The crystal structure of the Escherichia coli AmtB-GlnK complex reveals how GlnK regulates the ammonia channel

A regulatory protein-protein interaction governs glutamate biosynthesis in Bacillus subtilis: the glutamate dehydrogenase RocG moonlights in controlling the transcription factor GltC

Glutamate synthesis is the link between carbon and nitrogen metabolism. In Bacillus subtilis, glutamate is exclusively synthesized by the glutamate synthase encoded by the gltAB operon. The glutamate dehydrogenase RocG from B. subtilis is exclusively devoted to glutamate degradation rather than to its synthesis. The expression of the gltAB operon is induced by glucose and ammonium and strongly repressed by arginine. Regulation by glucose and arginine depends on the transcriptional activator protein GltC. The gltAB operon is constitutively expressed in a rocG mutant strain, but the molecular mechanism of negative control of gltAB expression by RocG has so far remained unknown. We studied the role of RocG in the intracellular accumulation of GltC. Furthermore, we considered the possibility that RocG might act as a transcription factor and be able to inhibit the expression of gltAB either by binding to the mRNA or to the promoter region of the gltAB operon. Finally, we asked whether a direct binding of RocG to GltC could be responsible for the inhibition of GltC. The genetic and biochemical data presented here show that the glutamate dehydrogenase RocG is able to bind to and concomitantly inactivate the activator protein GltC. This regulatory mechanism by the bifunctional enzyme RocG allows the tight control of glutamate metabolism by the availability of carbon and nitrogen sources.

Membrane sequestration of PII proteins and nitrogenase regulation in the photosynthetic bacterium Rhodobacter capsulatus

Both Rhodobacter capsulatus PII homologs GlnB and GlnK were found to be necessary for the proper regulation of nitrogenase activity and modification in response to an ammonium shock. As previously reported for several other bacteria, ammonium addition triggered the AmtB-dependent association of GlnK with the R. capsulatus membrane. Native polyacrylamide gel electrophoresis analysis indicates that the modification/demodification of one PII homolog is aberrant in the absence of the other. In a glnK mutant, more GlnB was found to be membrane associated under these conditions. In a glnB mutant, GlnK fails to be significantly sequestered by AmtB, even though it appears to be fully deuridylylated. Additionally, the ammonium-induced enhanced sequestration by AmtB of the unmodifiable GlnK variant GlnK-Y51F follows the wild-type GlnK pattern with a high level in the cytoplasm without the addition of ammonium and an increased level in the membrane fraction after ammonium treatment. These results suggest that factors other than PII modification are driving its association with AmtB in the membrane in R. capsulatus.

The higher plant PII signal transduction protein: structure, function and properties

The PII carbon/nitrogen sensing protein was discovered in Escherichia coli (Migula 1895) Castellani and Chalmers 1919, over 40 years ago. Orthologues have been discovered in three kingdoms of life making it one of the most ancient and conserved signaling proteins known. Recent advances in the field have established its primary binding partner in plants as N-acetyl glutamate kinase and the crystal structure has revealed features unique to plants that likely contribute to its function in vivo. Here, we review the properties, function, and novel structural features of this chloroplast-localized metabolic sensor of higher plants.

Regulation of NH4+ transport by essential cross talk between AMT monomers through the carboxyl tails

Ammonium transport across plant plasma membranes is facilitated by AMT/Rh-type ammonium transporters (AMTs), which also have homologs in most organisms. In the roots of the plant Arabidopsis (Arabidopsis thaliana), AMTs have been identified that function directly in the high-affinity NH4+ acquisition from soil. Here, we show that AtAMT1;2 has a distinct role, as it is located in the plasma membrane of the root endodermis. AtAMT1;2 functions as a comparatively low-affinity NH4+ transporter. Mutations at the highly conserved carboxyl terminus (C terminus) of AMTs, including one that mimics phosphorylation at a putative phosphorylation site, impair NH4+ transport activity. Coexpressing these mutants along with wild-type AtAMT1;2 substantially reduced the activity of the wild-type transporter. A molecular model of AtAMT1;2 provides a plausible explanation for the dominant inhibition, as the C terminus of one monomer directly contacts the neighboring subunit. It is suggested that part of the cytoplasmic C terminus of a single monomer can gate the AMT trimer. This regulatory mechanism for rapid and efficient inactivation of NH4+ transporters may apply to several AMT members to prevent excess influx of cytotoxic ammonium.