Porin from the outer membrane of Escherichia coli: immunological characterization of native and heat-dissociated forms.

Antisera against porin oligomers isolated from the outer membrane of Escherichia coli O26K60 and against porin monomers from the same bacterial strain were elicited in rabbits by intramuscular administration with Freund's complete adjuvant. Antibodies against native porin oligomers reacted strongly with porin oligomers, as revealed by sodium dodecyl sulphate-polyacrylamide gel immunoperoxidase (SGIP) analysis, the enzyme-linked immunosorbent assay (ELISA) and immunodiffusion, but showed no significant reaction with denatured monomers. The antibodies were completely absorbed by the intact outer membrane-peptidoglycan complex, which suggests that they were directed against antigenic determinants expressed on the outside of the intact outer membrane. Antibodies directed against denatured porin monomers reacted strongly with monomers in all tests but reacted only very weakly with porin oligomers. They were not absorbed by the native porin situated in the intact outer membrane. This indicates that the major antigenic determinants of the denatured porin monomer are hardly related to those of the native trimer situated in the intact outer membrane. The antigenic determinants of the denatured monomer seem to become fully expressed only after dissociation and denaturation of the porin. It is concluded that the immunological relationship of denatured porin monomers derived from many strains of E. coli and other Enterobacteriaceae which was reported in previous studies may not indicate that native porin trimers of these strains are also related.