Association of protein with the cell wall of Streptococcus mutans

Cell walls from Streptococcus mutans were prepared by conventional technique and subjected to a series of extraction procedures involving classical protein solvents. The extracted walls contained several non-peptidoglycan amino acids and were also amenable to radiolabeling with [125I]sodium iodide and chloramine T. The cell walls could be chemically modified with tetranitromethane and diazo-1H-tetrazole, suggesting the presence of tyrosine or histidine or both. Flourescence spectra of the walls revealed the presence of either tyrosine or tryptophan. Several proteases, including pronase, trypsin, subtilisin, and proteinase K, removed some of the label from the walls. In contrast, treatment of the walls with salts or denaturants did not result in the solubilization of label. When the walls were solubilized with mutanolysin and subjected to chromatography, three peaks of radioactivity with apparent molecular weights of 73,000, 39,000, and 9,600 were observed. Wall digests subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a single band of radioactivity corresponding to an apparent molecular weight of 79,000. Isoelectric focusing of labeled wall digest gave rise to two major bands of radioactivity with isoelectric points of approximately 2.4 and 5.6. The results suggest that the cell wall of S. mutans contains tightly and possibley covalently bound polypeptide molecules. We propose that the cell wall polypeptides of S. mutans serve as factors in the attachment of the bacteria to smooth surfaces.