Structure function relationships in diphtheria toxin channels: II. A residue responsible for the channel's dependence on trans pH

Ion-conducting channels formed in lipid bilayers by diphtheria toxin are highly pH dependent. Among other properties, the channel's single channel conductance and selectivity depend on proton concentrations on either side of the membrane. We have previously shown that a 61 amino acid fragment of DT is sufficient to form a channel having the same pH-dependent single channel properties as that of the intact toxin. This region corresponds to an a-helical hairpin in the recently published crystal structure of DT in solution; the hairpin contains two α-helices, each long enough to span a membrane, connected by a loop of about nine residues. This paper reports on the single channel effects of mutations which alter the two negatively charged residues in this loop. Changing Glutamate 349 to neutral glutamine or to positive lysine has no effect on the DT channel's single channel conductance or selectivity. In contrast, mutations of Aspartate 352 to neutral asparagine (DT-D352N) or positive lysine (DT-D352K) cause progressive reductions in single channel conductance at pH 5.3 cis/7.2 trans (in 1 m KCl), consistent with this group interacting electrostatically with ions in the channel. The cation selectivity of these mutant channels is also reduced from that of wild-type channels, a direction consistent with residue 352 influencing permeant ions via electrostatic forces. When both sides of the membrane are at pH 4, the conductance difference between wild-type and DT-D352N channels is minimal, suggesting that Asp 352 (in the wild type) is neutral at this pH. Differences observed between wild-type and DT-D352N channels at pH 4.0 cis/7.2 trans (with a high concentration of permeant buffer in the cis compartment) imply that residue 352 is on or near the trans side of the membrane. Comparing the conductances of wild-type and DT-D352K channels at large (cis) positive voltages supports this conclusion. The trans location of position 352 severely constrains the number of possible membrane topologies for this region.