Vesicular transport is not required for the cytoplasmic pool of cholera toxin to interact with the stimulatory alpha subunit of the heterotrimeric g protein.

Cholera toxin (CT) moves from the cell surface to the endoplasmic reticulum (ER) by retrograde vesicular transport. The catalytic A1 polypeptide of CT (CTA1) then crosses the ER membrane, enters the cytosol, ADP-ribosylates the stimulatory alpha subunit of the heterotrimeric G protein (Gsalpha) at the cytoplasmic face of the plasma membrane, and activates adenylate cyclase. The cytosolic pool of CTA1 may reach the plasma membrane and its Gsalpha target by traveling on anterograde-directed transport vesicles. We examined this possibility with the use of a plasmid-based transfection system that directed newly synthesized CTA1 to either the ER lumen or the cytosol of CHO cells. Such a system allowed us to bypass the CT retrograde trafficking itinerary from the cell surface to the ER. Previous work has shown that the ER-localized pool of CTA1 is rapidly exported from the ER to the cytosol. Expression of CTA1 in either the ER or the cytosol led to the activation of Gsalpha, and Gsalpha activation was not inhibited in transfected cells exposed to drugs that inhibit vesicular traffic. Thus, anterograde transport from the ER to the plasma membrane is not required for the cytotoxic action of CTA1.