Involved in the regulation of nitrogen metabolism. Regulates the activity of its targets by protein-protein interaction in response to the nitrogen status of the cell. Involved in the regulation of the ammonium transporter AmtB so as to optimize ammonium uptake under all growth conditions. 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. 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. In response to elevation of the extracellular ammonium concentration, interacts and forms a complex with AmtB. 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. Uridylylated/deuridylylated by GlnD. Fully uridylylated in nitrogen-limited conditions and deuridylylated when extracellular ammonium increases. Belongs to the P(II) protein family.