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Protein PAB, an albumin-binding bacterial surface protein promoting growth and virulence
M de Château1, E Holst, L Björck
1Department of Cell and Molecular Biology, Lund University, P. O. Box 94, S-221 00 Lund, Sweden.
Abstract:
The anaerobic bacterium Peptostreptococcus magnus is a human commensal and pathogen. Previous work has shown that strains of P. magnus isolated from patients with gynecological disease (vaginosis) frequently express an immunoglobulin (Ig) light chain-binding protein called protein L. Here we report that strains isolated from localized suppurative infections bind human serum albumin (HSA), whereas commensal isolates bind neither Ig nor HSA. The HSA-binding protein PAB was extracted from the bacterial surface or isolated from the culture supernatant of the P. magnus strain ALB8. Protein PAB was shown to have two homologous HSA-binding domains, GA and uGA. GA is absent in the sequence of a related protein from another P. magnus strain and shows a high degree of homology to the HSA-binding domains of streptococcal protein G. Therefore GA is believed to have recently been shuffled as a module from genes of other bacterial species into the protein PAB gene. This GA module was shown to exhibit a much higher affinity for HSA than uGA and was also found to be present in all of the isolates tested from localized suppurative infections, indicating a role in virulence. Moreover, when peptostreptococci or streptococci expressing the GA module were grown in the presence of HSA, the growth rate was substantially increased. Thus, the HSA binding activity of the GA module adds selective advantages to the bacteria, which increases their virulence in the case of P. magnus strains.
Insights
Peptostreptococcus magnus strains causing infections bind human serum albumin (HSA) using a protein called PAB. A specific domain, GA, enhances bacterial virulence by increasing growth with HSA, suggesting its role in disease.
Area of Science:
- Microbiology
- Bacterial Pathogenesis
- Molecular Biology
Background:
- Peptostreptococcus magnus is a bacterium found in humans, acting as both a commensal and a pathogen.
- Certain P. magnus strains from gynecological infections express protein L, which binds immunoglobulin light chains.
- Strains from suppurative infections exhibit different surface protein expressions compared to commensal strains.
Purpose of the Study:
- To investigate the surface protein interactions of Peptostreptococcus magnus strains isolated from different clinical conditions.
- To identify and characterize the protein responsible for human serum albumin (HSA) binding in virulent P. magnus strains.
- To elucidate the role of HSA-binding proteins in the virulence and growth of P. magnus.
Main Methods:
- Isolation and characterization of bacterial strains from various infection sites and commensal sources.
- Extraction and purification of the human serum albumin (HSA)-binding protein PAB from P. magnus.
- Analysis of protein domains (GA and uGA) within PAB using sequence homology and binding affinity assays.
- Growth rate experiments of bacteria in the presence of HSA.
Main Results:
- Strains from localized suppurative infections bind human serum albumin (HSA), unlike commensal strains.
- The HSA-binding protein PAB possesses two domains, GA and uGA, with GA showing higher affinity for HSA.
- The GA domain, homologous to streptococcal protein G domains, was found in all tested suppurative isolates and is believed to have been acquired horizontally.
- Bacterial growth rate significantly increased when grown in the presence of HSA, particularly for strains expressing the GA module.
Conclusions:
- The GA module of protein PAB plays a crucial role in the virulence of Peptostreptococcus magnus by facilitating HSA binding.
- HSA binding provides a selective advantage, enhancing bacterial growth and contributing to the pathogenesis of suppurative infections.
- The horizontal transfer of the GA module from other bacterial species likely contributed to the evolution of virulence in P. magnus.