Cholera toxin is a protein produced by cholera bacteria. This protein is responsible for the disease cholera.

Cholera toxin enters the epithelial cells that line the walls of the intestines and then binds to and causes allosteric change in the alpha subunit of membrane proteins called G-proteins. This change in the G protein prevents it from reverting to its inactive state (by preventing it from hydrolyzing GTP to GDP). Because G proteins are responsible for regulating the flow of ions through transmembrane proteins that span the cell membrane, the modified, always-active form causes sodium ions to flow out of the cells. This causes water to flow out of the cells (see osmosis) and into the intestine. The net result is what the cell biology text "Essential Cell Biology" (used by students in Physiological Foundations at the Johns Hopkins University) calls, "catastrophic diarrhea".

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