amtB

AmtB is a membrane protein of the ammonium transporter family (Amt/MEP) found in bacteria and archaea. Encoded by the amtB gene, it functions as an ammonium transporter in the inner membrane and contributes to nitrogen acquisition under limited ammonium conditions. AmtB proteins typically form homotrimers, with each subunit contributing to a central pore that enables ammonium passage across the cytoplasmic membrane.

Substrate and transport: AmtB mediates uptake of ammonium and can transport NH3 and NH4+ depending on pH. It plays a key role in nitrogen metabolism in organisms living in nitrogen-poor environments and often operates as a high-affinity transporter when external ammonium is scarce, complementing other assimilatory pathways.

Structure and mechanism: Structural studies show AmtB as a trimer of roughly 11-transmembrane-helix subunits. The pore interior is hydrophobic, and a conserved twin-histidine motif participates in the deprotonation of NH4+ to NH3, enabling diffusion of neutral ammonia through the channel while protons are managed by adjacent residues. The exact proton transfer pathway is a subject of ongoing research, but the overall model involves NH3 transit through the pore with selective gating.

Regulation and significance: In cells, AmtB activity is modulated by nitrogen status and regulatory proteins such as GlnK, which can inhibit transport under ammonium-replete conditions. AmtB is widely studied as a model for membrane transport and structure–function relationships within the Amt/MEP family, informing understanding of ammonium uptake in bacteria and archaea.