Crystal structure of the archaeal ammonium transporter Amt-1 from Archaeoglobus fulgidus

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Abstract
Ammonium transporters (Amts) are integral membrane proteins found in all kingdoms of life that fulfill an essential function in the uptake of reduced nitrogen for biosynthetic purposes. Amt-1 is one of three Amts encoded in the genome of the hyperthermophilic archaeon Archaeoglobus fulgidus. The crystal structure of Amt-1 shows a compact trimer with 11 transmembrane helices per monomer and a central channel for substrate conduction in each monomer, similar to the known crystal structure of AmtB from Escherichia coli. Xenon derivatization has been used to identify apolar regions of Amt-1, emphasizing not only the hydrophobicity of the substrate channel but also the unexpected presence of extensive internal cavities that should be detrimental for protein stability. The substrates ammonium and methylammonium have been used for cocrystallization experiments with Amt-1, but the identification of binding sites that are distinct from water positions is not unambiguous. The well ordered cytoplasmic C terminus of the protein in the Amt-1 structure has allowed for the construction of a docking model between Amt-1 and a homology model for its physiological interaction partner, the P(II) protein GlnB-1. In this model, GlnB-1 binds tightly to the cytoplasmic face of the transporter, effectively blocking conduction through the three individual substrate channels.