Role of N-terminal domain of streptokinase in protein transport

Streptokinase (SK), an extracellular protein of several haemolytic strains of Streptococcus, is utilized as a potent thrombolytic agent for the treatment of various myocardial disorders. Functional properties of SK remain unchanged when the first 13 N-terminal amino acid (aa) residues are removed. At present, role of this segment in protein structure function is unclear. skc gene encoding for the mature SK and its deletion variant, lacking its first 13 aa residues, were cloned and expressed in E. coli. Full length SK, deprived of any leader sequences, was able to translocate slowly, across the cyto-plasmic and outer membranes of E.coli. Whereas, SK derivative, devoid of its first 13 N-terminal aa residues, could not do so. Cell fractionation studies as well as genetic evidences utilizing alkaline phosphatase fusion, point towards the existence of additional information for protein transport, within the N-terminal domain of SK. To further investigate the role of this region in protein secretion, genetic fusions were created in between full length and 13 aa deleted SK with OmpA leader peptide. Studies on kinetics of SK export from E.coli, revealed that translocation of protein is 3-4 times faster when the first 13 N-terminal residues of SK are intact. On the basis of results obtained, it has been proposed that the N-terminus of mature SK maintains the export competent status of protein and, thus, confer speed and efficiency upon the translocation process of streptokinase.

Analysis of autoantibodies to plasminogen in the serum of patients with rheumatoid arthritis

Sera from patients with rheumatoid arthritis containing high titers of anti-streptokinase antibodies were found to contain anti-plasminogen antibodies of the IgG and IgA classes. High titers of anti-plasminogen autoantibodies of the IgA class were also found in sera from patients with systemic lupus erythematosus and Sjögren syndrome. Studies of the immune response to thrombolytic therapy with streptokinase in patients with no prior history of autoimmune disease suggest a strong correlation between streptokinase administration and the appearance of autoantibodies to plasminogen of the IgA class. The IgA anti-plasminogen autoantibody is specific for an epitope in a region of plasminogen which binds streptokinase and the IgG autoantibody reacts with an epitope in the C-terminal region corresponding to the catalytic domain of the plasminogen zymogen. Our findings suggest a different origin for the two classes of anti-plasminogen immunoglobulins in rheumatoid arthritis patients. Since plasminogen binding to rheumatoid synovial fibroblasts is enhanced, the high titers of both classes of anti-plasminogen autoantibodies may add to the localization and perpetuation of the immune response. We suggest that plasminogen may be a target of the immune response in autoimmune disease.

Domain structure, stability and interactions in streptokinase

The structural organization of streptokinase was established through detailed study of its denaturation by differential scanning calorimetry. Streptokinase exhibited a complex endotherm whose shape was sensitive to changing pH. In all cases the endotherms were easily described by four two-state transitions indicating unambiguously the presence of four independently folded domains in the molecule. Two of them were slightly destabilized by lowering pH from 7.0 to 3.8 while the other two were stabilized in this pH range. Two proteolytic fragments of streptokinase were examined, a 37-kDa fragment beginning at Ile1 with a cleavage following Phe62, and a 17-kDa fragment beginning at Lys 147. At pH 8.5, three two-state transitions were observed in the former and two in the latter indicating this many domains in each and suggesting that the fragments are formed by a step-wise removal of individual domains from the parent molecule. Comparison of the melting of these fragments with that of streptokinase allowed the first two transitions in the parent protein to be assigned to the melting of two NH2-terminal domains and the two higher-temperature transitions to the melting of the two COOH-terminal domains. The latter two domains strongly interact with each other since the absence of the most stable extreme COOH-terminal domain in both fragments resulted in a strong destabilization of its neighbor whose melting occurred with a midpoint near room temperature. The two NH2-terminal domains seem to be more independent. One of them melts similarly in the parent protein and both fragments while the other, formed by the 1-146 region, is less stable in the 37-kDa fragment. This destabilization is most probably due to the cleavage after Phe62 which, based on the sequence similarity of streptokinase with serine proteases, may be part of a surface-oriented loop.

Limiting systemic plasminogenolysis reduces the bleeding potential for tissue-type plasminogen activators but not for streptokinase

Clinical experience suggests that thrombolytic-induced bleeding is associated with systemic activation of the thrombolytic system. Using fibrin specific variants of tissue-type plasminogen activator (t-PA) and making use of the apparent fibrin specificity of streptokinase (SK) in the rabbit we tested the hypothesis that minimizing systemic plasmin production and fibrinogenolysis will decrease hemorrhages in models of peripheral bleeding and embolic stroke. t-PA consumed 51% of the available fibrinogen; caused cerebral bleeds and increased peripheral bleeding time. Fibrin-specific variants of t-PA depleted less than 20% of the fibrinogen and did not cause peripheral or cerebral bleeding. However, an equipotent dose of SK converted only 12% of the available fibrinogen but increased bleeding time and caused hemorrhagic conversion in 75% of embolic stroke model animals treated. The data suggest that bleeding associated with tissue-type plasminogen activators is linked to systemic plasmin generation and subsequent fibrinogenolysis. This hypothesis does not explain the mechanism(s) of SK-induced bleeding.

RcsC-mediated induction of colanic acid by secretion of streptokinase in Escherichia coli K-12

The introduction of a plasmid containing skc (streptokinase-coding gene) fused with ompA signal sequence into Escherichia coli K-12 strains, rendered the bacteria mucoid. Measurement of the synthesis of beta-galactosidase from a cps-lacZ fusion (lacZ fusion to a gene necessary for capsule synthesis) showed that the mucoid phenotype was due to induction of the capsular polysaccharide colanic acid synthesis. The introduction of a plasmid carrying skc fused with malE (gene encoding maltose-binding protein) also induced cps-lacZ expression, but intracellular expression of streptokinase in E. coli did not. The cps expression by secretion of streptokinase was diminished to the basal level in a cps-lacZ strain carrying a rcsC mutation. These results show that the secretion of streptokinase in E. coli induces colanic acid synthesis through the RcsC-dependent pathway.

Analogs of human plasminogen that are labeled with fluorescence probes at the catalytic site of the zymogen. Preparation, characterization, and interaction with streptokinase

Fluorescent analogs of the proteinase zymogen, plasminogen (Pg), which are specifically inactivated and labeled at the catalytic site have been prepared and characterized as probes of the mechanisms of Pg activation. The active site induced non-proteolytically in Pg by streptokinase (SK) was inactivated stoichiometrically with the thioester peptide chloromethyl ketone. N alpha-[(acetylthio)acetyl]-(D-Phe)-Phe-Arg-CH2Cl; the thiol group generated subsequently on the incorporated inhibitor with NH2OH was quantitatively labeled with the fluorescence probe, 2-((4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid; and the labeled Pg was separated from SK. Cleavage of labeled [Glu]Pg1 by urokinase-type plasminogen activator (uPA) was accompanied by a fluorescence enhancement (delta Fmax/Fo) of 2.0, and formation of 1% plasmin (Pm) activity. Comparison of labeled and native [Glu]Pg1 as uPA substrates showed that activation of labeled [Glu]Pg1 generated [Glu]Pm1 as the major product, while native [Glu]Pg1 was activated at a faster rate and produced [Lys]Pm1 because of concurrent proteolysis by plasmin. When a mixture of labeled and native Pg was activated, to include plasmin-feedback reactions, the zymogens were activated at equivalent rates. The lack of potential proteolytic activity of the Pg derivatives allowed their interactions with SK to be studied under equilibrium binding conditions. SK bound to labeled [Glu]Pg1, and [Lys]Pg1 with dissociation constants of 590 +/- 110 and 110 and 11 +/- 7 nM, and fluorescence enhancements of 3.1 +/- 0.1 and 1.6 +/- 0.1, respectively. Characterization of the interaction of SK with native [Glu]Pg1 by the use of labeled [Glu]Pg1 as a probe indicated a approximately 6-fold higher affinity of SK for the native Pg zymogen compared to the labeled Pg analog. Saturating levels of epsilon-aminocaproic acid reduced the affinity of SK for labeled [Glu]Pg1 by approximately 2-fold and lowered the fluorescence enhancement to 1.8 +/- 0.1, whereas the affinity of SK for labeled [Lys]Pg1 was reduced by approximately 98-fold with little effect on the enhancement. These results demonstrate that occupation of lysine binding sites modulates the affinity of SK for Pg and the changes in the environment of the catalytic site associated with SK-induced conformational activation. Together, these studies show that the labeled Pg derivatives behave as analogs of native Pg which report functionally significant changes in the environment of the catalytic site of the zymogen.

ATP-regulated activity of the plasmin-streptokinase complex: a novel mechanism involving phosphorylation of streptokinase

Streptokinase, an extracellular protein produced by Streptococci, is capable of activating the human fibrinolytic zymogen plasminogen. The rate of amidolytic activity of the plasminogen-streptokinase complex is greatly diminished by micromolar concentrations of ATP and heparin oligosaccharides. In addition, the plasminogen activator activity of the plasminogen-streptokinase complex is also inhibited by these effectors. ATP and heparin oligosaccharides show structural similarity, suggesting that the inhibition is caused by binding of these molecules to a common newly formed binding pocket in streptokinase, which appears after interaction with plasminogen. Addition of the bivalent cations Ca2+ and Mg2+ reverses the inhibition caused by ATP and heparin. In the presence of ATP and bivalent cations, the complex between plasminogen and streptokinase develops an autophosphorylating activity whose target is the sequence LTSRPAHG in the 4.5 kDa streptokinase N-terminal peptide, which is an early autolysis peptide. This streptokinase N-terminal peptide, which is essential for streptokinase activating activity, may serve, once phosphorylated, in mechanisms related to the pathogenicity of Streptococci. These studies suggest a critical role for plasminogen in regulating the activity of the streptokinase molecule.

Streptokinase contains two independent plasminogen-binding sites

Streptokinase (SK) exerts its thrombolytic effect by activating plasminogen (PG) indirectly, after the formation of an equimolar complex with either PG or plasmin (PN). The location and nature of the PG/PN-binding sites in SK have been explored using limited proteolysis with immobilized trypsin. Employing Western blotting with radiolabeled PG after SDS-PAGE of total tryptic digest, three fragments of MW 7 kD, 19 kD and 31 kD were found to possess PG-binding ability. Each of these fragments was then isolated by reverse phase HPLC and characterised with respect to its sequence, as well as its PG-binding properties by ELISA. These analyses revealed that in addition to a PG-binding site in the region 143-293 reported recently in the literature, there is another distinct, high-affinity and independent PG-binding site, located in the N-terminal region (residues 1-59) of SK. Using a synthetic peptide, the N-terminally located PG-binding-site has been further localised to the region 37-51 of SK. Further, we demonstrate that the PG-binding of this peptide is not mediated through the lysine-binding sites ("Kringles") of PG. This stretch contains a short sequence (LTSRPA) that is also present in the PG-binding domain of human fibronectin.

Comparison of human VDAC1 with streptococcal streptokinase and bovine bactericidal permeability increasing protein: role of structural information in identifying functionally significant domains

Comparison of the primary amino acid sequence of the human X-linked voltage-dependent anion channel, with other sequences in data base searches, identified regions of similarity in streptococcal streptokinase and bovine bactericidal permeability increasing protein. These regions of similarity were in different areas of the protein and were relatively short. However, examination of an empirically derived structural model of the channel showed that each region of similarity in streptokinase and bovine bactericidal permeability increasing protein corresponded to contiguous transmembrane domains within the channel protein. We speculate that these transmembrane domains may be functionally significant for streptokinase and bovine bactericidal permeability increasing protein. These investigations demonstrate the need for incorporation of information regarding secondary, tertiary, and quaternary structures, as well as function, in algorithms used for database searches.

Secretion of streptokinase fusion proteins from Escherichia coli cells through the hemolysin transporter

The hemolysin (HlyA) secretion system was used to achieve the sec-independent secretion of streptokinase (Skc) originating from Streptococcus equisimilis into the medium by Escherichia coli cells. The in-frame fusions of the skc gene, either possessing or lacking a region encoding the signal peptide (SP) with the 3'-end of the hlyA gene of various lengths were analysed. All hybrids retained Skc activity. Hybrid proteins devoided of the N-terminal SP, regardless of length of the hlyA secretion signal (62 vs. 194 amino acids), were secreted into the medium by the E. coli HlyA transporter at similar levels. Considerable amounts of hybrid proteins were still, however, associated with E. coli cells, mainly in the degraded form.

Structure and function of microplasminogen: II. Determinants of activation by urokinase and by the bacterial activator streptokinase

We have used a group of human microplasminogens (mPlg), modified by residue substitutions, insertions, deletions, and chain breaks (1) to study the determinants of productive interactions with two plasminogen activators, urokinase (uPA), and streptokinase (SK); (2) to explore the basis of species specificity in the zymogen-SK complex activity; and (3) to compare active SK complex formation in mPlg and microplasmin (mPlm). Modifications within the disulfide-bonded loop containing the activation site and the adjacent hexadecapeptide upstream sequence showed that uPA recognition elements encompassed R29 at the activation site and multiple elements extending upstream to perhaps 13 residues, all maintained in specific conformational register by surrounding pairs of disulfide bonds. A generally parallel pattern of structural requirements was observed for active zymogen-SK complex formation. Changes within the loop downstream of the activation site were tolerated well by uPA and poorly by SK. The introduction of selected short bovine (Plg) sequences in human mPlg reduced the activity of the resulting SK complexes. The requirements for active SK complex formation are different for mPlg and mPlm.

Differential effects of staphylokinase, streptokinase and tissue-type plasminogen activator on the lysis of retracted human plasma clots and fibrinolytic plasma parameters in vitro

A novel plasma clot lysis system was used to compare the fibrinolytic characteristics of staphylokinase, streptokinase and tissue-type plasminogen activator. 125I-fibrinogen-labelled human plasma clots were formed on needles and mechanically compressed after spontaneous retraction. This model is relatively resistant to lysis and differentiates between fibrin-specific and non-fibrin-specific plasminogen activators. The novel plasminogen activator, recombinant staphylokinase, produced high rates of clot lysis without markedly influencing fibrinogen, plasminogen and alpha 2-antiplasmin in the plasma containing the clots. At equimolar concentrations, streptokinase markedly depleted these parameters in plasma despite low clot lysis rates. Tissue-type plasminogen activator showed relatively high lysis rates at low concentrations, but at higher concentrations, plasminogen depletion caused a decrease in clot lysis. Staphylokinase can be characterised as a fibrin-specific and plasminogen-saving fibrinolytic agent with a high clot lysis potential.

Identification of a plasminogen binding region in streptokinase that is necessary for the creation of a functional streptokinase-plasminogen activator complex

Streptokinase is a plasminogen activator widely used to treat patients with myocardial infarction. However, streptokinase is not a protease, and must first bind and interact with plasminogen to form an enzymatic complex. By measuring the binding of recombinant streptokinase fragments to plasminogen, we have sought, first, to identify a plasminogen binding region in streptokinase and, second, to explore the relation between binding (via this region) and the generation of a functional streptokinase--plasminogen activator complex. Recombinant streptokinase bound in a saturable and specific manner to human Glu-plasminogen with a dissociation constant of 4.2 x 10(-10) M. Recombinant streptokinase fragments spanning amino acids 1-127 and 1-253 could not be shown to bind to Glu-plasminogen, whereas fragments spanning amino acids 1-352, 120-352, and 244-414 bound tightly to plasminogen and each fragment completely inhibited the binding of full-length streptokinase to plasminogen. Although these latter streptokinase fragments formed a complex with plasminogen, enzymatic assays indicated that none of them was capable of generating an active site. When the streptokinase region shared by these three fragments, spanning residues 244-352, was expressed, it also bound plasminogen and competitively inhibited the formation of a functional plasminogen activator complex by full-length streptokinase. Taken together, these data indicate that streptokinase binds to plasminogen with high affinity, that a primary binding region for plasminogen is located within amino acids 244-352, and that binding via this region is necessary for the generation of a functional plasminogen activator complex.

Effect of fucoidan during activation of human plasminogen

Fucoidan [sulfated poly (L-fucopyranose)] was compared with 6-aminohexanoic acid (6-AH) or CNBr-cleaved fibrinogen (CNBr-Fbg) alone or in combination in enhancing the activation of glutamic plasminogen (Glu-Plg) or lysine plasminogen (Lys-Plg) by two-chain tissue plasminogen activator (t-PA) or LMwt-urokinase or by streptokinase. Fucoidan enhanced the t-PA activation of Glu-Plg or Lys-Plg at Plg concentrations greater than 75nM, while stimulation by CNBr-Fbg of t-PA activation followed saturation kinetics of Michaelis-Menton. During t-PA activation of Glu-Plg, a high degree of synergism was observed between 6-AH and fucoidan while the enhancement by CNBr-Fbg was not influenced by fucoidan and was reversed by 6-AH. Fucoidan alone at higher concentrations was effective in enhancing the activation of Glu-Plg by urokinase while the combination of fucoidan and 6-AH showed additive effect in enhancing the activation of Lys-Plg. The activation of Glu-Plg by streptokinase was reversed by fucoidan in a manner similar to that reported for 6-AH. The results are interpreted to suggest that CNBr-Fbg and 6-AH compete with each other for the same lysine binding sites (LBS) on the Plg molecule while fucoidan acted synergistically with 6-AH in enhancing the t-PA activation of Glu-Plg by a different mechanism. The double reciprocal plot for the interaction of Glu-Plg and urokinase also showed a significantly higher affinity between the two in presence of fucoidan.

Structural domains of streptokinase involved in the interaction with plasminogen

Two fragments of recombinant streptokinase, comprising amino acids Val143-Lys293 (17-kDa rSK) or Val143-Lys386 (26-kDa rSK), were cloned and expressed in Escherichia coli, purified to homogeneity and their interactions with plasmin(ogen) were evaluated. Both 17-kDa rSK and 26-kDa rSK bound to plasminogen with a 1:1 stoichiometry and with affinity constants of 3.0 x 10(8) M-1 and 12 x 10(8) M-1, respectively, as compared to 6.3 x 10(8) M-1 for the binding of intact recombinant streptokinase to plasminogen. Binding of 17-kDa rSK to plasminogen-Sepharose was displaced by addition of increasing concentrations of recombinant streptokinase, whereas bound recombinant streptokinase was not displayed by 17-kDa rSK. In equimolar mixtures of plasminogen and 26-kDa rSK, the appearance of amidolytic activity as monitored with a chromogenic substrate, was significantly delayed compared to the equimolar mixture with recombinant streptokinase (60% of the maximal activity after 30 min, compared to maximum activity within < or = 2 min). In contrast, no amidolytic activity was generated in equimolar mixtures of plasminogen and 17-kDa rSK. Plasminogen was rapidly activated by catalytic amounts (1:100 molar ratio) of recombinant streptokinase (60-70% within 10-15 min), whereas only 4% of the plasminogen was activated within 60 min with 26-kDa rSK, and no plasmin was generated with 17-kDa rSK. Complexes of plasmin with 17-kDa rSK or with 26-kDa rSK were very rapidly inhibited by alpha 2-antiplasmin (apparent second-order inhibition rate constant of approximately 2 x 10(7) M-1 s-1), whereas the complex with recombinant streptokinase was resistant to inhibition. With 26-kDa rSK, inhibition by alpha 2-antiplasmin resulted in dissociation of the complexes and recycling of functionally active 26-kDa rSK to other plasminogen molecules; 17-kDa rSK, in contrast, remained associated with the plasmin-alpha 2-antiplasmin complex. These findings suggest that different regions of the streptokinase molecule are involved in binding to plasminogen, in active-site exposure, and in impairment of the inhibition of plasmin by alpha 2-antiplasmin. Thus, the 17-kDa region spanning Val143-Lys293 in streptokinase mediates its binding to plasminogen but does not induce activation. Furthermore, this region does not interfere with the inhibition of the complex with plasmin by alpha 2-antiplasmin.

Analysis of the interaction of group A streptococci with fibrinogen, streptokinase and plasminogen

Group A streptococci demonstrate a number of distinct ways to interact with the human fibrinolytic system to acquire unregulatable cell-surface enzymatic activity. Interactions between bacteria, fibrinogen, streptokinase and plasminogen resulted in acquisition of cell-associated enzymatic activity that can lyse fibrin clots despite the presence of the major physiological plasmin inhibitor, alpha 2-antiplasmin. Western blot analysis of extracted streptococcal surface proteins suggested that binding of fibrinogen to M or M-related proteins mediated the capture of streptokinase-plasminogen complexes to the bacteria. The enzymatic complex formed by reaction of bacteria with fibrinogen, streptokinase and plasminogen was found to be more stable in human plasma than pre-formed plasmin bound directly to the same bacteria strain.

A role for fibrinogen in the streptokinase-dependent acquisition of plasmin(ogen) by group A streptococci

Acquisition of plasmin(ogen) by group A streptococci occurs by two distinct pathways. In addition to the well-characterized direct interaction of plasmin with cell-surface receptors on group A streptococci, a second pathway dependent on streptokinase and a nonplasminogen factor(s) present in human plasma was identified. The role of streptokinase in the second pathway was not merely as a plasminogen activator, since substitution of the plasminogen activator urokinase did not result in the capture of plasmin(ogen) by bacteria in the presence of plasminogen-depleted plasma. However, if streptokinase was added to plasmin that had been generated by treatment of plasminogen with urokinase, the ability of the bacteria to capture plasmin in the presence of plasminogen-depleted plasma was restored. Fibrinogen present in human plasma was identified as the major factor required for streptokinase-dependent uptake of plasmin(ogen) by group A streptococci.

Streptokinase activity among group A streptococci in relation to streptokinase genotype, plasminogen binding, and disease manifestations

Certain genotypic variants of streptokinase (ska) of beta-hemolytic streptococci group A have been associated with acute post-streptococcal glomerulonephritis (APSGN). In our earlier studies on strains isolated from Ethiopian children with various streptococcal disease manifestation, we reported an even distribution of streptokinase genotypes with no association to disease patterns. Considering the possibility that strains could differ in their ability to secrete the protein, levels of streptokinase activity in culture supernatants of these strains were determined by a plasminogen activation assay using a synthetic tripeptide, H-D-valyl-leucyl-lysin-p-nitroaniline, as a substrate. Of the 53 streptococcal group A strains, ten (19%), which belonged to genotype ska4 and ska8, did not activate human plasminogen. These strains did not activate bovine, sheep, horse, rabbit or porcine plasminogens either. They represented at least five M protein and non-typeable serotypes, and were characterized by high human plasminogen binding activity. Six of the 53 strains (11%) harbouring genotype ska3 and ska7 showed low levels of human plasminogen activation. Strains of ska1 and ska2, 37/53, activated human plasminogen at a higher level (p < 0.005). Levels of plasminogen activation were not significantly different among the ska1 and ska2 strains associated with various streptococcal disease manifestations. Antibody levels against streptokinase were higher (p < 0.05) in convalescent sera from acute rheumatic fever and APSGN patients in comparison with sera from other patient categories and healthy controls. Streptokinase genotype and in vitro streptokinase production do not correlate directly to streptococcal disease manifestation, indicating a probable significance of additional streptococcal and/or host factors in the initiation of APSGN.

Substitution of arginine 719 for glutamic acid in human plasminogen substantially reduces its affinity for streptokinase

In isolation human plasminogen possesses no enzymatic activity, yet upon formation of an equimolar complex with the bacterial protein streptokinase, it acquires a plasminogen activator function. The region(s) of plasminogen and of streptokinase which mediate complex formation has (have) not been previously published. Here it is reported that a single-residue substitution (Arg719-->Glu) in the serine protease domain of full-length Glu-plasminogen substantially reduces its affinity for streptokinase. The plasminogen variant displays no other significant differences from the wild-type molecule with respect to activation by two-chain urokinase-type plasminogen activator, recognition by monoclonal antibodies, or ability to undergo conformational change. It is concluded that Arg719 in human plasminogen is an important determinant of the streptokinase binding site, although further sites are likely to contribute both to the affinity of plasminogen for streptokinase and to mechanisms by which the active site is formed within the complex.

Streptokinase activates plasminogen bound to human group C and G streptococci through M-like proteins

An ability to interact with plasminogen or plasmin could provide micro-organisms with a mechanism for invasion. Thus, group A, C and G streptococci secrete streptokinase which binds and activates plasminogen. Some streptococci also express surface structures which bind plasminogen without causing its activation. Plasminogen-binding surface proteins were extracted from one group C and one group G streptococcal isolate. Both proteins were found to bind plasmin, fibrinogen and serum albumin in addition to plasminogen. Gene fragments encoding the streptococcal proteins were amplified by PCR and were subsequently cloned and expressed in Escherichia coli. DNA sequence determination revealed for both genes open reading frames encoding proteins which contained repetitive domains and a carboxyl-terminal unrepeated region that were typical of M and M-like proteins. Though the amino-terminal regions of the group C and G streptococcal proteins demonstrated a rather high overall similarity between themselves, they were not similar to the variable regions of other M-like proteins with one exception: there was a 46% identity between the first 22 amino acids of the group G streptococcal protein and the corresponding sequence of PAM, the plasminogen-binding M-like protein of type M53 group A streptococci. Like the proteins extracted from the streptococci, the recombinant proteins bound plasminogen, fibrinogen and albumin. The three plasma proteins bound to separate sites on the streptococcal M-like proteins. Plasminogen bound by the group C and G streptococcal proteins was readily activated by streptokinase, providing evidence for a functional link between the secreted plasminogen-activator and proteins exposed on the bacterial surface.

Engineering and production of streptokinase in a Bacillus subtilis expression-secretion system

Streptokinase is one of the major blood-clot-dissolving agents used in many medical treatments. With the cloned streptokinase gene (skc) available, production of the secreted streptokinase from various Bacillus subtilis strains was studied. The use of the six-extracellular-protease-deficient strain, WB600, greatly improved the production yield of the secreted streptokinase. A modified skc which has the original skc promoter and signal sequence replaced with the B. subtilis levansucrase promoter and signal sequence was also constructed. B. subtilis carrying either the wild-type or the modified skc produces streptokinase at a comparable level. Even with WB600 as the expression host, a C-terminally-processed streptokinase was also observed. Through region-specific combinatorial mutagenesis around the C-terminal processing sites, streptokinase derivatives resistant to C-terminal degradation were engineered. One of the derivatives showed a 2.5-fold increase in specific activity and would potentially be a better thrombolytic agent.

[Inhibition of esterase by L-lysine, the activator and fibrinolytic activity of the plasmin-streptokinase activator complex]

The effect of L-lysine on some reactions catalysed by plasmin and the plasmin-streptokinase activator complex has been studied. The constants for competitive inhibition by L-lysine of the benzyloxycarbonyl-L-lysine p-nitrophenyl ester hydrolysis by the activator (Ki 116 mM), of the plasminogen activation by the activator (Ki 8 mM) and of fibrinolysis by plasmin (Ki 3 mM) were determined. It was found that L-lysine at concentrations below 0.05 M, which do not affect the activator's esterase activity, does inhibit fibrinolysis by plasmin and the activator complex. The effect of L-lysine on fibrinolysis under the action of the activator is complex: it inhibits both the activation of clot-entrapped plasminogen by the activator and lysis of fibrin by the plasmin formed. This inhibitory action of L-lysine is largely related to the fact that it lowers the sorption of the activator, plasminogen and plasmin on fibrin, competing with fibrin for their lysine-binding sites as well as worsens the activator-plasminogen binding.