Identification and characterization of a surface protein-releasing activity in Streptococcus mutans and other pathogenic streptococci

Abstract

Surface proteins of Streptococcus mutans have been reported to be released into the culture filtrate at concentrations that vary with the growth conditions. The reason for this is not clear. The present study attempts to investigate the mechanism of the protein release. The results showed that whole cells and raffinose-stabilized protoplasts of S. mutans NG8, when incubated in buffers, were capable of releasing their surface proteins in a pH-dependent manner with optimal release at pH 5 to 6. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that the released proteins were very complex. Two proteins, adhesin P1, which has been previously shown to interact with a human salivary agglutinin, and glucosyltransferase have been identified among the released proteins. The release of adhesin P1 and other proteins was found to be inhibited by heat, Cu2+,Zn2+, and thiol-blocking reagents. The inhibition by heat and Cu2+ was irreversible, whereas that by the thiol-blocking reagents was reversible. EDTA, phenylmethylsulfonyl fluoride, and N-p-tosyl-L-lysyl-chloromethyl ketone had no effect on the release of P1, indicating that the release was probably not due to proteolytic activity. Adhesin P1 from Cu(2+)-inactivated S. mutans NG8 protoplasts could be released by mixing with fresh whole cells and protoplasts, but not the culture filtrate, of a P1-negative mutant of NG8, suggesting that the enzyme is located on the cell surface. This P1-releasing activity was also detected in two other strains of S. mutans and one strain each of S. gordonii, S. agalactiae, S. pneumoniae, and S. pyogenes. The biological role(s) of this enzyme activity remains to be determined. However, owing to its ability to release virulent surface proteins from the cell, it may play an important role in cell surface modulation among the pathogenic streptococci.