Figure 1.

AFM images show that VacA associates only with supported bilayers containing anionic phospholipids at pH < 5. (a) An eggPC bilayer after 1 hr incubation with ≈30 μg/ml VacA in 1 mM citric acid, pH ≈ 3.5. There is no VacA association with the bilayer, but there are aggregates directly adsorbed to mica within bilayer defects (dark regions). Similar results were obtained with bilayers composed of other zwitterionic lipids (see Table 1) at all pH values examined from pH 3.5 to 7.2 or with bilayers containing negatively charged lipids at pH > 5. (b) A high density of VacA oligomers is seen associated with bilayers made of HL (a total lipid extract including anionic phospholipids) under the conditions in a but at pH ≈ 3. Individual VacA oligomers can be clearly discerned in this image. (c) After adsorption of VacA to HL bilayers, raising the pH to 7 after removal of unbound VacA promoted the formation of two-dimensional crystals of VacA oligomers. Notice that the central rings of VacA oligomers are clearly visualized both in the randomly distributed oligomers and within the crystals. [Bar = 400 nm (a) and 200 nm (b and c).]