The Multivalent Role of Fibronectin-Binding Proteins A and B (FnBPA and FnBPB) of Staphylococcus aureus in Host Infections

Abstract

Staphylococcus aureus, one of the most important human pathogens, is the causative agent of several infectious diseases including sepsis, pneumonia, osteomyelitis, endocarditis and soft tissue infections. This pathogenicity is due to a multitude of virulence factors including several cell wall-anchored proteins (CWA). CWA proteins have modular structures with distinct domains binding different ligands. The majority of S. aureus strains express two CWA fibronectin (Fn)-binding adhesins FnBPA and FnBPB (Fn-binding proteins A and B), which are encoded by closely related genes. The N-terminus of FnBPA and FnBPB comprises an A domain which binds ligands such as fibrinogen, elastin and plasminogen. The A domain of FnBPB also interacts with histones and this binding results in the neutralization of the antimicrobial activity of these molecules. The C-terminal moiety of these adhesins comprises a long, intrinsically disordered domain composed of 11/10 fibronectin-binding repeats. These repetitive motifs of FnBPs promote invasion of cells that are not usually phagocytic via a mechanism by which they interact with integrin α₅β₁ through a Fn mediated-bridge. The FnBPA and FnBPB A domains engage in homophilic cell-cell interactions and promote biofilm formation and enhance platelet aggregation. In this review we update the current understanding of the structure and functional properties of FnBPs and emphasize the role they may have in the staphylococcal infections.

Keywords: FnBPA; FnBPB; Staphylococcus aureus; adhesin; binding mechanism; extracellular matrix protein; invasin; virulence factor.

FIGURE 1

Domain organization of fibronectin-binding protein A and B (FnBPA and FnBPB). The primary translation products of FnBPA and FnBPB proteins contain a signal sequence (S) at the N-terminus and a sorting signal (SS) at the C-terminus. The N-terminal A region of both proteins contains three separately folded subdomains, termed N1, N2 and N3. N2 and N3 form IgG-like folds that combined bind ligands such as fibrinogen by the DLL mechanism. Located distal to the A domain is an unstructured region consisting of tandemly arranged motifs (11 in FnBPA and 10 in FnBPB) that individually bind to the N-terminal domain of fibronectin. Also shown are the locations of proline rich region (PRR) and wall spanning region (W).

FIGURE 2

Schematic representation of FnBPs ligands. (A) Fibronectin. Each Fn chain consists of three different types of modules with distinct folds: FnI, FnII and FnIII. Sets of adjacent modules form functional protease-stable domains, including the N-terminal domain, the gelatin-binding domain and the cell-binding domain. The cell-binding domain harbors the RGD motif within the 10th FnIII module and “synergy site” in the 9th FnIII module, both involved in interaction with integrin α₅β₁. Cellular Fn includes two variable proportions of alternatively spliced FnIII modules EDB and EDA (extracellular domains B and A) and one connecting segment (IIICS). Binding sites for different binding partners are indicated. (B) Fibrinogen. Fbg is composed of three pairs of non-identical chains named Aα, Bβ and γ. The three chains are connected by disulfide bonds and are in hand to hand conformation forming a central E domain from which fibrinopetides A and B are released by the action of thrombin. The E domain is connected to the C-terminal D domains by coiled-coil region. The C-terminus of the γ chain includes the binding sites for integrin GPIIb/IIIa and FnBPA and FnFPB. (C) Plasminogen. Circulating Plg comprises at the N-terminal end a signal peptide (SP) and a preactivation region (PAP) followed by five consecutive disulphide-bonded kringle domains (K1-K5) and a serine protease domain (P). The K4 domain is supposed to be the binding site for FnBPs. The catalytic triad in the protease domain formed by residues His⁶⁰³, Asp⁶⁴⁶ and Ser⁷⁴¹ is also reported. (D) Tropoelastin. Tropoelastin is a protein comprising a signal peptide (SP) followed by alternating hydrophobic and cross-linking regions. The protein is functinctinally composed of an N-terminal domain that confers spring-like properties to the molecule and a C-terminal domain recognizing the α_vβ₃ integrin. The domains are connected by a bridge region that contains sites for the coarcevation process.

FIGURE 3

Schematic representation of the binding mechanisms of Fbg and Fn. (A) Dock, lock and latch mechanism. The N2 and N3 subdomains of the MSCRAMM form a open trench where a Fbg peptide (red) can be inserted. After ligand binding, the C-terminal unstructured extension of the N3 subdomain undergoes conformational changes so that the segment covers the bound peptide and locks it in place (magenta) and binds to a β-strand in the N2 subdomain (green). Adapted from Geoghegan and Foster, 2017. (B) Tandem β-zipper mechanism. The Fn-binding repeats form a disordered region between the A domain and the C-terminal end of FnBPs. Each linear repeat (red) can complex with the type I modules (green) of the N-terminal domain forming additional short β-strands and extending the triple-stranded β-sheet on each module. Adapted from Matthews and Potts (2013).