Borrelia outer surface protein C is capable of human fibrinogen binding

Outer surface protein C (OspC) is one of the most abundant surface lipoproteins produced during early infection by the Borrelia spirochete, the causative agent of Lyme disease. The high sequence variability of the ospC gene results in the production of several and strongly divergent OspC types. One of the known roles of OspC is the recruitment of blood components, including complement regulators, to facilitate the bloodstream survival of Borrelia at an essential stage of host infection. Here, we identify and describe a new interaction between OspC and human fibrinogen. To test the ability of OspC to bind fibrinogen, we developed a microscale thermophoresis assay using four fluorescently labeled types of OspC. We show that OspC binds fibrinogen tightly, with nanomolar K_d , and that the binding depends on the OspC type. The binding assays combined with SAXS studies allowed us to map the OspC-binding site on the fibrinogen molecule. Spectrometric measurements of fibrinogen clotting in the presence of OspC indicate that OspC negatively influences the clot formation process. Taken together, our findings are consistent with the hypothesis that OspC interacts with blood protein partners to facilitate Borrelia spreading by the hematogenous route.