The role of proline 345 in diphtheria toxin translocation

Diphtheria toxin (DT) can translocate across endosomal membranes in response to low pH. Buried hydrophobic domains localized in the 37-kDa toxin B chain become exposed in response to acidic conditions and are thought to participate in the membrane translocation process. The crystal structure of DT has revealed a structurally distinct translocation domain composed of nine alpha-helices with their interconnecting loops (Choe, S., Bennett, M., Fujii, G., Curmi, P., Kantardjieff, K., Collier, R., and Eisenberg, D. (1992) Nature 357, 216-222). Two of these alpha-helices, TH8 and TH9, are unusually apolar and constitute the central core of the translocation domain. It has been proposed that these domains and the highly charged interconnecting loop undergo a conformation change under acidic conditions producing a dagger-like structure capable of inserting into the membrane thus initiating the translocation process. Proline 345 occupies a strategic location at the end of the TH8 alpha-helix. Proline residues have the ability to undergo a cis-trans isomerization reaction and because of this have been proposed to play a role in the conformational change that is a prerequisite for toxin translocation. The role of the proline at position 345 in membrane translocation was investigated. Pro was mutagenized to Glu and to Gly using a two-step recombinant polymerase chain reaction procedure, and the mutant proteins were expressed in vitro. Glu, an alpha-helix former, and Gly, an alpha-helix breaker, were selected for mutagenesis to distinguish between a structural role for Pro as an alpha-helix breaker and alternative roles, perhaps involving cis-trans isomerization-related conformational changes. Replacing Pro at position 345 with Glu or Gly resulted in a 99% reduction in toxicity to Vero cells. The enzymatic and binding activity of the toxin were not altered by the mutations. Instead, the reduction in toxicity is due to decreased translocation ability, suggesting that the Pro at position 345 plays a specific role in toxin membrane translocation.