BINDING OF HUMAN PLASMINOGEN TO BORRELIA-BURGDORFERI

We studied the binding of plasminogen to Borrelia burgdorferi, a spirochete which causes Lyme disease and produces no endogenous proteases which digest extracellular matrix proteins, Using I-125-labeled plasminogen, we demonstrated that B. burgdorferi bound human plasminogen and that this binding was inhibitable with unlabeled plasminogen. I-125-labeled plasminogen binding by B. burgdorferi was also inhibited by the lysine analog E-aminocaproic acid. There was no significant difference in the binding of Glu- or Lys-plasminogen to B, burgdorferi, Binding of plasminogen was similar in low-passage (infectious) and high-passage (noninfectious) isolates of B, burgdorferi. Plasminogen bound to the surface of B. burgdorferi could be converted into plasmin by a human urokinase-type plasminogen activator, I-125-labeled plasminogen ligand blots of borrelial membrane proteins demonstrated two prominent binding proteins at similar to 70 and similar to 30 kDa. By Western blot (immunoblot), the 30-kDa protein was found to be outer surface protein A (Osp A) of B. burgdorferi. I-125-labeled plasminogen binding to both the 70-kDa protein and Osp A was inhibited by similar to 90% with a 1,000-fold excess of unlabeled plasminogen. By scanning densitometry, the 70-kDa band bound >10 time more I-125-labeled plasminogen than did Osp A, An Osp A-deficient mutant of B. burgdorferi and wild-type B, burgdorferi bound equal amounts of I-125-labeled plasminogen. Ligand blots of membrane proteins from an Osp A-deficient mutant showed association of I-125-labeled plasminogen at only the 70-kDa protein. Two-dimensional gel electrophoresis showed that the 70-kDa protein had a pi of similar to 5.3, clearly separable from Osp A. The association of host plasmin(ogen) with borrelial surface proteins provides a mechanism by which B. burgdorferi can digest extracellular matrix and disseminate.