Involved in the uptake of ammonium/ammonia (NH(4)(+)/NH(3)) (PubMed:14668330, PubMed:15876187, PubMed:15563598, PubMed:15361618, PubMed:16910683, PubMed:30211659, PubMed:32662768). Transport is electrogenic (PubMed:30211659, PubMed:32662768). Following sequestration of NH(4)(+) at the periplasmic face, NH(4)(+) is deprotonated and neutral NH(3) is transported into the cytoplasm (PubMed:15876187, PubMed:16910683, PubMed:18362341, PubMed:19278252, PubMed:32662768). Neutral NH(3) and charged H(+) are carried separately across the membrane on a unique two-lane pathway, before recombining to NH(4)(+) inside the cell (PubMed:32662768). In vitro can also transport the larger analogs methylamine and methylammonium (PubMed:11847102, PubMed:14668330, PubMed:12023896, PubMed:15876187, PubMed:17998534, PubMed:18362341). Also acts as a sensor of the extracellular ammonium concentration (PubMed:14668330). In the presence of high extracellular ammonium concentrations, transport activity is inhibited by interaction with the regulatory protein GlnK (PubMed:11847102, PubMed:14668330, PubMed:16864585). Ammonium transport through AmtB is required for GlnK sequestration (PubMed:14668330). Formation of the GlnK-AmtB complex is influenced by intracellular pools of the effector molecules ATP, ADP, Mg(2+) and 2-oxoglutarate (PubMed:16864585, PubMed:20639578). The rapid drop in the 2-oxoglutarate pool upon ammonium influx and a simultaneous, but transient, change in the ATP/ADP ratio promotes AmtB-GlnK complex formation (PubMed:20639578). The GlnK-AmtB interaction is also controlled by the level of intracellular glutamine and the uridylylation status of GlnK (PubMed:14668330). Inhibited by imidazole and thallium, which likely act by competitive binding to the periplasmic ammonium binding site (PubMed:18362341). Transport activity is independent of the membrane potential and of ATP hydrolysis (PubMed:15876187). Homotrimer (PubMed:12023896, PubMed:15563598, PubMed:15361618, PubMed:17040913). In response to elevation of the extracellular ammonium concentration, interacts and forms a complex with GlnK (PubMed:10637624, PubMed:11847102, PubMed:14668330, PubMed:17220269, PubMed:17190799, PubMed:16864585, PubMed:20639578). Interacts with GlnK with a stoichiometry of AmtB(3):GlnK(3) (PubMed:17220269, PubMed:17190799). Sequestration of GlnK is reversible (PubMed:14668330). The C-terminal cytoplasmic domain plays a significant role in normal function and it might also mediate co-operativity between the three subunits (PubMed:17453422). The C-terminal cytoplasmic domain is also required for interaction with the T-loop of GlnK (PubMed:11847102, PubMed:17190799). Specifically binds 1-palmitoyl-2-oleoyl phosphatidylglycerol (POPG), which increases protein stability (PubMed:24899312). POPG is an essential cofactor for transport activity and, in the absence of POPG, AmtB cannot complete the full translocation cycle (PubMed:30211659). Can also bind phosphatidylethanolamine and cardiolipin-like molecules, leading to positive allosteric modulation (PubMed:29507234). Belongs to the ammonia transporter channel (TC 1.A.11.2) family. Methylammonium (MeA) has been widely used to measure ammonium transport activity because the radioactive tracer [14C]MeA is commercially available. However, MeA is a poor substrate analog for AmtB, not well suited to elucidate the mechanistic details of AmtB activity.