alpha-Enolase of Streptococcus pneumoniae is a plasmin(ogen)-binding protein displayed on the bacterial cell surface

Mol Microbiol. 2001 Jun;40(6):1273-87. doi: 10.1046/j.1365-2958.2001.02448.x.


Binding of human plasminogen to Streptococcus pneumoniae and its subsequent activation promotes penetration of bacteria through reconstituted basement membranes. In this study, we have characterized a novel pneumococcal surface protein with a molecular mass of 47 kDa, designated Eno, which specifically binds human plasmin(ogen), exhibits alpha-enolase activity and is necessary for viability. Using enzyme assays, we have confirmed the alpha-enolase activity of both pneumococcal surface-displayed Eno and purified recombinant Eno protein. Immunoelectron microscopy indicated the presence of Eno in the cytoplasm as well as on the surface of encapsulated and unencapsulated pneumococci. Plasminogen-binding activity was demonstrated with whole pneumococcal cells and purified Eno protein. Binding of activated plasminogen was also shown for Eno; however, the affinity for plasmin is significantly reduced compared with plasminogen. Results from competitive inhibition assays indicate that binding is mediated through the lysine binding sites in plasmin(ogen). Carboxypeptidase B treatment and amino acid substitutions of the C-terminal lysyl residues of Eno indicated that the C-terminal lysine is pivotal for plasmin(ogen)-binding activity. Eno is ubiquitously distributed among pneumococcal serotypes, and binding experiments suggested the reassociation of secreted Eno to the bacterial cell surface. The reassociation was also confirmed by immunoelectron microscopy. The results suggest a mechanism of plasminogen activation for human pathogens that might contribute to their virulence potential in invasive infectious processes.

MeSH terms

  • Amino Acid Sequence
  • Bacterial Outer Membrane Proteins / metabolism
  • Carboxypeptidase B
  • Carboxypeptidases / chemistry
  • Carboxypeptidases / metabolism
  • Cell Membrane / metabolism*
  • Cell Survival
  • Cross Reactions
  • Cytoplasm / metabolism
  • Fibrinolysin / metabolism*
  • Gene Expression Regulation, Bacterial
  • Humans
  • Lysine / genetics
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Phosphopyruvate Hydratase / genetics
  • Phosphopyruvate Hydratase / immunology
  • Phosphopyruvate Hydratase / metabolism*
  • Plasminogen / metabolism*
  • Sequence Analysis, Protein
  • Serotyping
  • Streptococcus pneumoniae / classification
  • Streptococcus pneumoniae / enzymology*
  • Streptococcus pneumoniae / metabolism
  • Streptococcus pyogenes / enzymology


  • Bacterial Outer Membrane Proteins
  • Plasminogen
  • Carboxypeptidases
  • Carboxypeptidase B
  • Fibrinolysin
  • Phosphopyruvate Hydratase
  • Lysine