Describe the structure and function of cholera toxins.

Structure: Cholera toxin is composed of a single A subunit and five B subunits. The A subunit is responsible for enzymatic activity, while the B subunits bind to the host cell receptor. Function: Cholera toxin activates adenylate cyclase, increasing cAMP levels, which leads to hypersecretion of electrolytes and water into the intestinal lumen, causing severe diarrhea.