The B subunit of cholera toxin (CtxB) binds to a GM1-ganglioside receptor, a ubiquitous glycolipid cell surface receptor. This binding is widely accepted to initiate toxin action by triggering uptake and delivery of the toxin A subunit into cells. The beta chain has no toxic activity by itself. The holotoxin consists of a pentameric ring of B subunits whose central pore is occupied by the A subunit. The A subunit contains two chains, A1 and A2, linked by a disulfide bridge. The A subunit (and Cholera toxin) activates the adenylate cyclase enzyme in cells of the intestinal mucosa leading to increased levels of intracellular cAMP.