The thermostability of natural variants of bacterial plasminogen-activator staphylokinase

Lipid modification of staphylokinase and its implications on stability and activity

Thrombolytic agents are routinely used to dissolve blood clot by activating fibrinolytic system. Among different thrombolytic agents available, staphylokinase (SAK) is gaining much attention because of their fibrin specificity and reduced inhibition by α2 antiplasmin. Though SAK had exhibited less circulatory half life, they are equipotent to tissue plasminogen activator and streptokinase and had shown more potency for clot dissolution during retracted thrombi. In this study, SAK was lipid modified at the N-terminal by a protein engineering approach to enhance its stability and activity. Native SAK as well as the gene encoding SAK with lipobox was cloned into E. coli GJ1158 using pRSET-B expression vector for higher expression. The lipid modification of SAK was confirmed by a mobility shift of 1.3 kDa against the 15.5 kDa of native SAK using tricine SDS-PAGE. Lipid modification of SAK was confirmed by LC MS/MS. The secondary structure analysis was carried out using circular dichroism and deconvoluted fourier transform infrared spectroscopy. LMSAK was found to have a slightly higher denaturation temperature compared to SAK. The improved stablility and activity of lipid modified SAK was studied by heated plasma agar plate assay and mouse tail bleeding test.

High-level expression of a biologically active staphylokinase in Pichia pastoris

Staphylokinase (SAK) as the third generation thrombolytic molecule is a promising agent for the treatment of thrombosis. SAK variant of SAKфC was expressed in Pichia pastoris strains KM71H and GS115. The codon adaptation index of SAK was improved from 0.75 to 0.89. The expression of recombinant SAK (rSAK) reached to its maximum (310 mg/L of the culture medium) after 48-hr stimulation with 3% methanol and remained steady until day 5. The maximum activity of the enzyme was at pH 8.6 and 37°C. It was highly active at temperatures 20-37°C and pH ranges of 6.8-9 (relative residual activity more than 80%). It was determined that rSAK was 73.8% of the total proteins secreted by P. pastoris KM71H into the culture media. The specific activities of rSAK were measured as 9,002 and 21,042 U/mg for the nonpurified and purified proteins, respectively. The quantity of the purified protein (>99% purity) was 720 µg/mL with a purification factor of 2.34. Western blot analysis showed two bands of nearly 22 and 18.6 kDa. It was concluded that P. pastoris is a proper host for expression of biologically active and endotoxin-free rSAK due to its high expression and low protein impurity in culture supernatant.

Staphylokinase reduces plasmin formation by endogenous plasminogen activators

Hyperfibrinolysis is a consequence of imbalance between fibrinolytic activators and their inhibitors. Increased levels of circulating plasminogen (Plg) activators such as tissue- or urokinase-type plasminogen activators (tPA or uPA respectively) are the most common causes of hyperfibrinolysis, occasionally causing major hemorrhages. We found that staphylokinase (SAK), a well-known Plg activator of bacterial origin, inhibits Plg activation mediated by endogenous tPA and uPA. Furthermore, mixture of SAK with tPA led to a significantly reduced Plg-dependent fibrinolysis. This inhibitory effect was exerted through direct action of SAK on Plg rather than indirectly on tPA or uPA. Inhibition of Plg activation by SAK is readily abrogated by interaction of SAK with human neutrophil peptides (HNPs). Finally, we show that NH2-terminal residues of SAK are important for the inhibitory effect of SAK on tPA- and uPA-mediated Plg activation. In conclusion, SAK reduces tPA/uPA-mediated Plg activation by means of SAK.Plg complex formation, consequently downregulating tPA/uPA-induced fibrinolysis.

Glycosylation of asparagine-28 of recombinant staphylokinase with high-mannose-type oligosaccharides results in a protein with highly attenuated plasminogen activator activity

The properties of recombinant staphylokinase (SakSTAR) expressed in Pichia pastoris cells have been determined. The single consensus N-linked oligosaccharide linkage site in SakSTAR (at Asn28 of the mature protein) was occupied in approximately 50% of the expressed protein with high-mannose-type oligosaccharides. The majority of these glycans ranged in polymerization state from Man8GlcNAc2 to Man14GlcNAc2, with the predominant species being Man10GlcNAc2 and Man11GlcNAc2. Glycosylated SakSTAR (SakSTARg) did not differ from its aglycosyl form in its aggregation state in solution, its thermal denaturation properties, its ability to form a complex with human plasmin (hPm), the amidolytic properties of the respective SakSTAR-hPm complexes, or its ability to liberate the amino-terminal decapeptide required for formation of a functional SakSTAR-hPm plasminogen activator complex. However, this latter complex with SakSTARg showed a greatly reduced ability to activate human plasminogen (hPg) as compared with the same complex with the aglycosyl form of SakSTAR. We conclude that glycosylation at Asn28 does not affect the structural properties of SakSTAR or its ability to participate in the formation of an active enzymatic complex with hPm, but it is detrimental to the ability of the SakSTAR-hPm complex to serve as a hPg activator. This is likely due to restricted access of hPg to the active site of the SakSTARg-hPm complex.

NH2-terminal structural motifs in staphylokinase required for plasminogen activation

Staphylokinase (Sak) forms an inactive 1:1 stoichiometric complex with plasminogen which requires both conversion of plasminogen to plasmin and hydrolysis of the Lys10-Lys11 peptide bond of Sak to become a potent plasminogen activator (Schlott, B., Guhrs, K.-H., Hartmann, M., Rocker, A., and Collen, D. (1997) J. Biol. Chem. 272, 6067-6072). Exposure of a positively charged NH2-terminal amino acid after hydrolysis of Sak is a major determinant of the plasminogen-activating potential, but in itself is neither necessary nor sufficient. Here, the structural motifs of the NH2-terminal region Lys11-Gly-Asp-Asp-Ala-Ser16-Tyr-Phe-Glu of processed Sak, required for plasminogen activating potential, were studied by deletion and substitution mutagenesis. Expression in Escherichia coli of variants with deletion of 11, 14, 15, or 16 NH2-terminal amino acids yielded correctly processed but inactive molecules. Expression of their homologues with the NH2-terminal amino acid substituted with Lys-generated derivatives from which the NH2-terminal initiation Met was no longer removed, yielding inactive (</= 10%) Sak42DDeltaN11(M),G12K, active (>50%) Sak42DDeltaN14(M), A15K and Sak42DDeltaN15(M),S16K, and inactive Sak42DDeltaN16(M),Y17K. Lys variants without NH2-terminal Met, generated from fusion proteins in which a His6 tag and a factor Xa recognition sequence were linked to the NH2 terminus of the Sak variants, were indistinguishable from their NH2-terminal Met-containing counterparts. All variants studied had intact affinities for plasminogen as measured by biospecific interaction analysis. The activity of Sak42DDeltaN11(M),G12K could be restored by additional substitution of both Asp13 and Asp14 with Asn, yielding active Sak42DDeltaN11(M),G12K, D13N, D14N, whereas substitution in Sak42DDeltaN16(M),Y17K of Phe18 and Glu19 with Asn yielded inactive Sak42DDeltaN16(M),Y17K,F18N,E19N. These data, in combination with the recent finding that the 20 NH2-terminal amino acids of Sak lack secondary structure, suggest that the NH2-terminal region of Sak is not required for binding to plasmin/plasminogen, but that a positively charged amino acid in the ultimate or penultimate NH2-terminal position corresponding to amino acids 11-16 of this flexible region participates in the reconfiguration of the active site of the plasmin molecule to endow it with plasminogen-activating potential.

Nuclear magnetic resonance solution structure of the plasminogen-activator protein staphylokinase

Staphylokinase, a 15.5 kDa protein from Staphylococcus aureus, is a plasminogen activator which is currently undergoing clinical trials for the therapy of myocardial infarction and peripheral thrombosis. The three-dimensional (3D) NMR solution structure has been determined by multidimensional heteronuclear NMR spectroscopy on uniformly 15N- and 15N,13C-labeled samples of staphylokinase. Structural constraints were obtained from 82 3JHNH alpha as well as 22 3JNH beta scalar coupling constants and 2345 NOE cross-peaks, derived from 15N-edited and 13C-edited 3D NOE spectra. NOE cross-peak assignments were confirmed by analysis of ¿15N,13C¿-edited and ¿13C,13C¿-edited 4D NOE spectra. The structure is presented as a family of 20 conformers which show an average rmsd of 1.02 +/- 0.15 A from the mean structure for the backbone atoms. The tertiary structure of staphylokinase shows a well-defined global structure consisting of a central 13-residue alpha-helix flanked by a two-stranded beta-sheet, both of which are located above a five-stranded beta-sheet. Two of the connecting loops exhibit a higher conformational heterogeneity. Overall, staphylokinase shows a strong asymmetry of hydrophilic and hydrophobic surfaces. The N-terminal sequence, including Lys10 which is the site of the initial proteolytic cleavage during activation of plasminogen, folds back onto the protein core, thereby shielding amino acids with functional importance in the plasminogen activation process. From a comparison of the structure with mutational studies, a binding region for plasminogen is proposed.

A novel variant of staphylokinase gene from Staphylococcus aureus ATCC 29213

The processes of hemostasis and thrombolysis are elegantly regulated in order to ensure normal functions of vascular system. A search for new plasminogen activators as thrombolytic agents has been carried out for the purpose of clinical applications to modulate thrombolytic processes. In the current work, several strains and clinical isolates of Staphylococcus aureus were screened for the fibrinolytic activity. The DNA sequences of staphylokinase gene in the strains expressing 15 kDa protein with staphylokinase activity were determined and subsequently compared with three known staphylokinase gene sequences. From the sequence comparison a new variant of staphylokinase gene has been identified in ATCC 29213 strain. The gene product needs to be further characterized and tested for the therapeutic potential.

Three-dimensional structure of staphylokinase, a plasminogen activator with therapeutic potential

The three-dimensional structure of staphylokinase has been determined at 1.8 A. The puntative site of interaction with plasminogen was identified and epitopes were mapped.

Recombinant staphylokinase variants with altered immunoreactivity. I: Construction and characterization

BACKGROUND

Recombinant staphylokinase offers promise for thrombolytic therapy in acute myocardial infarction, but it is immunogenic. Although reduced immunogenicity of heterologous proteinaceous drugs by protein engineering has not previously been reported, an attempt was made to achieve this in staphylokinase by site-specific mutagenesis.

METHODS AND RESULTS

Biospecific interaction analysis of a panel of 17 murine monoclonal antibodies against recombinant staphylokinase (SakSTAR variant) identified three nonoverlapping immunodominant epitopes, two of which could be eliminated by substitution mutagenesis of clusters of two or three charged amino acids with alanine. Circulating anti-staphylokinase antibodies elceted in patients by treatment with SakSTAR were incompletely (< 90%) absorbed by these mutants. Therefore, the combination variants K35A,E38A,K74A,E75A,R77A (SakSTAR.M38) and K74A,E75A,R77A,E80A,D82A (SakSTAR.M89) were constructed, expressed in Escherichia coli, highly purified by ion-exchange and hydrophobic interaction chromatography, and characterized. These variants had specific activities that were approximately half that of SakSTAR, and they combined the reduced reactivity with the panels of monoclonal antibodies of their parent molecules. Absorption of circulating antibodies elicited in patients by treatment with SakSTAR was incomplete in 13 of 16 patients (median values, 68% and 65% with SakSTAR.M38 and SakSTAR.M89, respectively).

CONCLUSIONS

SakSTAR contains three immunodominant epitopes, two of which were eliminated by site-directed mutagenesis, yielding combination mutants with relatively maintained specific activities that were not recognized by a significant fraction of the antibodies elicited in patients by treatment with wildtype SakSTAR. These mutants appear to be suitable for more detailed investigation of their thrombolytic and antigenic properties.

A correlation between thermal stability and structural features of staphylokinase and selected mutants: a Fourier-transform infrared study

Variants of recombinant staphylokinase (Sak) were investigated by Fourier-transform infrared spectroscopy: Sak (wild type), Sak-M26A, Sak-M26L, and Sak-G34S/R36G/R43H (Sak-B). Estimation of the secondary structure and hydrogen-deuterium exchange experiments revealed the existence of fast-exchanging and strongly solvent-exposed fractions of the helical structures in the two samples Sak and Sak-M26L. These two samples are also thermally less stable with unfolding transition temperatures of 43.7 degrees C (Sak) and 43.5 degrees C (Sak-M26L), respectively. On contrast, Sak-M26A and Sak-G34S/R36G/R43H have a slower hydrogen-deuterium exchange, have a smaller solvent-exposed portion of the helical part, and are more resistant against thermal unfolding; the transition temperatures are 51.7 degrees C and 59.3 degrees C, respectively. The secondary structure analysis was performed by two different approaches, by curve-fitting after band narrowing and by pattern recognition (factor analysis) based upon reference spectra of proteins with known crystal structure. Within the limits of the used methods, we are unable to detect significant differences in the secondary structure of the four variants of Sak. According to the results of the factor analysis, the portions of secondary structure elements were obtained to 16-20% alpha-helix, 28-30% beta-sheet, 23-27% turns, 28-30% irregular (random) and other structure. The sharp differences in the specific plasminogen-activating capacity (Sak, Sak-G34S/R36G/R43H and Sak-M26L are fully active, but Sak-M26A does not form a stable complex with plasminogen) are not reflected in the structural features revealed by the infrared spectra of this study.