How the loop and middle regions influence the properties of Helicobacter pylori VacA channels

VacA is a pore-forming cytotoxin produced by Helicobacter pylori in several strain-specific isoforms, which have been classified in two main families, m1 and m2, according to the sequence of a variable "midregion." Both forms are associated with gastric pathologies and can induce vacuolation of cult ured cells. The comparison of two representative toxins, m1 17874 and m2 95 54, has indicated that the m2 form is less powerful in vacuolation assays a nd that its effects are more strongly cell type dependent. To rationalize t hese differences and to investigate structure-function relationships in thi s toxin, we have compared the properties of the channels formed by these tw o variants and by a construct derived from 17874 by deleting a loop that co nnects the two toxin domains, which is shorter in 9554 than in 17874. Altho ugh the channels formed by all three proteins are similar, m2 9554 channels have, on average, a lower conductance and are less anion-selective and mor e voltage-dependent than the ml pores. Furthermore, the rate of incorporati on of 9554 VacA into planar bilayers depends on lipid composition much more strongly than that of 17874. The comparison with the behavior of the loop deletion mutant indicates that this latter property, as well as a portion o f the conductance decrease, may be attributed to the reduction in loop leng th. The differences in pore properties are proposed to account in part for the different cytotoxicity exhibited by the two toxin isoforms. We furtherm ore present evidence suggesting that the conformation of the membrane-embed ded toxin may be influenced by the lipid composition of the membrane itself .