Borrelia burgdorferi enolase is a surface-exposed plasminogen binding protein

Borrelia burgdorferi is the causative agent of Lyme disease, the most commonly reported arthropod-borne disease in the United States. B. burgdorferi is a highly invasive bacterium, yet lacks extracellular protease activity. In order to aid in its dissemination, B. burgdorferi binds plasminogen, a component of the hosts' fibrinolytic system. Plasminogen bound to the surface of B. burgdorferi can then be activated to the protease plasmin, facilitating the bacterium's penetration of endothelial cell layers and degradation of extracellular matrix components. Enolases are highly conserved proteins with no sorting sequences or lipoprotein anchor sites, yet many bacteria have enolases bound to their outer surfaces. B. burgdorferi enolase is both a cytoplasmic and membrane associated protein. Enolases from other pathogenic bacteria are known to bind plasminogen. We confirmed the surface localization of B. burgdorferi enolase by in situ protease degradation assay and immunoelectron microscopy. We then demonstrated that B. burgdorferi enolase binds plasminogen in a dose-dependent manner. Lysine residues were critical for binding of plasminogen to enolase, as the lysine analog εaminocaproic acid significantly inhibited binding. Ionic interactions did not play a significant role in plasminogen binding by enolase, as excess NaCl had no effects on the interaction. Plasminogen bound to recombinant enolase could be converted to active plasmin. We conclude that B. burgdorferi enolase is a moonlighting cytoplasmic protein which also associates with the bacterial outer surface and facilitates binding to host plasminogen.