Orientation of the diphtheria toxin channel in lipid bilayers

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.

Inositol 1,4,5-trisphosphate directly opens diphtheria toxin channels

Inositol 1,4,5-trisphosphate (IP3) is a soluble second messenger, which acts by cooperatively releasing Ca2+ into the cytosol from non-mitochondrial stores, probably by activating Ca(2+)-permeable channels. We demonstrate that submicromolar concentrations of IP3 can directly open the Ca(2+)-permeable diphtheria toxin channel, and that this occurs by IP3 binding to a specific site on the toxin protein. This provides a model for IP3-induced Ca2+ release and suggests that IP3-induced channel opening may play a role in diphtheria intoxication and in protein translocation across membranes.