Protein PAB, an albumin-binding bacterial surface protein promoting growth and virulence.

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.