Pentameric Pleasures: Structures of Cholera Toxin and E. coli Heat-labile Enterotoxin. EA Merritt¹, S Sarfaty², W Minke¹, CLMJ Verlinde², IK Feil¹, WGJ Hol^{1,2 1}Dept of Biological Structure, University of Washington, Seattle WA 98195 ²HHMI, University of Washington, Seattle WA 98195

Cholera toxin and the closely related E. Coli heat-labile enterotoxin are 85 kDalton hexameric assemblies consisting of a single 240 residue A subunit and five identical 103 residue B subunits. The B subunits assemble into a regular pentamer containing a central pore, through which the C-terminal tail of the A subunit extends to hold the AB₅ holotoxin together.

These toxins are attractive targets for structure-based drug design, and in the course of pursuing this target we have refined structures of both the AB₅ holotoxin and the B₅ pentamer in many different space groups and unit cells. The 5-fold symmetry of the B-pentamer, together with asymmetric units containing up to 260 kDalton in some cases, present both opportunity and challenge to the crystallographer.