Three-Dimensional Structure of Cholera Toxin Penetrating a Lipid Membrane

Two-dimensional crystals of cholera toxin bound to receptors in a lipid membrane give diffraction extending to 15 Å resolution. Three-dimensional structure determination reveals a ring of five B subunits on the membrane surface, with one-third of the A subunit occupying the center of the ring. The remaining mass of the A subunit appears to penetrate the hydrophobic interior of the membrane. Cleavage of a disulfide bond in the A subunit, which activates the toxin, causes a major conformational change, with the A subunit mostly exiting from the B ring.