Fragment-based screening identifies inhibitors of ATPase activity and of hexamer formation of Cagα from the Helicobacter pylori type IV secretion system.

Type IV secretion systems are multiprotein complexes that mediate the translocation of macromolecules across the bacterial cell envelope. In Helicobacter pylori a type IV secretion system encoded by the cag pathogenicity island encodes 27 proteins and most are essential for virulence. We here present the identification and characterization of inhibitors of Cagα, a hexameric ATPase and member of the family of VirB11-like proteins that is essential for translocation of the CagA cytotoxin into mammalian cells. We conducted fragment-based screening using a differential scanning fluorimetry assay and identified 16 molecules that stabilize the protein suggesting that they bind Cagα. Several molecules affect binding of ADP and four of them inhibit the ATPase activity. Analysis of enzyme kinetics suggests that their mode of action is non-competitive, suggesting that they do not bind to the active site. Cross-linking suggests that the active molecules change protein conformation and gel filtration and transmission electron microscopy show that molecule 1G2 dissociates the Cagα hexamer. Addition of the molecule 1G2 inhibits the induction of interleukin-8 production in gastric cancer cells after co-incubation with H. pylori suggesting that it inhibits Cagα in vivo. Our results reveal a novel mechanism for the inhibition of the ATPase activity of VirB11-like proteins.