THE 3-DIMENSIONAL CRYSTAL-STRUCTURE OF CHOLERA-TOXIN

The clinical manifestations of cholera are largely attributable to the actions of a secreted hexameric ABS enterotoxin (choleragen). We have independently solved and refined the three-dimensional structure of c holeragen at 2.5 Angstrom resolution. The structure of the crystalline toxin closely resembles that described for the heat-labile enterotoxi n from Escherichia coli (LT) with which it shares 80% sequence homolog y. In both cases, the wedge-shaped A subunit is loosely held high abov e the plane of the pentameric B subunits by the tethering A2 chain. Th e most striking difference between the two toxins occurs at the carbox yl terminus of the A2 chain. Whereas the last 14 residues of the A2 ch ain of LT threading through the central pore of the B-5 assembly form an extended chain with a terminal loop, the A2 chain of choleragen rem ains a nearly continuous alpha-helix throughout its length. The four c arboxyl-terminal residues of the A2 chain (KDEL sequence), disordered in the crystal structure of LT, are clearly visible in choleragen's el ectron-density map. In the accompanying article we describe the three- dimensional Structure of the isolated B pentamer of cholera toxin (cho leragenoid). Comparison of the crystalline coordinates of choleragen, choleragenoid, and LT provides a solid three-dimensional foundation fo r further experimental investigation. These structures, along with tho se of related toxins from Shigella dysenteria and Bordetella pertussis , offer a first step towards the rational design of new vaccines and a nti-microbial agents. (C) 1995 Academic Press Limited