Publication

Journal	Proceedings of the National Academy of Sciences
Volume (Issue)	113(37)
Page(s)	10358 - 10363
Title of proceedings	Proceedings of the National Academy of Sciences
DOI	10.1073/pnas.1605442113

Pathogenic enterobacteria need to survive the extreme acidity of the stomach to successfully colonize the human gut. Enteric bacteria circumvent the gastric acid barrier by activating extreme acid-resistance responses, such as the arginine-dependent acid resistance system. In this response, l -arginine is decarboxylated to agmatine, thereby consuming one proton from the cytoplasm. In Escherichia coli , the l -arginine/agmatine antiporter AdiC facilitates the export of agmatine in exchange of l -arginine, thus providing substrates for further removal of protons from the cytoplasm and balancing the intracellular pH. We have solved the crystal structures of wild-type AdiC in the presence and absence of the substrate agmatine at 2.6-Å and 2.2-Å resolution, respectively. The high-resolution structures made possible the identification of crucial water molecules in the substrate-binding sites, unveiling their functional roles for agmatine release and structure stabilization, which was further corroborated by molecular dynamics simulations. Structural analysis combined with site-directed mutagenesis and the scintillation proximity radioligand binding assay improved our understanding of substrate binding and specificity of the wild-type l -arginine/agmatine antiporter AdiC. Finally, we present a potential mechanism for conformational changes of the AdiC transport cycle involved in the release of agmatine into the periplasmic space of E. coli .