Regulation of plant NR activity by reversible phosphorylation, 14-3-3 proteins and proteolysis

Abstract

This review highlights progress in dissecting how plant nitrate reductase (NR) activity is regulated by Ca2+, protein kinases, protein kinase kinases, protein phosphatases, 14-3-3 proteins and protease(s). The signalling components that regulate NR have also been discovered to target other enzymes of metabolism, vesicle trafficking and cellular signalling. Extracellular sugars exert a major impact on the 14-3-3-binding status and stability of many target proteins, including NR in plants, whereas other stimuli affect the regulation of some targets and not others. We thus begin to see how selective or global switches in cellular behaviour are triggered by regulatory networks in response to different environmental stimuli. Surprisingly, the question of how changes in NR activity actually affect the rate of nitrate assimilation is turning out to be a tough problem.

Quantitative Circadian Phosphoproteomic Analysis of Arabidopsis Reveals Extensive Clock Control of Key Components in Physiological, Metabolic, and Signaling Pathways

“…In higher plants, NR inactivation is mediated by the phosphorylation of a conserved serine residue (S534) (supplemental Table S3) and binding of 14 -3-3 proteins in the presence of divalent cations or polyamines (81). Mutation of this phosphorylation site in Arabidopsis to aspartate (82) results in the complete abolition of activation/inactivation in response to light/dark transitions or other treatments known to regulate the activation state of NR.…”

Network Analysis of Enzyme Activities and Metabolite Levels and Their Relationship to Biomass in a Large Panel ofArabidopsisAccessions

“…NR is a key site for the regulation of nitrate assimilation. In addition to transcriptional and translational regulation (Nussaume et al, 1995;Kaiser and Huber, 2001;MacKintosh and Meek, 2001;Lea et al, 2006), NR is subject to posttranslational regulation involving phosphorylation followed by the Mg 2+ -dependent binding of a 14-3-3-protein (Nussaume et al, 1995;Kaiser and Huber, 2001;MacKintosh and Meek, 2001;Lea et al, 2006). One possible explanation for the divergent responses may be that NR activity is mainly regulated by posttranslational mechanisms.…”

Light Regulation of Nitrate uptake, Assimilation and Metabolism

“…Generally NR is inactivated in darkness by phosphorylation, and activated by dephosphorylation in the light Spill 1991, MacKintosh 1992). However, the system is more complex than simply phosphorylation and dephosphorylation because members of the 14-3-3 protein family also bind to phosphorylated NR (reviewed by Kaiser and Huber 2001, Lillo and Appenroth 2001, MacKintosh and Meek 2001). 14-3-3 proteins belong to a highly conserved protein family with regulatory roles in plant, fungal and mammalian cells (MacKintosh and Meek 2001).…”

“…However, the system is more complex than simply phosphorylation and dephosphorylation because members of the 14-3-3 protein family also bind to phosphorylated NR (reviewed by Kaiser and Huber 2001, Lillo and Appenroth 2001, MacKintosh and Meek 2001). 14-3-3 proteins belong to a highly conserved protein family with regulatory roles in plant, fungal and mammalian cells (MacKintosh and Meek 2001). It is after the binding of these 14-3-3 proteins that phosphorylated NR is actually inhibited, and inhibition is only observed in the presence of cations.…”