Analyses of gonococcal lipopolysaccharide in whole-cell lysates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis: stable association of lipopolysaccharide with the major outer membrane protein (protein I) of Neisseria gonorrhoeae

Abstract

The lipopolysaccharide (LPS) of Neisseria gonorrhoeae whole-cell lysates and proteinase K-digested lysates was examined and compared with purified homologous LPS by a method which preferentially stains LPS in polyacrylamide gels. The silver-stained profile of gonococcal LPS in the proteinase K-digested lysate was similar to that of homologous purified LPS; however, the LPS profile in whole-cell lysates was much smaller than that of digested lysates or purified LPS. Conditions of solubilization did not affect these differences. Since it is known that LPS migrates in a unique fashion in second-dimension electrophoresis, the location of LPS in the whole-cell lysates was probed by second-dimension sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a variety of stains and radiolabels. Results from these experiments indicated a stable and reproducible association of LPS with proteins ranging between 23,000 to 36,000 in Mr, in particular major outer membrane protein I. In addition to staining with the silver method, which preferentially stains LPS, the putative LPS was resistant to digestion by proteinase K, did not stain with Coomassie brilliant blue, and was not labeled extrinsically with 125I (Iodogen method) or intrinsically with [35S]methionine. Analysis of two-dimensional gels by immunoblotting with rabbit antisera prepared from protein I bands removed from a polyacrylamide gel revealed the presence of antigens in the same area of the gel (below proteins that were 23,000 to 36,000 in Mr). Antibodies to constituents which migrated below the diagonal were essentially removed by adsorption of antisera with purified LPS, as were antibodies to homologous LPS and LPS in proteinase K-digested whole-cell lysates. Immunoblotting with a monoclonal antibody specific for LPS demonstrated reactivity of the antibody with LPS and with the protein I band. On the basis of these data, we conclude that protein I and perhaps other proteins in the whole-cell lysate are stably associated with LPS; this complex is resistant to dissociation in sodium dodecyl sulfate at high temperature (approximately 100 degrees C) but does, for unknown reasons, dissociate with electrophoresis in the second dimension. The association of LPS with protein antigens in sodium dodecyl sulfate-polyacrylamide gels adds another dimension of complexity to analysis of these antigens by immunoelectroblotting. Furthermore, the tight association of LPS with the major outer membrane protein I may alter the nature of the immune response generated by "purified" protein I vaccine antigens. The possible role of protein-LPS complexes in the pathogenesis of gonorrhea is discussed.