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Structure and Mechanism of an Amino Acid Antiporter

Antiporter Antics Bacteria that survive in the acidic environment of the stomach have mechanisms to maintain a high intracellular pH. In Escherichia coli, glutamate (Glu) and arginine (Arg) are decarboxylated intracellularly and the reaction products are exchanged with extracellular Glu and Arg. Gao et al. (p. 1565, published online 28 May) now report a crystal structure of AdiC, an arginine:agmatine antiporter from E. coli. AdiC exhibits the same fold as that of the Na+-coupled symporters, including LeuT. It contains 12 transmembrane segments, forms a homodimer, and exists in an outward-facing, open conformation in the crystals. The structure, together with biochemical data, suggests how the antiporter senses the pH and responds to transport the reaction product agmatine out of the cell and Arg into the cell. The architecture of a membrane protein channel helps explain how Escherichia coli survive the acidic conditions of the stomach. Virulent enteric pathogens such as Escherichia coli strain O157:H7 rely on acid-resistance (AR) systems to survive the acidic environment in the stomach. A major component of AR is an arginine-dependent arginine:agmatine antiporter that expels intracellular protons. Here, we report the crystal structure of AdiC, the arginine:agmatine antiporter from E. coli O157:H7 and a member of the amino acid/polyamine/organocation (APC) superfamily of transporters at 3.6 Å resolution. The overall fold is similar to that of several Na+-coupled symporters. AdiC contains 12 transmembrane segments, forms a homodimer, and exists in an outward-facing, open conformation in the crystals. A conserved, acidic pocket opens to the periplasm. Structural and biochemical analysis reveals the essential ligand-binding residues, defines the transport route, and suggests a conserved mechanism for the antiporter activity.