Expression of the major outer membrane protein of Chlamydia trachomatis in Escherichia coli

The major outer membrane protein (MOMP) of Chlamydia trachomatis was expressed in Escherichia coli. To assess whether it assembled into a conformationally correct structure at the cell surface, we characterized the recombinant MOMP (rMOMP) by Western immunoblot analysis, indirect immunofluorescence, and immunoprecipitation with monoclonal antibodies (MAbs) that recognize contiguous and conformational MOMP epitopes. Western blot analysis showed that most of the rMOMP comigrated with authentic monomer MOMP, indicating that its signal peptide was recognized and cleaved by E. coli. The rMOMP could not be detected on the cell surface of viable or formalin-killed E. coli organisms by indirect immunofluorescence staining with a MAb specific for a MOMP contiguous epitope. In contrast, the same MAb readily stained rMOMP-expressing E. coli cells that had been permeabilized by methanol fixation. A MAb that recognizes a conformational MOMP epitope and reacted strongly with formalin- or methanol-fixed elementary bodies failed to stain formalin- or methanol-fixed E. coli expressing rMOMP. Moreover, this MAb did not immunoprecipitate rMOMP from expressing E. coli cells even though it precipitated the authentic protein from lysates of C. trachomatis elementary bodies. Therefore we concluded that rMOMP was not localized to the E. coli cell surface and was not recognizable by a conformation-dependent antibody. These results indicate that rMOMP expressed by E. coli is unlikely to serve as an accurate model of MOMP structure and function. They also question the utility of rMOMP as a source of immunogen for eliciting neutralizing antibodies against conformational antigenic sites of the protein.