Nitrogen storage regulation by PII protein: lessons learned from taxonomic outliers

Abstract

The paper 'Interaction of N-acetyl-L-glutamate kinase with the PII signal transducer in the non-photosynthetic alga Polytomella parva: Co-evolution towards a hetero-oligomeric enzyme' by Selim et al. highlights how the study of a true taxonomic oddity, the heterotrophic unicellular alga P. parva, has been instrumental in uncovering the large potential for adaptive variation in the signaling complex of PII with the enzyme N-acetylglutamate kinase (NAGK). This complex modifies the regulatory properties of NAGK, allowing nitrogen stockpiling as arginine. In P. parva, a stable PII-NAGK complex is formed which lacks regulation by canonical PII effectors but which exhibits novel adaptive responses to nitrogen abundance mediated by glutamine, a neo-effector of PII proteins of photosynthetic eukaryotes.Comment on: https://doi.PII is an ancient and very widespread homotrimeric signaling protein that was discovered in the sixties of the past century while studying in Escherichia coli the regulation of glutamine synthetase (GS) [1]. After about half-century of studies, it would appear unlikely that PII could still conceal important secrets. However, Selim et al. show [2] that this is not the case, by studying a taxonomic outlier: The heterotrophic nonphotosynthetic unicellular alga Polytomella parva, a close relative of Chlamydomonas reinhardtii that although retaining the plastid, lacks photosynthetic pigments and plastidic DNA. PII regulationIn the GS regulation studies, E. coli PII was found to exhibit a post-translational modification which affects a long flexible loop called the T-loop. This modification (a uridylylation) strongly influences PII effects on its target, ATase, the bifunctional enzyme responsible for adenylylating/deadenylylating GS [1], showing that changes in the T-loop are crucial for transducing PII signals to its targets. PII targets are proteins, either enzymes, channels for metabolites like ammonium, or direct or indirect gene expression regulators [3]. From the few known structures of PII complexes with its targets, it appears that the flexible T-loop adapts its conformation to the target proteins, resulting in interaction and in the induction of functional changes on the target [3]. While post-translational modifications of the T-loop (uridylylation, adenylylation, and phosphorylation) occur in few organisms, allosteric control of PII by small molecules is general. PII effectors act by influencing in a given way the conformation of the T-loop, favoring or disfavoring the Abbreviations GS, glutamine synthetase; NAGK, acetylglutamate kinase and N-acetyl-L-glutamate kinase.

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“…Another effector molecule that interferes with the PII-NAGK complex is arginine. Arginine binds to allosteric sites in NAGK, one site per each subunit, which are occupied in an anti-cooperative manner, thereby inhibiting its activity 21 , 31 . In complex with PII, NAGK feedback-inhibition by arginine is strongly relieved, presumably because the PII complex lowers the affinity of Arg to the allosteric binding sites.…”

“…As a consequence, interaction between PII and PipX is highly sensitive to fluctuations in the ATP/ADP ratio whereas PII -NAGK complex responds mainly to changes to the 2-oxoglutarate levels, as long as sufficient ATP is present. This allows PipX to sense the energy status via PII interaction whereas at the same time, NAGK can read out the level of 2-oxoglutarate (a status reporter of the carbon/nitrogen balance), provided that PII is present in excess over the receptors, which appears to be the case 21 .…”