Low pH triggers the translocation domain of diphtheria toxin (T-domain), which contains 10 α helices, to insert into a planar lipid bilayer membrane, form a transmembrane channel, and translocate the attached catalytic domain across the membrane. Three T-domain helices, corresponding to TH5, TH8, and TH9 in the aqueous crystal structure, form transmembrane segments in the open-channel state; the amino-terminal region, TH1-TH4, translocates across the membrane to the trans side. Residues near either end of the TH6-TH7 segment are not translocated, remaining on the cis side of the membrane; because the intervening 25-residue sequence is too short to form a transmembrane α-helical hairpin, it was concluded that the TH6-TH7 segment resides at the cis interface. Now we have examined this segment further, using the substituted-cysteine accessibility method. We constructed a series of 18 mutant T-domains with single cysteine residues at positions in TH6-TH7, monitored their channel formation in planar lipid bilayers, and probed for an effect of thiol-specific reagents on the channel conductance. For 10 of the mutants, the reagent caused a change in the single-channel conductance, indicating that the introduced cysteine residue was exposed within the channel lumen. For several of these mutants, we verified that the reactions occurred primarily in the open state, rather than in the flicker-closed state. We also established that blocking of the channel by an amino-terminal hexahistidine tag could protect mutants from reaction. Finally, we compared the reaction rates of reagent added to the cis and trans sides to quantify the residue's accessibility from either side. This analysis revealed abrupt changes in cis- versus trans-side accessibility, suggesting that the TH6-TH7 segment forms a constriction that occupies a small portion of the total channel length. We also determined that this constriction is located near the middle of the TH8 helix.
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