A fibrinogen receptor from group B Streptococcus interacts with fibrinogen by repetitive units with novel ligand binding sites

Group B Streptococcus (GBS) is a frequent cause of bacterial sepsis and meningitis in neonates. During the course of infection, GBS colonizes and invades a number of host compartments, thereby interacting with different host proteins. In the present report, we describe the isolation of the fbsA gene, which encodes a fibrinogen receptor from GBS. The deduced FbsA protein is characterized by repetitive units, each 16 amino acids in length. Sequencing of the fbsA gene from five different GBS strains revealed significant variation in the number of repeat-encoding units. The deletion of the fbsA gene in the genome of GBS 6313 completely abolished fibrinogen binding, suggesting that FbsA is the major fibrinogen receptor in this strain. Growth of the fbsA deletion mutant in human blood was significantly impaired, indicating that FbsA protects GBS from opsonophagocytosis. In Western blot experiments with truncated FbsA proteins, the repeat region of FbsA was identified as mediating fibrinogen binding. Using synthetic peptides, even a single repeat unit of FbsA was demonstrated to bind to fibrinogen. Spot membrane analysis and competitive binding experiments with peptides carrying single amino acid substitutions allowed the prediction of a fibrinogen-binding motif with the consensus sequence G-N/S/T-V-L-A/E/M/Q-R-R-X-K/R/W-A/D/E/N/Q-A/F/I/L/V/Y-X-X-K/R-X-X.