Topography of the TH5 Segment in the Diphtheria Toxin T-Domain Channel

The geometry of diphtheria toxoid CRM197 channel assessed by thiazolium salts and nonelectrolytes

The geometry of the channel formed by nontoxic derivative of diphtheria toxin CRM197 in lipid bilayer was determined using the dependence of single-channel conductance upon the hydrodynamic radii of different nonelectrolytes. It was found that the cis entrance of CRM197 channel on the side of membrane to which the toxoid was added at pH 4.8 and the trans entrance on the opposite side at pH 6.0 had effective radii of 3.90 and 3.48 Å, respectively. The 3-alkyloxycarbonylmethyl-5-(2-hydroxyethyl)-4-methyl-1,3-thiazolium salts reversibly reduced current via CRM197 channels. The potency of the blockers increased with increasing length of alkyl chain at symmetric pH 6.0 and remained high and stable at pH 4.8 on the cis side. Comparative analysis of CRM197 and amphotericin B pore size with the inhibitory action of thiazolium salts revealed a significant increase in CRM197 pore dimension at pH 6.0. Addition of thiazolium salt with nine carbons alkyl tail increased by ∼30% the viability of human carcinoma cells A431 treated with diphtheria toxin.

Bioinformatic discovery of a toxin family in Chryseobacterium piperi with sequence similarity to botulinum neurotoxins

Clostridial neurotoxins (CNTs), which include botulinum neurotoxins (BoNTs) and tetanus neurotoxin (TeNT), are the most potent toxins known to science and are the causative agents of botulism and tetanus, respectively. The evolutionary origins of CNTs and their relationships to other proteins remains an intriguing question. Here we present a large-scale bioinformatic screen for putative toxin genes in all currently available genomes. We detect a total of 311 protein sequences displaying at least partial homology to BoNTs, including 161 predicted toxin sequences that have never been characterized. We focus on a novel toxin family from Chryseobacterium piperi with homology to BoNTs. We resequenced the genome of C. piperi to confirm and further analyze the genomic context of these toxins, and also examined their potential toxicity by expression of the protease domain of one C. piperi toxin in human cells. Our analysis suggests that these C. piperi sequences encode a novel family of metalloprotease toxins that are distantly related to BoNTs with similar domain architecture. These toxins target a yet unknown class of substrates, potentially reflecting divergence in substrate specificity between the metalloprotease domains of these toxins and the related metalloprotease domain of clostridial neurotoxins.

Cellular Entry of the Diphtheria Toxin Does Not Require the Formation of the Open-Channel State by Its Translocation Domain

Cellular entry of diphtheria toxin is a multistage process involving receptor targeting, endocytosis, and translocation of the catalytic domain across the endosomal membrane into the cytosol. The latter is ensured by the translocation (T) domain of the toxin, capable of undergoing conformational refolding and membrane insertion in response to the acidification of the endosomal environment. While numerous now classical studies have demonstrated the formation of an ion-conducting conformation-the Open-Channel State (OCS)-as the final step of the refolding pathway, it remains unclear whether this channel constitutes an in vivo translocation pathway or is a byproduct of the translocation. To address this question, we measure functional activity of known OCS-blocking mutants with H-to-Q replacements of C-terminal histidines of the T-domain. We also test the ability of these mutants to translocate their own N-terminus across lipid bilayers of model vesicles. The results of both experiments indicate that translocation activity does not correlate with previously published OCS activity. Finally, we determined the topology of TH5 helix in membrane-inserted T-domain using W281 fluorescence and its depth-dependent quenching by brominated lipids. Our results indicate that while TH5 becomes a transbilayer helix in a wild-type protein, it fails to insert in the case of the OCS-blocking mutant H322Q. We conclude that the formation of the OCS is not necessary for the functional translocation by the T-domain, at least in the histidine-replacement mutants, suggesting that the OCS is unlikely to constitute a translocation pathway for the cellular entry of diphtheria toxin in vivo.