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

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 NHsub 4sup +/NHsub 3, a binding site for NHsub 4sup +, and a 20 angstrom-long hydrophobic channel that lowers the NHsub 4sup + pKsub a to below 6 and conducts NHsub 3. Favorable interactions for NHsub 3 are seen within the channel and use conserved histidines. Reconstitution of AmtB into vesicles shows that AmtB conducts uncharged NHsub 3. [PUBLICATION ABSTRACT]