Multiple Interactions of FbsA, a Surface Protein from Streptococcus agalactiae, with Fibrinogen:  Affinity, Stoichiometry, and Structural Characterization

Abstract

Streptococcus agalactiae is an etiological agent of several infective diseases in humans. We previously demonstrated that FbsA, a fibrinogen-binding protein expressed by this bacterium, elicits a fibrinogen-dependent aggregation of platelets. In the present communication, we show that the binding of FbsA to fibrinogen is specific and saturable, and that the FbsA-binding site resides in the D region of fibrinogen. In accordance with the repetitive nature of the protein, we found that FbsA contains multiple binding sites for fibrinogen. By using several biophysical methods, we provide evidence that the addition of FbsA induces extensive fibrinogen aggregation and has noticeable effects on thrombin-catalyzed fibrin clot formation. Fibrinogen aggregation was also found to depend on FbsA concentration and on the number of FbsA repeat units. Scanning electron microscopy evidentiated that, while fibrin clot is made of a fine fibrillar network, FbsA-induced Fbg aggregates consist of thicker fibers organized in a cage-like structure. The structural difference of the two structures was further indicated by the diverse immunological reactivity and capability to bind tissue-type plasminogen activator or plasminogen. The mechanisms of FbsA-induced fibrinogen aggregation and fibrin polymerization followed distinct pathways since Fbg assembly was not inhibited by GPRP, a specific inhibitor of fibrin polymerization. This finding was supported by the different sensitivity of the aggregates to the disruptive effects of urea and guanidine hydrochloride. We suggest that FbsA and fibrinogen play complementary roles in contributing to thrombogenesis associated with S. agalactiae infection.

Characterization of Fibrinogen Binding by Glycoproteins Srr1 and Srr2 of Streptococcus agalactiae

“…These adhesins appear to be structurally unrelated to each other or the SRR proteins, and neither protein is associated with a DLL binding mechanism. The binding site on fibrinogen for FbsA is contained within the D fragment (60) but has not been further characterized. No binding site for FbsB has as yet been identified.…”

Role of the Serine-Rich Surface Glycoprotein Srr1 of Streptococcus agalactiae in the Pathogenesis of Infective Endocarditis

“…A number of bacterial surface structures have been shown to mediate binding to fibrinogen, such as ClfA, ClfB, FnbA and Efb of S. aureus , and the Fss proteins of Enterococcus faecalis [33]–[38]. We recently identified Srr1 of GBS as a fibrinogen-binding protein that was important for bacterial attachment to microvascular endothelial cells and CNS invasion [26].…”

Extensive Adaptive Changes Occur in the Transcriptome of Streptococcus agalactiae (Group B Streptococcus) in Response to Incubation with Human Blood

“…For example, the gene ( fbsA ) encoding the major GBS receptor for fibrinogen was significantly up-regulated in blood. FbsA potentiates thrombin-catalyzed fibrin clot formation [37]. Notably, a fbsA mutant grows poorly in human blood and loses its ability to induce platelet aggregation [38], [39].…”