Cloning, Sequencing and Functional Overexpression of the Streptococcus equisimilis H46A gapC Gene Encoding a Glyceraldehyde‐3‐phosphate Dehydrogenase that also Functions as a Plasmin(ogen)‐Binding Protein

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Abstract
We previously identified DNA sequences involved in the function of the complex promoter of the streptokinase gene from Streptococcus equisimilis H46A, a human serogroup C strain known to express this gene at a high level. As a prerequisite to understanding possible mechanisms that control the balance between the plasminogen activating and plasmin(ogen) binding capacities of H46A, we describe here its gapC gene encoding glyceraldehyde-3-phosphate dehydrogenase (GraP -DH, EC 1.2.1.12), a glycolytic enzyme apparently transported to the cell surface where it functions as a plasmin(ogen) binding protein. The gapC gene was cloned and sequenced and found to code for a 336-amino-acid polypeptide (≈35.9 kDa) exhibiting 94.9% sequence identity to the Plr protein from Streptococcus pyogenes shown by others to be capable of plasmin binding [Lottenberg, R., Broder, C. C., Boyle, M. D., Kain, S. J., Schroeder, B. L. & Curtiss, R. III (1992) J. Bacteriol. 174, 5204–5210. To study the properties of the GapC protein, its gene was inducibly overexpressed in Escherichia coli from QIAexpress expression plasmids to yield the authentic GapC or (His)6 GapC carrying a hexahistidyl N-terminus to permit affinity purification. Both proteins were functionally active, exhibiting specific GraP -DH activities of about 80 kat/mol (≈130 U/mg) after purification. Their binding parameters [association (ka) and dissociation (kd) rate constants, and equilibrium dissociation constants (Kd= kd/ka] for the interaction with human Glu-plasminogen and plasmin were determined by real-time biospecific interaction analysis using the Pharmacia BIAcore instrument. For comparative purposes, the commercial GraP -DH from Bacillus stearothermophilus (BstGraP -DH), a nonpathogenic organism, was included in these experiments. The Kd, values for binding of plasminogen to GapC, (His)6 GapC and BstGraP -DH were 220 nM, 260 nM and 520 nM, respectively, as compared to 25 nM, 17 nM and 98 nM, respectively, for the binding to plasmin. These data show that both the zymogen and active enzyme posse low-affinity binding sites for the gapC gene product and that the hexahistidyl terminus does not affect its function. Prior limited treatment with plasmin enhanced the subsequent plasminogen binding capacity of all three GraP -DHs, presumably by the exposure of new C-terminal lysine residues for binding to the zymogen.

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