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 (BibA_126-398) and a low-resolution small-angle X-ray scattering (SAXS) structure of the full-length N-terminal domain (BibA_34-400) are described. The BibA_126-398 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 BibA_34-400 delayed the degradation of the protein, and circular dichroism spectroscopy of BibA_34-400 suggested a similar secondary-structure composition to that observed in the crystallized BibA_126-398 fragment. A model was generated for the 92 N-terminal residues (BibA_34-125) using structural similarity prediction programs, and a BibA_34-400 model was generated by combining the coordinates of BibA_34-126 and BibA_126-398. The X-ray structure of BibA_126-398 and the model of BibA_34-400 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.