Abstract
Cholera toxin (CT) and the type I heat-labile enterotoxin (LT-I) of E. coli, the causative agents in cholera and traveler's diarrhea, are structurally and functionally similar. Both have homopentameric B subunits which bind to specific cell surface receptors and monomeric A subunits which persistantly activate adenylylcyclase in target cells. In the human intestinal cell, the rise in cyclic AMP mediates the pathophysiological effects of the toxins. Both toxins bind with the A subunit facing away from the membrane, and during a characteristic lag phase, the holotoxins are internalized and undergo inter-cellular processing which leads to release of the A1 peptide. The latter is an ADP-ribosyltransferase which modifies the subunit of stimulatory G protein (GSα), keeping adenylylcyclase in an activated state. Exposure of cells to brefeldin A, a disruptor of the Golgi apparatus, blocks the release of the A1 peptide and thus prevents the activation of adenylylcyclase. This suggests that an intact Golgi apparatus is required for processing and activation of the toxins.
The ganglioside GM1 has been identified as the only natural, functional receptor for cholera toxin and also can serve as a functional receptor for LT-I. Intestinal cells of several species including man have additional receptors for LT-I which have been identified as galactoproteins with polylactosaminyl-glycan determinants required for toxin binding. Paradoxically, both toxins can bind to neoganglio-proteins containing GM1-oligosacharride, which have been generated on the surface of GM1-deficient cells, but are unable to activate adenylylcyclase. Activation can occur in the presence of chloroquine but with a delayed lag phase. This suggests that the toxins, when bound to the neoganglioproteins, are entering a different intracellular pathway then when bound to their natural receptors. The influence of receptor structure on toxin action is further indicated by a series of neogangliolipids synthesized from GM1-oligosacharride and different lipids. When taken up by GM1-deficient cells, they support toxin binding but mediate varying toxin activity. For one series of neoglycolipids, the latter is inversely related to the distance between the oligosaccharide and the lipid moiety.
