Matrix-assisted laser desorption ionization-time of flight mass spectrometry analysis identified enolase as a cell surface component of Streptococcus mutans, which was confirmed by enzyme-linked immunosorbent assay, Western blotting, and transmission electron microscopy. Surface enolase was demonstrated to bind to human plasminogen and salivary mucin MG2. The results suggested a role for enolase in S. mutans attachment, clearance, or breach of the bloodstream barrier.Streptococcus mutans is an alpha-hemolytic and nongroupable streptococcus, which causes dental caries and occasionally bacterial endocarditis (BE) (20). Some surface proteins of S. mutans are virulence factors with functions critical to viability, such as adhesion (2). Adhesion is one of the most important steps in the pathogenicity of microorganisms, and bacterial cell surface-mediated interactions with salivary proteins may be the first step in S. mutans attachment to a tooth surface coated with salivary pellicle or saliva-coated plaque (11). Liu et al. reported that S. mutans binds to salivary mucins, but the specific molecule responsible for this binding was unknown (18). Salivary mucin MG2 is a 180-kDa glycoprotein (22). Mucin MG2 binds to several oral microbes, including S. mutans, Actinobacillus actinomycetemcomitans, and Candida albicans, and forms heterotypic complexes with salivary proteins, including secretory immunoglobulin A (SIgA) and lactoferrin (4,15,18,19,29).Enolase is a 47-kDa cytoplasmic enzyme in the glycolytic pathway (21). There are three distinct isoforms (␣, , and ␥) of enolase, each with the same molecular mass. ␣-Enolase is the isoform typically found in bacteria. Surface ␣-enolase has been recently identified on Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumonia, Streptococcus oralis, and A. actinomycetemcomitans (3,13,16,25,32). Surface enolase of S. pyogenes binds and activates human plasminogen, a molecule which plays a crucial role in fibrinolysis, homeostasis, and the degradation of extracellular matrix (8). Proteins, such as enolase, with exposed carboxyl-terminal lysines on the cell surface can bind and promote plasminogen activation (33).In the present study, we identified ␣-enolase from a cell surface protein preparation of S. mutans A32-2 by two-dimensional polyacrylamide gel electrophoresis (PAGE) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis. The results of enzyme-linked immunosorbent assay (ELISA), Western blot, and transmission electron microscopy (TEM) confirmed ␣-enolase on the cell surface as well as in the cytoplasm of S. mutans. Results of Western blot, TEM, and ELISA experiments demonstrate that S. mutans surface enolase binds to human plasminogen and human salivary mucin MG2.S. mutans A32-2 was isolated from a highly-caries-active patient and expresses significantly more surface proteins than isolates from caries-free subjects (26). Surface protein, cell wall, and cytoplasmic samples were purified according to methods ...
