Role of surface displayed pneumococcal enolase in plasmin-mediated degradation of extracellular matrix and transmigration

The enolase is one of the non classical surface proteins displaying plasminogen-binding activity on the surface of Streptococcus pneumoniae. The impact of the internal plasminogen-binding motif FYDKERKVY of enolase, which is located between amino acids 248 and 256, on plasmin-mediated degradation of extracellular matrix (ECM) and pneumococcal transmigration was investigated. In the presence of host-derived plasminogen activators (PA) tissue-type PA or urokinase PA and plasminogen S. pneumoniae expressing wild-type enolase efficiently degraded reconstituted basement membrane or ECM secreted by NCI-H292 lung epithelial cells. In contrast, pneumococcal mutants with amino acid substitutions in the nine residue plasminogen-binding motif of enolase showed significantly reduced degradation of ECM and basement membrane. In particular, wild-type pneumococci covered with serine proteolytic activity potentiated dissolution of laminin, fibronectin or fibrin as shown by electron microscopy. Moreover, wild-type pneumococci, but not enolase mutants which lack a functional internal plasminogen-binding site, efficiently transmigrated through a network of fibrin fibrils. In conclusion, these results provide evidence that the nine residue plasminogen-binding motif of enolase is the key cofactor for plasmin-mediated pneumococcal degradation and transmigration through host ECM.