FnbA

FnbA, short for fibronectin-binding protein A, is a surface-anchored adhesin of Staphylococcus aureus. It belongs to the MSCRAMM family (microbial surface components recognizing adhesive matrix molecules) and mediates binding to fibronectin, an abundant extracellular matrix glycoprotein. The fnbA gene encodes FnBPA, and its close paralog FnBPB is encoded by fnbB. Both proteins are anchored to the bacterial cell wall by an LPXTG motif and sortase A, projecting from the surface to interact with host ligands.

Structure and binding: FnBPA contains an N-terminal A-domain followed by fibronectin-binding repeats that together recognize fibronectin. This organization enables effective attachment to host tissues that expose fibronectin, such as damaged epithelium and connective tissue.

Function: By binding fibronectin, FnBPA promotes initial adherence to host cells and surfaces. Through fibronectin bridging to host cell integrins, particularly certain α5β1-mediated pathways, the bacterium can be internalized by nonphagocytic cells. FnBPs also contribute to biofilm formation under specific conditions by enhancing surface attachment.

Regulation and role in disease: Expression of fnbA and fnbB is modulated by global regulatory networks, including sarA and agr, and responding to environmental cues. The combined activity of FnBPA and FnBPB can enhance adhesion and invasion. FnBPs are implicated in a range of S. aureus infections, including endocarditis, osteomyelitis, sepsis, and skin and soft tissue infections.

Clinical relevance: FnBPs are explored as potential vaccine targets and diagnostic markers. Antibodies against FnBPs can inhibit adhesion, and continued research seeks to clarify their roles in pathogenesis and host interaction.