A protein antigen, I/II, was purified from cells and culture supernatants of (serotype ) by solubilization in urea followed by ion exchange chromatography and gel filtration. Immunological activity was retained after further purification by preparative sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The sedimentation coefficient was estimated to be approximately 8.7S by sucrose gradient centrifugation. The use of staining procedures, as well as the linear migration of this protein through different concentrations of acrylamide during SDS-PAGE, indicated that the antigen is probably not a glycoprotein. A lower molecular weight protein containing the free antigen I determinant was shown to have extensive homology with intact antigen I/II which implied that the former was a degradation product of the intact 185000 dalton antigen I/II. Antigen II, although previously defined by its resistance to proteases, could be further digested with trypsin after denaturation by SDS-PAGE. Antigen I/II could not be correlated with a group of glucosyltransferases isolated from whole cells and culture supernatants. The cell surface location of antigen I/II was established by the lactoperoxidase-catalysed iodination of intact cells followed by protein analysis using SDS-PAGE. Previously, the potential importance of antigen I/II has been established by its immunogenicity and capacity to induce a protective immune response against dental caries.


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