Novel structure of the N-terminal helical domain of BibA, a group B streptococcus immunogenic bacterial adhesin

BibA, a group B streptococcus (GBS) surface protein, has been shown to protect the pathogen from phagocytic killing by sequestering a complement inhibitor: C4b-binding protein (C4BP). Here, the X-ray crystallographic structure of a GBS BibA fragment (BibA126-398) and a low-resolution small-angle X-ray scattering (SAXS) structure of the full-length N-terminal domain (BibA34-400) are described. The BibA126-398 fragment crystal structure displayed a novel and predominantly helical structure. The tertiary arrangement of helices forms four antiparallel three-helix-bundle-motif repeats, with one long helix from a bundle extending into the next. Multiple mutations on recombinant BibA34-400 delayed the degradation of the protein, and circular dichroism spectroscopy of BibA34-400 suggested a similar secondary-structure composition to that observed in the crystallized BibA126-398 fragment. A model was generated for the 92 N-terminal residues (BibA34-125) using structural similarity prediction programs, and a BibA34-400 model was generated by combining the coordinates of BibA34-126 and BibA126-398. The X-ray structure of BibA126-398 and the model of BibA34-400 fitted well into the calculated SAXS envelope. One possible binding site for the BibA N-terminal domain was localized to the N-terminal CCP (complement-control protein) domains of the C4BP α-chain, as indicated by the decreased binding of BibA to a ΔCCP1 C4BP α-chain mutant. In summary, it is suggested that the GBS surface protein BibA, which consists of three antiparallel α-helical-bundle motifs, is unique and belongs to a new class of Gram-positive surface adhesins.

Association between maternal Group B Streptococcus surface-protein antibody concentrations and invasive disease in their infants

OBJECTIVES

Group B Streptococcus (GBS) surface-proteins have been shown to be immunogenic and potential vaccine candidates. We aim to determine the association between maternal IgG antibodies to select GBS surface-proteins and invasive GBS disease in their infants.

METHODS

Using a matched case-control study, maternal antibody levels for GBS-immunogenic bacterial adhesin, fibrinogen-binding protein A and pilus-island (PI) PI-1, PI-2a, PI-2b were compared between infants with invasive GBS disease and well-baby controls.

RESULTS

The absolute risk of disease did not differ between cases and colonized controls with increasing antibody concentrations for these surface-proteins. There was, however, a relative risk reduction in invasive disease associated with fibrinogen-binding protein A, with an adjusted odds ratio of 0.04 (95% CI: 0.01-0.69) at antibody levels ≥10,000 AU/ml.

CONCLUSION

We have not demonstrated an association between naturally occurring fibrinogen-binding protein A, GBS-immunogenic bacterial adhesin, and PI surface-protein antibodies and the risk of invasive disease in young infants. These surface-proteins may not be suitable GBS vaccine candidates.