title: Substrate selectivity in arginine-dependent acid resistance in enteric bacteria.
abstract: To successfully colonize the human gut, enteric bacteria must activate acid resistance systems to survive the extreme acidity (pH 1.5-3.5) of the stomach. The antiporter AdiC is the master orchestrator of the arginine-dependent system. Upon acid shock, it imports extracellular arginine (Arg) into the cytoplasm, providing the substrate for arginine decarboxylases, which consume a cellular proton ending up in a C-H bond of the decarboxylated product agmatine (Agm(2+)). Agm(2+) and the "virtual" proton it carries are exported via AdiC subsequently. It is widely accepted that AdiC counters intracellular acidification by continuously pumping out virtual protons. However, in the gastric environment, Arg is present in two carboxyl-protonation forms, Arg(+) and Arg(2+). Virtual proton pumping can only be achieved by Arg(+)/Agm(2+) exchange, whereas Arg(2+)/Agm(2+) exchange would produce no net proton movement. This study experimentally asks which exchange AdiC catalyzes, an issue previously unapproachable due to the absence of a reconstituted system mimicking the situation of bacteria in the stomach. Here, using an oriented liposome system able to hold a three-unit pH gradient, we demonstrate that Arg/Agm exchange by AdiC is strongly electrogenic with positive charge moved outward, and thus that AdiC mainly mediates Arg(+)/Agm(2+) exchange to support effective virtual proton pumping. Further experiments reveal a mechanistic surprise--that AdiC selects Arg(+) against Arg(2+) on the basis of gross valence, rather than by local scrutiny of protonation states of the carboxyl group, as had been suggested by Arg-bound AdiC crystal structures.
authors.affiliation: Department of Biochemistry, and Howard Hughes Medical Institute, Brandeis University, Waltham, MA 02454, USA.
authors.forename: Ming-Feng
authors.initials: MF
authors.lastname: Tsai
authors.forename: Christopher
authors.initials: C
authors.lastname: Miller
mesh_terms: D026905:Amino Acid Transport Systems / Q000737:chemistry*; D017920:Antiporters / Q000737:chemistry*; D001120:Arginine / Q000737:chemistry; D001426:Bacterial Proteins / Q000737:chemistry*; D001692:Biological Transport; D002247:Carbon Isotopes / Q000737:chemistry; D002384:Catalysis; D002956:Citrulline / Q000737:chemistry; D004755:Enterobacteriaceae / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D008081:Liposomes / Q000737:chemistry; D011188:Potassium / Q000737:chemistry; D011487:Protein Conformation; D011522:Protons; D013379:Substrate Specificity
chemical_list: D026905:Amino Acid Transport Systems; D017920:Antiporters; D001426:Bacterial Proteins; D002247:Carbon Isotopes; D008081:Liposomes; D011522:Protons; D002956:Citrulline; D001120:Arginine; D011188:Potassium