Mechanism of Ammonia Transport by Amt/MEP/Rh: Structure of AmtB at 1.35 Å

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Abstract
The first structure of an ammonia channel from the Amt/MEP/Rh protein superfamily, determined to 1.35 angstrom resolution, shows it to be a channel that spans the membrane 11 times. Two structurally similar halves span the membrane with opposite polarity. Structures with and without ammonia or methyl ammonia show a vestibule that recruits NH 4+/NH3, a binding site for NH 4+, and a 20 angstrom–long hydrophobic channel that lowers the NH 4+ pKa to below 6 and conducts NH3. Favorable interactions for NH3 are seen within the channel and use conserved histidines. Reconstitution of AmtB into vesicles shows that AmtB conducts uncharged NH3.