Subunits of the heat-labile enterotoxin of Escherichia coli (LT) assemble in the periplasm and are secreted through the outer membrane in Vibrio cholerae. Deletions or substitutions of residues at the carboxyl terminus of the B subunit (EtxB) result in mutant polypeptides that assemble into normal pentamers at 30°C but cannot assemble at 42¼ in vivo. This defect may be suppressed by substitutions of single amino acid residues in regions that interact directly with the modified carboxyl terminus. Carboxyl terminal residues of EtxB thus appear to be required for formation or stabilization of an assembly intermediate of B subunit pentamerization but are not essential for the stability of the final pentamer.

Secretion of the cholera toxin (CT) or of EtxB through the outer membrane of V. cholerae requires the functions of several genes that display extensive similarities to genes required for macromolecular translocation in other Gram-negative bacteria. One of the gene products required seems to be a cytoplasmic protein containing ATP-binding domains. It may be a protein involved in the regulatory signal transduction.