Interazione molecolare del plasminogeno con le proteine FnBPs di Staphylococcus aureus e il suo ruolo nell'invasione batterica

Staphylococcus aureus, a Gram-positive pathogenic bacterium, represents the etiologic agent of potentially harmful human health diseases such as sepsis, endocarditis and necrotising pneumonia, and is also responsible for many less severe clinical manifestations such as epithelial and mucosal infections. Nowadays, the emergence of methicillin- and vancomycin-resistant S. aureus strains, is of great concern. Despite the advance in antibiotic development, treating these infections remains a huge challenge for the scientific community. The main class of staphylococcal proteins responsible for host tissue colonisation is represented by the cell wall-anchored (CWA) proteins. Among them, the major family is the MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules). Like several other invasive pathogens, S. aureus can capture human plasma protein plasminogen (PLG), a 92 kDa zymogen, and trigger its conversion to catalytically active plasmin mediated by host plasminogen activators including tissue plasminogen activator (tPA). In this study, I have show the molecular interaction between the Fibronectin-binding proteins A and B (FnBPA and B) and human PLG. Analysis of protein-ligand binding indicates a high-affinity interaction in the nanomolar range between Plasminogen and FnBPs A domain, this revealed that fibronectin-binding proteins are the main PLG receptors expressed on the surface of the bacterium. In particular the Kringle 4 of PLG was predicted to be the binding domain for fibronectin-binding proteins. PLG bound to surface-coated FnBPs proteins can be converted into active plasmin by tPA and has been shown that Plasmin is a serine protease able to degrade a variety of proteins such as fibronectin, a component of the extracellular matrix. PLG activation is also important for the tissue barrier degradation that allows cell migration. Through activation of metalloproteases, plasmin-coated bacteria can break down extracellular matrix and basement membranes either directly or indirectly thereby facilitating bacterial spread. To determine whether PLG captured by FnBPs proteins on the S. aureus surface and activated by tPA can mediate staphylococcal transmigration across endothelial cell monolayers, HUVEC cells were layered on the upper porous membrane of the Transwell® system and allowed to grow to confluent monolayers. The results suggest that, through activation of PLG, plamsin can break down extracellular matrices and basement membranes facilitating the likelihood of bacterial spread and fibronectin-binding proteins A and B are the key players in the plasmin-mediated transmigration of S. aureus across endothelial cell monolayers.