Our proposed research focuses on defining the mechanism of action of the TlpC chemoreceptor in modulating colonization and pathogenesis of the ulcer-causing bacterium Helicobacter pylori. TlpC plays important and exciting roles in H. pylori pathogenesis, sensing host lactate and allowing H. pylori to use it to resist a previ- ously-unappreciated innate immune challenge, complement. The central tenet that guides this work is that un- derstanding chemotaxis signals will lend new insight into the nature of host-pathogen interactions. A gap re- mains, however in our understanding of how H. pylori uses lactate to promote complement resistance, how TlpC integrates lactate and other ligand sensing, and how and where TlpC-mediated sensing promotes in vivo growth. Continued existence of this gap prevents us from gaining a full understanding of H. pylori’s pathogenic mechanisms and, in the long term, creating new drugs to thwart these processes. Millions of people worldwide and in the U.S. are infected by H. pylori and suffer from its associated diseases—ulcers and gastric cancer. Gastric cancer is the fourth highest cause of cancer deaths worldwide. H. pylori is here to stay based on recent studies that show H. pylori incidence has stabilized in the developed world. Furthermore, current therapies to cure H. pylori infection fail with unacceptable frequency, e.g., recent estimates in the United States have found that 20-25% of infected individuals are not cured by the current therapeutic regime. New drug targets are des- perately needed. The specific objective of this application is to dissect TlpC signal transduction and the role of it and its sensed compounds in gastric colonization and disease. Our central hypothesis is that the TlpC- sensed compounds play fundamental roles in H. pylori colonization, and are sensed cooperatively using both subdomains of TlpC’s dCACHE ligand binding domain structure. Our hypothesis has been formulated from preliminary data using crystallization of TlpC, analyzing H. pylori ’s response to lactate and host complement, and determining the role of chemotaxis and TlpC in vivo. Our approach has three Aims, which combine H. py- lori molecular biology, mouse models, and high resolution protein crystallography. In Aim 1, we dissect the role of lactate in H. pylori colonization including how it promotes growth and complement resistance. In Aim 2, we determine how H. pylori TlpC integrates information from multiple ligands into a chemotaxis response. In Aim 3, we define how chemotaxis underlies in vivo population control. The proposed research is innovative in that it will create new knowledge about the functions of chemotaxis during bacterial pathogenesis, the role of comple- ment in the stomach, and high resolution information about how dCACHE ligand binding domains bind ligands. The proposed research is significant because both in our understanding of H. pylori pathogenesis but also for advancing our understanding of dCACHE liga...