Crystal structure of PfbA, a surface adhesin of Streptococcus pneumoniae, provides hints into its interaction with fibronectin

PfbA is a surface adhesin and invasin of Streptococcus pneumoniae that binds to human fibronectin and plasminogen of the host extracellular matrix. It is a virulence factor for its pathogenesis. The crystal structure of recombinant PfbA150–607 from S. pneumoniae strain R6, was determined using multiwavelength anomalous dispersion (MAD) method and refined to 1.90Å resolution. The structure of rPfbA150–607 revealed that residues Thr150 to Lys570 form a rigid parallel beta helix, followed by a short disordered region (571–607) that consists of beta hairpins. The structural organization of the beta helix resembles that of polysaccharide-modifying enzymes. The structural and sequence features essential for fibronectin-binding observed in the well characterized fibronectin-binding proteins such as FnBPA of Staphylococcus aureus, SfbI of Streptococcus pyogenes and BBK32 of Borrelia burgdorferi has been found in rPfbA150–607. Based on this, it is predicted that the disordered region following the beta helix could be the fibronectin-binding region in PfbA. PfbA150–607 contains relatively high number of surface exposed lysines and these residues are probably involved in binding plasmin(ogen) as observed in other plasminogen-binding proteins.