Involved in the regulation of nitrogen metabolism (PubMed:8843440, PubMed:10760266, PubMed:11847102, PubMed:12366843, PubMed:14668330, PubMed:28538158). Regulates the activity of its targets by protein-protein interaction in response to the nitrogen status of the cell (PubMed:8843440, PubMed:10760266, PubMed:11847102, PubMed:14668330). Involved in the regulation of the ammonium transporter AmtB so as to optimize ammonium uptake under all growth conditions (PubMed:11847102, PubMed:14668330, PubMed:16864585). In nitrogen-limited conditions, GlnK does not interact with AmtB, which remains active and imports ammonium. When extracellular ammonium increases, GlnK associates tightly with AmtB in the inner membrane, thereby inhibiting the transporter activity (PubMed:11847102, PubMed:14668330, PubMed:16864585). Also involved in the regulation of the glutamine synthetase adenylyltransferase/adenylyl-removing (AT/AR) enzyme GlnE and the glutamine synthetase GlnA (PubMed:8843440, PubMed:10760266, PubMed:28538158). In nitrogen-limited conditions, formation of uridylylated GlnB(PII)/GlnK heterotrimers may fine-regulate the activation of GlnA: uridylylated heterotrimers stimulate the deadenylation and the activation of GlnA whereas uridylylated GlnK homotrimers do not stimulate, or hardly stimulate, the deadenylation of GlnA (PubMed:10760266). In addition, regulates the expression of Ntr genes during nitrogen starvation, probably via the control of the levels of phosphorylated NRI during different stages of the response to nitrogen limitation (PubMed:12366843). 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). ADP orients the surface of GlnK for AmtB blockade (PubMed:17190799). The GlnK-AmtB interaction is also controlled by the level of intracellular glutamine and the uridylylation status of GlnK (PubMed:11847102, PubMed:14668330, PubMed:16864585). The uridylylation state of GlnK influences the dynamics of its interaction with AmtB, but association and dissociation of the complex in response to cellular nitrogen status can occur in the absence of GlnK post-translational modification (PubMed:25566239). Homotrimer (PubMed:10760266, PubMed:9733647, PubMed:17220269). In response to elevation of the extracellular ammonium concentration, interacts and forms a complex with AmtB (PubMed:11847102, PubMed:14668330, PubMed:17220269, PubMed:17190799, PubMed:16864585, PubMed:20639578). Binds to the cytoplasmic face of AmtB with a stoichiometry of AmtB(3):GlnK(3) (PubMed:17220269, PubMed:17190799). Sequestration by AmtB is reversible (PubMed:14668330). In nitrogen-poor medium, can form heterotrimers with GlnB (PII protein) (PubMed:10760266). During nitrogen limitation, GlnK is predominantly in its fully uridylylated state in the cytoplasmic fraction. In response to nitrogen shock, GlnK is deuridylylated rapidly and associates tightly with AmtB in the inner membrane. Expression is regulated by the nitrogen status (PubMed:8843440). May be regulated through the NRI/NRII two-component regulatory system (PubMed:8843440). Interacts with AmtB almost exclusively via a long surface loop containing Tyr-51 (T-loop), the tip of which inserts deeply into the cytoplasmic pore exit, blocking ammonia conduction (PubMed:17220269, PubMed:17190799). Uridylylation probably sterically blocks the T-loop region from interacting with AmtB (PubMed:17190799). Uridylylated/deuridylylated by GlnD (PubMed:8843440, PubMed:11847102). Fully uridylylated in nitrogen-limited conditions and deuridylylated when extracellular ammonium increases (PubMed:11847102, PubMed:20639578). Uridylylation abrogates binding to AmtB (PubMed:11847102, PubMed:16864585). Mutant lacking this gene shows very high expression of the glnK and nac promoters in nitrogen-starved cells, and has a severe defect in the growth rate recovery in ammonia after nitrogen starvation. Belongs to the P(II) protein family.