Phosphoglycerate kinase inhibits epithelial cell invasion by group B streptococci

Group B streptococci (GBS) are opportunistic human pathogens that cause infection and invasive disease in newborns, pregnant women and non-pregnant adults. The internalization of GBS into eukaryotic cells occurs in an actin-microfilament dependant process. The objective of our study was to understand what host cell and/or bacterial factors may be involved in this process. We focused on α-actinin, an actin binding protein closely associated with cytoplamsic F-actin in the eukaryotic cell, to determine if it is involved in actin recruitment upon GBS internalization. Initial work revealed that GBS does not recruit α-actinin. However, it was found that α-actinin antibodies bound to the surface of the GBS, suggesting GBS possess surface-exposed actin binding protein(s). Slide agglutination experiments revealed that when the bacteria were emulsified with F-actin, visible agglutination occurred, further suggesting the presence of an actin binding protein on the GBS cell. Western blot analysis found that anti-α-actinin antibodies bound to a 42 kDa protein; mass spectra analysis identified this protein as GBS phosphoglycerate kinase (PGK). Competitive binding assays suggest that the PGK-actin interaction is not a factor in the initial binding of GBS to epithelial cells, however, treating epithelial cells with PGK prior to performing an invasion assay inhibited GBS internalization. This occurred in a dose dependant manner with 10 μg/mL of PGK inhibiting invasion by over 70%, and 50 μg/mL PGK inhibits GBS invasion completely.