In vitro trimerization of outer membrane protein PhoE

The folding of outer membrane protein PhoE of E coli into its native trimeric structure was studied in vitro by using monoclonal antibodies, which recognize cell-surface exposed, conformational epitopes of the protein. These antibodies were able to precipitate the in vitro synthesized PhoE protein, showing that the conformational epitopes are formed in vitro. From analysis by SDS--polyacrylamide gel electrophoresis, it appeared that the precipitated protein represents a folded monomer. The signal sequence interferes with the formation of the conformational epitopes. Outer membranes were required to induce the formation of the stable trimeric form of the protein. This trimerization was not accompanied by insertion into the outer membranes.