Novel fold and assembly of the repetitive B region of the Staphylococcus aureus collagen-binding surface protein

Background: The Staphylococcus aureus collagen-binding protein Cna mediates bacterial adherence to collagen. The primary sequence of Cna has a non-repetitive collagen-binding A region, followed by the repetitive B region. The B region has one to four 23 kDa repeat units (B₁-B₄), depending on the strain of origin. The affinity of the A region for collagen is independent of the B region. However, the B repeat units have been suggested to serve as a 'stalk' that projects the A region from the bacterial surface and thus facilitate bacterial adherence to collagen. To understand the biological role of these B-region repeats we determined their three- dimensional structure. Results: B₁ has two domains (D₁ and D₂) placed side-by-side. D₁ and D₂ have similar secondary structure and exhibit a unique fold that resembles but is the inverse of the immunoglobulin-like (IgG-like) domains. Comparison with similar immunoglobulin superfamily (IgSF) structures shows novel packing arrangements between the D₁ and D₂ domains. In the B₁B₂ crystal structure, an omission of a single glycine residue in the D₂-D₃ linker loop, compared to the D₁-D₂ and D₃-D₄ linker loops, resulted in projection of the D₃ and D₄ in a spatially new orientation. We also present a model for B₁B₂B₃B₄. Conclusions: The B region of the Cna collagen adhesin has a novel fold that is reminiscent of but is inverse in nature to the IgG fold. This B region assembly could effectively provide the needed flexibility and stability for presenting the ligand binding A region away from the bacterial cell surface.