Intoxication of zebrafish and mammalian cells by cholera toxin depends on the flotillin/reggie proteins but not Derlin-1 or -2.

Cholera toxin (CT) causes the massive secretory diarrhea associated with epidemic cholera. To induce disease, CT enters the cytosol of host cells by co-opting a lipid-based sorting pathway from the plasma membrane, through the trans-Golgi network (TGN), and into the endoplasmic reticulum (ER). In the ER, a portion of the toxin is unfolded and retro-translocated to the cytosol. Here, we established zebrafish as a genetic model of intoxication and examined the Derlin and flotillin proteins, which are thought to be usurped by CT for retro-translocation and lipid sorting, respectively. Using antisense morpholino oligomers and siRNA, we found that depletion of Derlin-1, a component of the Hrd-1 retro-translocation complex, was dispensable for CT-induced toxicity. In contrast, the lipid raft-associated proteins flotillin-1 and -2 were required. We found that in mammalian cells, CT intoxication was dependent on the flotillins for trafficking between plasma membrane/endosomes and two pathways into the ER, only one of which appears to intersect the TGN. These results revise current models for CT intoxication and implicate protein scaffolding of lipid rafts in the endosomal sorting of the toxin-GM1 complex.