THE ACID ACTIVATION OF HELICOBACTER-PYLORI TOXIN VACA - STRUCTURAL AND MEMBRANE-BINDING STUDIES

The cell vacuolating activity of the protein toxin VacA, released by H elicobacter pylori, is strongly increased in vitro by exposure to acid ic pH followed by neutralization. This short acid exposure does not in crease significantly the binding of VacA to cell or to lipid membranes . However, membrane photolabeling with photoactivatable radioactive ph ospholipids and ANS binding studies show that VacA transiently exposed to pH equal or lower than 5 changes conformation and exposes on its s urface hydrophobic segments. Both the 32 and the 58 kDa subunits of th e toxin insert in the lipid bilayer and interact with the fatty acid c hains of phospholipids, Membrane binding and penetration are enhanced by incubating target cells or liposomes with the toxin at mild acidic pH values, similar to those present around H. pylori on the stomach mu cosa. These findings are discussed with respect to the critical step i n cell intoxication consisting in the translocation of the active toxi n domain into the cell cytosol. We suggest that membrane translocation takes place at the plasma membrane level. (C) 1998 Academic Press.