Purification of the B-subunit oligomer of Escherichia coli heat-labile enterotoxin by heterologous expression and secretion in a marine vibrio.

Heat-labile enterotoxins (Etx) are plasmid-encoded, multimeric proteins produced by certain diarrheagenic strains of Escherichia coli. The nontoxic, receptor-binding B subunit (EtxB) of such toxins may be useful as a component of vaccines against enterotoxigenic E. coli, or as a carrier for the delivery of heterologous epitopes to the mucosal immune system. Here we describe a simple method for the purification of EtxB from a marine vibrio harboring a broad-host range controlled expression vector containing the etxB gene. Induction of EtxB resulted in its specific secretion to the medium, to a concentration of greater than 25 mg/liter of culture. The techniques of ultrafiltration and hydrophobic interaction chromatography were used to purify EtxB to homogeneity from the medium of this organism (with a yield of 60.7%). EtxB-epitope fusion proteins were also successfully expressed and secreted in this marine vibrio, suggesting that this system may be of general use in the preparation of EtxB-based vaccines.