Major component of the acid-resistance (AR) system allowing enteric pathogens to survive the acidic environment in the stomach. Exchanges extracellular arginine for its intracellular decarboxylation product agmatine (Agm) thereby expelling intracellular protons. Probably undergoes several conformational states in order to translocate the substrate across the membrane; keeps the substrate accessible to only 1 side of the membrane at a time by opening and closing 3 membrane-internal gates. agmatine(in) + L-arginine(out) = agmatine(out) + L-arginine(in) Homodimer; each subunit has its own individual transport capacity. Each subunit has 12 transmembrane (TM) helices; TM1 and TM6 are interrupted by short non-helical Gly-rich loops in the middle of their transmembrane spans. Each subunit has a central cavity which binds substrate. Belongs to the amino acid-polyamine-organocation (APC) superfamily. Basic amino acid/polyamine antiporter (APA) (TC 2.A.3.2) family.