Structural and Functional Analysis of H. pylori toxin VacA

Helicobacter pylori is a Gram-negative bacterium that colonizes the human stomach and contributes to peptic ulceration and gastric adenocarcinoma. Gastric cancer is the second leading cause of cancer-related deaths worldwide. H. pylori secretes a pore-forming exotoxin known as vacuolating toxin (VacA). VacA has important roles in H. pylori colonization of the human stomach and in the pathogenesis of H. pylori-associated gastroduodenal diseases. VacA is secreted as an 88 kDa toxin that oligomerizes to form channels in various cell membranes. Understanding the molecular basis for the role of VacA in pathogenesis has been complicated by VacA's ability to trigger a pleiotropic set of cellular responses in multiple cell types, including gastric epithelial cells and immune cells such as T-lymphocytes. Although the ability to interact with cell membranes, oligomerize, form channels, and be specifically trafficked are all required for VacA function, the mechanisms underlying each of these processes are not understood. The vacA genotype of a H. pylori strain is strongly associated with the capacity to induce peptic ulceration and the development of H. pylori-associated gastric cancer. Sequence variations within specific toxin domains have been proposed to alter VacA's ability to form pores and bind receptors; however, there is no mechanistic explanation for why certain VacA variants are more pathogenic than others. To address the molecular function of VacA, we will define how VacA oligomerizes on lipids and the mechanism of VacA pore formation, determine the structural basis of VacA receptor binding, and define VacA trafficking pathways in gastric epithelial cells. In addition, we will examine how variations in VacA sequence alter these events. The goal of this proposal is to illuminate the critical mechanisms required for VacA activity, providing a necessary platform for the future development of new therapeutic approaches for gastric diseases attributable to H. pylori.