Membrane sequestration of the signal transduction protein GlnK by the ammonium transporter AmtB

The Amt proteins are ammonium transporters that are conserved throughout all domains of life, being found in bacteria, archaea and eukarya. In bacteria and archaea, the Amt structural genes (amtB) are invariably linked to glnK, which encodes a member of the PII signal transduction protein family, proteins that regulate enzyme activity and gene expression in response to the intracellular nitrogen status. We have now shown that in Escherichia coli and Azotobacter vinelandii, GlnK binds to the membrane in an AmtB-dependent manner and that GlnK acts as a negative regulator of the transport activity of AmtB. Membrane binding is dependent on the uridylylation state of GlnK and is modulated according to the cellular nitrogen status such that it is maximal in nitrogen-sufficient situations. The membrane sequestration of GlnK by AmtB represents a novel form of signal transduction in which an integral membrane transport protein functions to link the extracellular ammonium concentration to the intracellular responses to nitrogen status. The results also offer new insights into the evolution of PII proteins and a rationale for their trigonal symmetry.