The vacuolating toxin from Helicobacter pylori forms hexameric pores in lipid bilayers at low pH

Pathogenic strains of Helicobacter pylori secrete a cytotoxin, VacA, that i n the presence of weak bases, causes osmotic swelling of acidic intracellul ar compartments enriched in markers for late endosomes and lysosomes. The m olecular mechanisms by which VacA causes this vacuolation remain largely un known. At neutral pH, VacA is predominantly a water-soluble dodecamer forme d by two apposing hexamers. In this report, we show by using atomic force m icroscopy that below pH approximate to 5, VacA associates with anionic lipi d bilayers to form hexameric membrane-associated complexes. We propose that water-soluble dodecameric VacA proteins disassemble at low pH and reassemb le into membrane-spanning hexamers. The surface contour of the membrane-bou nd hexamer is strikingly similar to the outer surface of the soluble dodeca mer, suggesting that the VacA surface in contact with the membrane is burie d within the dodecamer before protonation. In addition, electrophysiologica l measurements indicate that, under the conditions determined by atomic for ce microscopy for membrane association, VacA forms pores across planar lipi d bilayers. This low pH-triggered pore formation is likely a critical step in VacA activity.