Functions as an ammonium and methylammonium transporter in the absence of glutamine (PubMed:14600241). Required for ammonium utilization at low concentrations or at low pH values, when ammonium is the single nitrogen source (PubMed:14600241). Required for binding of NrgB to the membrane (PubMed:14600241). Interaction between GlnK-AmtB complex and TnrA protects TnrA from proteolytic degradation (PubMed:17001076, PubMed:21435182). Interacts with NrgB for a correct localization of the latter. GlnK-AmtB complex interacts with TnrA. The nrgAB operon is activated by TnrA (PubMed:12823818). NrgB is required for full induction of the operon under conditions of ammonium limitation (PubMed:14600241, PubMed:10864496). Cells grow poorly at pH 5.5 and not at all at pH 5.0 with ammonium as their sole nitrogen source (PubMed:14600241). They are unable to transport the ammonium analog methylammonium, and thus probably also ammonium (PubMed:14600241). In the absence of nitrogen source, cells lacking this gene do not show degradation of TnrA, which is entirely soluble (PubMed:17001076, PubMed:21435182). Belongs to the ammonia transporter channel (TC 1.A.11.2) family.