E2AB_ECOLX 123 The biological activity of the toxin is produced by the A chain, which activates intracellular adenyl cyclase. MSSKKIIGAFVLMTGILSGQVYAGVSEHFRNICNQTTADIVAGVQLKKYIADVNTNTRGIYVVSNTGGVWYIPGGRDYPDNFLSGEIRKTAMAAILSDTKVNLCAKTSSSPNHIWAMELDRES Heat-labile enterotoxin IIA, B chain