TOWARDS THE CRYSTALLIZATION OF DIPHTHERIA TOXIN TRANSLOCATION MUTANTS P345C, P345E, and P345G Melinda Balbirnie, Ralf Landgraf, David Eisenberg, Department of Chemistry and Biochemistry, UCLA, 405 Hilgard Ave., LA, CA 90095

Diphtheria toxin is a 535 residue proenzyme that binds to and enters human cells, forms a pore across the endosomal membrane, and translocates a toxic domain into the cytoplasm which kills the cell. The translocation has been ascribed to a two-helical segment (helices 8 and 9). Proline 345 is located at the end of helix 8 and mutation at this position to cysteine, glutamate, or glycine abolishes translocation activity. The diphtheria toxin mutants were prepared in the laboratory of R. John Collier. A mutant diphtheria toxin with proline 345 replaced by cysteine has been expressed and purified. The aim of the present study is to develop an improved structure-based model of diphtheria toxin membrane translocation. We present our current progress on this project.