GlnK is the single PII-family signal transduction protein of Pseudomonas putida KT2440. PII proteins are small cytoplasmic homotrimers with a flexible T-loop that integrate cellular nitrogen and carbon/energy status by binding the effector metabolites ATP, ADP and 2-oxoglutarate. GlnK transduces these signals to partner proteins of the nitrogen regulatory (Ntr) system. Its modification state is controlled by GlnD-mediated reversible uridylylation/de-uridylylation at a conserved tyrosine (residue 51), responding to intracellular glutamine levels. In its appropriate state GlnK modulates the kinase/phosphatase balance of the sensor histidine kinase NtrB (NRII/GlnL), thereby controlling the phosphorylation state of the sigma-54-dependent enhancer-binding transcriptional activator NtrC (NRI) and the downstream nitrogen assimilation regulon. The glnK gene lies upstream of and is co-transcribed with the ammonium channel gene amtB (PP_5233), and PII proteins canonically also regulate ammonium uptake through direct interaction with AmtB-type transporters. In KT2440, glnK transcription is itself directly activated by NtrC at a sigma-54 promoter and is strongly induced under nitrogen limitation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000166 nucleotide binding | IEA GO_REF:0000104 | ACCEPT | Summary: PII proteins bind the nucleotides ATP and ADP in a cleft between subunits; nucleotide binding is central to their function as energy/nitrogen sensors. This general term is correct but is captured more specifically by the ATP binding annotation below. Reason: Consistent with PII-family biology and the UniProt Nucleotide-binding keyword; a true molecular property of the protein. |
| GO:0005524 ATP binding | IEA GO_REF:0000118 | ACCEPT | Summary: PII proteins bind ATP (and ADP) as part of their effector-sensing mechanism; the ATP/ADP ratio is a key allosteric input. This is the more informative specific nucleotide-binding term and represents a core molecular property. Reason: Well supported by conserved PII structural biology and InterPro/PANTHER family assignment. |
| GO:0005829 cytosol | IEA GO_REF:0000118 | ACCEPT | Summary: PII proteins are cytoplasmic signal transduction proteins; the default localization for GlnK is the cytosol, where it engages NtrB and other partners. Condition-dependent membrane association with AmtB occurs but the soluble cytosolic pool is the predominant and best-supported localization. Reason: Consistent with PII-family biology; no contradicting evidence. |
| GO:0006351 DNA-templated transcription | IEA GO_REF:0000104 | REMOVE | Summary: GlnK is not itself a component of the transcription machinery and does not directly carry out DNA-templated transcription. Its effect on transcription is indirect, via modulation of the NtrB/NtrC two-component system. This term is too general and mischaracterizes GlnK as part of the transcription process rather than a regulator of it; the regulatory aspect is captured by GO:0006355 and GO:0006808. Reason: Over-broad and not the function of GlnK; it acts on a signaling cascade, not on the transcription reaction itself. The regulation-of-transcription and regulation-of-nitrogen-utilization terms more accurately capture the biology. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: By controlling the NtrB/NtrC phosphorelay, GlnK ultimately influences sigma-54-dependent (NtrC-activated) transcription of nitrogen assimilation genes. This indirect regulatory role is accurate, though the nitrogen-specific term GO:0006808 is more biologically informative. Reason: Correct as a downstream consequence of GlnK signaling, but the regulation of nitrogen utilization term better captures the specific physiological role. |
| GO:0006808 regulation of nitrogen utilization | IEA GO_REF:0000120 | ACCEPT | Summary: This is the central biological process for GlnK. As the single PII protein of KT2440, it couples nitrogen/energy status to the Ntr regulatory cascade, controlling nitrogen assimilation and ammonium utilization. Strongly supported by the PII family role and by the gene's NtrC-dependent induction under nitrogen limitation. Reason: Accurately captures the core physiological function of GlnK. |
| GO:0030234 enzyme regulator activity | IEA GO_REF:0000120 | MODIFY | Summary: GlnK regulates the enzymatic activity of the bifunctional sensor kinase/phosphatase NtrB. The generic enzyme regulator activity term is correct but uninformative; the UniProt protein name ("Activator of NRII(GlnL/NtrB) phosphatase") and PII biology point to a more specific activity, modulation of NtrB's phosphatase activity. Reason: A more specific molecular function term better captures GlnK's documented role in stimulating the phosphatase activity of NtrB; replacing the generic enzyme regulator activity term improves informativeness. Proposed replacements: phosphatase activator activity |
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Download this section (compressed HTML)Q: Does P. putida KT2440 GlnK physically interact with and regulate the ammonium channel AmtB (PP_5233) in a uridylylation-dependent manner, as in enteric bacteria?
Q: Is KT2440 GlnK reversibly uridylylated by GlnD at Tyr51 in vivo, and what is the glutamine/2-oxoglutarate dependence of this modification?
Experiment: Co-purification or bacterial two-hybrid / pull-down assays between GlnK and NtrB, and between GlnK and AmtB, in KT2440 to confirm partner interactions.
Experiment: Mass spectrometry or anti-UMP detection of GlnK uridylylation state across nitrogen-replete and nitrogen-limited conditions, with a glnD mutant control.
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