melting to be observed. The serine protease module contains two more domains which at neutral pH melt cooperatively in a single transition but at low pH melt independently, accounting for the greater number of transitions observed there. Measurements with a 50-kDa fragment lacking the C-terminal half of the serine protease module and with a variant lacking the finger and EGF domains indicate that the serine protease domains interact strongly with and are stabilized by the finger and/or EGF domains in the intact protein. This interaction between domains located at opposite ends of the rtPA molecule produces a more compact structure. A better understanding of such interactions may enhance efforts to engineer plasminogen activators with improved thrombolytic properties.     

Increased expression of recombinant human tissue plasminogen activator in Leishmania tarentolae

Recombinant tissue plasminogen activator (rt-PA) is one of the most important thrombolytic agents for treating cardiovascular obstructions such as stroke. Glycoprotein rt-PA is a serine protease, consisting of 527 amino acids of which 35 are cysteine residues. A variety of recombinant protein expression systems have been developed for heterologous gene expression in prokaryotic and eukaryotic hosts. In recent years, Leishmania tarentolae has been considered because of its safety aspects and special attributes in expression of complex proteins. In this study, two expression cassettes, each one including two copies of t-PA cDNA, were used for integration into the L. tarentolae genome by electroporation. Transformed clones were selected in the presence of appropriate antibiotics. Expression of active rt-PA was confirmed by Western blot and Zymography tests. Real-time PCR analysis was applied to investigate the presence of multiple t-PA gene copies in the parasite genome. Correlation of t-PA gene dosage and production rate was confirmed with real-time PCR. It was shown that the expression level of rt-PA in L. tarentolae is at least 480 IU/mL of culture media. This concentration of rt-PA is seven times higher than what was reported in previous studies in L. tarentolae and some other eukaryotic systems.