Abstract
CELL BIOLOGY Cholera toxin is composed of two polypeptides linked by a disulfide bond. In order to intoxicate target intestinal epithelial cells, a fragment of the A chain, A1, must be transported across one of the host cell membranes to gain access to the cytosol. In the cytosol, A1 goes on to attack the cellular signal transduction apparatus and to stimulate opening of chloride channels, resulting in massive chloride and water secretion. The entry process for the toxin has been very well studied. However, Tsai et al. have now uncovered an unanticipated, critical step in cholera toxin activation. An enzyme in the endoplasmic reticulum, protein disulfide isomerase, appears to act as a kind of redox-regulated chaperone for incoming toxin (an assembly of five B chains and the disulfide-linked A1 and A2 peptides), promoting disassembly, reduction, and unfolding in order to allow translocation of the killer A1 chain across the membrane and into the cytosol; this sequence of steps recapitulates the biosynthesis of the toxin, except in reverse order. — SMH Cell 104 , 937 (2001).
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