Figure. Molecular structure of heat-labile enterotoxins. A and B polypeptides are translated from a single mRNA, with an excess of B polypeptides. The leader sequences of both allow them to be transported into the periplasm, where B subunits assemble into pentamers. A disulfide bridge spans the junction of the A1 and A2 segments of the A polypeptide, where a proteolytic nick occurs. The A2 subunit non-covalently associates with the central pore in the B pentamer.