PROTONATION OF HISTIDINE-132 PROMOTES OLIGOMERIZATION OF THE CHANNEL-FORMING TOXIN AEROLYSIN

Aerolysin is a bacterial toxin that binds to a receptor on eukaryotic cells and oligomerizes to form stable, SDS-resistant, noncovalent olig omers that insert into the plasma membrane and produce well-defined ch annels. Little is known about the mechanisms controlling this process, Here we show that the protonation of a single histidine is required f or oligomerization of aerolysin in solution. First we have investigate d the effect of pH on the activity of aerolysin. The toxin's ability t o disrupt human erythrocytes declined as the pH increased above 7.4. E xperiments with receptor-fret planar lipid bilayers demonstrated that the rate at which aerolysin formed channels also decreased with increa sing pH, although the conductance of preexisting channels was not affe cted. The reduction in the rate of channel formation was shown to be d ue to a decrease in the toxin's ability to oligomerize. Our data indic ate that the pH effect on activity is due to the deprotonation of a si ngle residue rather than a global effect of pH on the protein. In agre ement with our previous sire-directed mutagenesis studies, His-132 is most likely to be the target of this pH effect. This conclusion was re inforced by the fact that we could shift the pH dependence of the acti vity to lower pH values by mutating Asp-139. a residue less than 3 Ang strom away from; His-132 and likely to contribute to the unusually hig h pK(a) of this histidine.