Involved in the uptake of ammonium/ammonia (NH(4)(+)/NH(3)). Transport is electrogenic. Following sequestration of NH(4)(+) at the periplasmic face, NH(4)(+) is deprotonated and neutral NH(3) is transported into the cytoplasm. 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. In the presence of high extracellular ammonium concentrations, transport activity is inhibited by interaction with the regulatory protein GlnK. Formation of the GlnK-AmtB complex is influenced by intracellular pools of the effector molecules ATP, ADP, Mg(2+) and 2-oxoglutarate. The GlnK-AmtB interaction is also controlled by the level of intracellular glutamine and the uridylylation status of GlnK. Homotrimer (By similarity). In response to elevation of the extracellular ammonium concentration, interacts and forms a complex with GlnK (By similarity). Belongs to the ammonia transporter channel (TC 1.A.11.2) family.