Molecular interactions of Human Plasminogen with FnBPB, a Fibrinogen/Fibronectin-binding Protein from Staphylococcus aureus

Staphylococcus aureus is the causative agent of potentially harmful diseases, like sepsis, endocarditis and necrotizing pneumonia and it is also responsible of less severe clinical manifestations such as epithelial and mucosal-associated infections. Nowadays, the emergence of methicillin (MRSA) as well as vancomycin-resistant (VRSA) strains is of great concern. In spite of the advance in antibiotic development, treating these infections remains a huge challenge. The surface of Gram-positive pathogenic bacterium Staphylococcus aureus is decorated with cell wall-anchored (CWA) proteins, Like several other invasive pathogens S. aureus can capture plasminogen from the human host where it can be converted to plasmin by host plasminogen activators or by endogenously expressed staphylokinase. Sortase-anchored cell wall associated proteins are responsible for capturing the bulk of bound plasminogen. A mutant defective in the sortase enzyme responsible for anchoring CWA proteins to the S.aureus cell surface captured 10-fold less PLG compared to the wild type indicating that one or more CWA proteins is involved. Two cell wall associated proteins, the fibrinogen and fibronectin binding proteins A and B, were found to bind plasminogen and one of them, FnBPB, was studied in detail.Plasminogen bound to recombinant FnBPB with KD of 0.532 μM as determined by surface plasmon resonance. Plasminogen binding did not to occur by the same mechanism of Dock Lock and Latch through which FnBPB binds to fibrinogen. Indeed, FnBPB could bind both ligands simultaneously indicating that their binding sites do not overlap. The N3 subdomain of FnBPB contains the full plasminogen binding site and this comprises, at least in part, two conserved patches of surface-located lysine residues which were recognized by kringle 4 of the host protein. PLG-FnBPB interaction does not reduce the PLG ability to be activated by the activators as SAK or tPA. Indeed they can activate PLG bound on the surface of S.aureus USA 300 LAC and use it as a proteolytic reactor with the ability to digest the artificial substrate S2251 and human FBG, suggesting that binding of PLG to staphylococcal surface does not compromise its biological functionality. Moreover SAK both in cell surface and in TCA-precipitated culture supernatants of USA 300 LAC was detected, allowing to conclude that S.aureus can capture SAK secreted in the culture environment and acquire the ability to active adsorbed PLG in plasmin.