LIPID INTERACTION OF THE 37-KDA AND 58-KDA FRAGMENTS OF THE HELICOBACTER-PYLORI CYTOTOXIN

Helicobacter pylori cytotoxin vacA (95 kDa) causes a vacuolar degenera tion of epithelial cells. There is evidence that this protein toxin ac ts inside cells, and hence has to cross a cell membrane. This cytotoxi n is frequently obtained as two fragments of 58 kDa (p58) and 37 kDa ( p37) and it is available only in minute amounts. Here, its membrane in teraction was studied with the two fragments, produced in Escherichia coli. Light scattering and energy transfer experiments show that p37 a nd p58 cause aggregation and fusion of small unilamellar lipid vesicle s: only a reversible aggregation is induced at neutral pH, whereas at acid pH fusion also takes place. p58, but not p37, causes potassium ef flux from liposomes and this occurs only at acid pH. Hydrophobic photo labelling with photoactivatable phosphatidylcholines inserted into lip osomes shows that both fragments are labelled at neutral pH. The amoun t of labelling of the two fragments is much higher at acid pH, consist ent with a further penetration into the hydrophobic core of the lipid bilayer. Tryptophan fluorescence measurements indicate that the two fr agments undergo a pH-driven conformational change. These data are cons istent with cytotoxin entry in the cell cytosol via an intracellular a cidic compartment.