Schematic representation of the modular architecture of PfbA and structure of the parallel β-helix core domain.

<p>(A) The PfbA architecture include the FSIRK and LPXTG sequence motifs that are common Gram-positive export signal peptide and streptococcal cell-wall adherence features, respectively. The surface exposed α/β and cell-wall link domains are conserved across pneumococcal isolates. (B) Cartoon representation of the CAZyme-like 12-stranded β-helix domain solved by X-ray crystallography to 2.28 Å. The 90 residue (T139-Q230, coloured red) N-terminal initiator of the core domain is a conserved precursor of the parallel β-helix fold and includes the calcium-binding site established by the sidechains of E187 and H214 and the carbonyl oxygen of T225. A water coordinated by residues Q296, Q331, E333 and the carbonyl oxygen of G339 is reminiscent of the metal binding sites seen in polysaccharide lyases.