GlnK

GlnK is a regulatory protein of the PII family found in bacteria and archaea that participates in nitrogen metabolism. It is encoded by the glnK gene and is often located near or together with glnB, another PII protein, in bacterial genomes. GlnK typically forms a homotrimer and acts as a sensor of cellular nitrogen status, integrating signals from energy and carbon metabolism through nucleotide binding and from nitrogen availability through post-translational modification.

A key feature of GlnK function is its reversible uridylylation, performed by the uridylyltransferase GlnD in response to nitrogen conditions. The uridylylation state of GlnK influences its interactions with target proteins and regulatory pathways. When nitrogen is scarce, GlnD uridylylates GlnK, enabling regulatory interactions that adjust ammonium uptake and nitrogen-responsive gene expression. Under different nitrogen conditions, the modified state of GlnK alters its associations accordingly.

One major target of GlnK is the ammonium transporter AmtB; binding of GlnK to AmtB modulates ammonium transport across the membrane, helping the cell balance uptake with nitrogen availability. GlnK also interfaces with the NtrB/NtrC two-component regulatory system, affecting transcription of nitrogen-regulated genes and broader nitrogen assimilation pathways.

Distribution and function: GlnK is widespread in bacteria and archaea, where it provides overlapping or distinct regulatory roles alongside GlnB. In some organisms, GlnK can compensate for GlnB functions, while in others the two proteins have specialized regulatory roles. The glnK gene is commonly part of nitrogen-regulation networks and contributes to the coordination of nitrogen uptake and metabolism.