Major component of the acid-resistance (AR) system allowing enteric pathogens to survive the acidic environment in the stomach (Probable). Exchanges extracellular arginine for its intracellular decarboxylation product agmatine (Agm) thereby expelling intracellular protons (PubMed:12867448, PubMed:14594828, PubMed:19578361, PubMed:21368142). 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 (Probable). agmatine(in) + L-arginine(out) = agmatine(out) + L-arginine(in) Optimum pH is 2.5 for Arg-Agm exchange. Homodimer; each subunit has its own individual transport capacity. By acidic conditions, a monocistronic operon (at protein level). 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. Loss of arginine-dependent acid resistance. No coupled transport of arginine and agmatine (PubMed:12867448, PubMed:14594828). No effect on levels of AdiA (PubMed:12867448). Belongs to the amino acid-polyamine-organocation (APC) superfamily. Basic amino acid/polyamine antiporter (APA) (TC 2.A.3.2) family.