Moonlighting NAD<sup>+</sup> Malate Dehydrogenase Is Essential for Chloroplast Biogenesis

Smith, Steven M.

doi:10.1105/tpc.18.00474

Cited by 4 publications

(3 citation statements)

References 6 publications

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“…Among these enzymes, eight were involved in the CBC, while three were relevant to the C4-dicarboxylic acid cycle and the CAM pathway, namely, phosphoenolpyruvate carboxylase (PEPC) [EC:4.1.1.31], malate dehydrogenase (MDH) [EC:1.1.1.37], and alanine transaminase (ALT) [EC:2.6.1.2]. MDH catalyzes the reduction of NAD + to NADH 48 to facilitate the oxidation of malate to oxaloacetate. This process is part of some crucial metabolic pathways, for example, the citric acid cycle.…”

Section: Resultsmentioning

confidence: 99%

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

et al. 2020

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Lysine ubiquitination, a widely studied posttranslational modification, plays vital roles in various biological processes in eukaryotic cells. Although several studies have examined the plant ubiquitylome, no such research has been performed in tobacco, a model plant for molecular biology. Here, we comprehensively analyzed lysine ubiquitination in tobacco ( Nicotiana tabacum ) using LC–MS/MS along with highly sensitive immune-affinity purification. In total, 964 lysine-ubiquitinated (K ub ) sites were identified in 572 proteins. Extensive bioinformatics studies revealed the distribution of these proteins in various cellular locations, including the cytoplasm, chloroplast, nucleus, and plasma membrane. Notably, 25% of the K ub proteins were located in the chloroplast of which 21 were enzymatically involved in important pathways, that is, photosynthesis and carbon fixation. Western blot analysis indicated that TMV infection can cause changes in ubiquitination levels. This is the first comprehensive proteomic analysis of lysine ubiquitination in tobacco, illustrating the vital role of ubiquitination in various physiological and biochemical processes and representing a valuable addition to the existing landscape of lysine ubiquitination.

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Section: Resultsmentioning

confidence: 99%

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

et al. 2020

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“…The null mutants of plNAD-MDH in Arabidopsis are embryo-lethal, implying that this enzyme has an essential metabolic function during early embryogenesis [37,39]. Furthermore, a transgenic line with lowered plNAD-MDH resulting from gene silencing exhibited poor growth, pale leaves, disorganized chloroplasts and low respiration, indicating an important function at multiple stages of plant development [37,38,40]. Analysis of such leaves showed that chloroplast development is impaired in the absence of plNAD-MDH.…”

Section: The Multitasking Role Of Plnad-mdhmentioning

confidence: 99%

Malate Circulation: Linking Chloroplast Metabolism to Mitochondrial ROS

Zhao

Zhou

et al. 2020

Trends in Plant Science

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110

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In photosynthetic cells, chloroplasts and mitochondria are the sites of the core redox reactions underpinning energy metabolism. Such reactions generate reactive oxygen species (ROS) when oxygen is partially reduced. ROS signaling leads to responses by cells which enable them to adjust to changes in redox status. Recent studies in Arabidopsis thaliana reveal that chloroplast NADH can be used to generate malate which is exported to the mitochondrion where its oxidation regenerates NADH. Oxidation of this NADH produces mitochondrial ROS (mROS) which can activate signaling systems to modulate energy metabolism, and in certain cases can lead to programmed cell death (PCD). We propose the term 'malate circulation' to describe such redistribution of reducing equivalents to mediate energy homeostasis in the cell. The Challenge of Regulating Chloroplast Reducing Power The chloroplast is considered to be the main site of reactive oxygen species (ROS; see Glossary) production, because photosynthetic electron transfer reactions ultimately generate all the reducing equivalents required for assimilation and biosynthesis in the whole cell. Achieving the balance between the production and consumption of reducing power is vital since excess reducing equivalents result in ROS production, leading to oxidative signaling. Under stressful environmental conditions such as excess light or high temperature, elevated ROS signaling can lead to changes in cell function to adjust to the effects of excess reducing equivalents [1-3]. One way in which the chloroplast is thought to dissipate reducing equivalents is by means of the malate valve [4,5]. It is proposed that excess NADPH is consumed by plastidial NADPdependent malate dehydrogenase (plNADP-MDH) to produce malate which is exported from the chloroplast for subsequent oxidation. However, mutants lacking plNADP-MDH do not appear to be compromised under conditions of photosynthetic stress, suggesting that other mechanisms can operate [6-8]. New discoveries reveal that excess chloroplast NADH is consumed by plastidial NAD-dependent malate dehydrogenase (plNAD-MDH), and the malate is exported to the mitochondrion. There, malate oxidation generates NADH, which is then oxidized by Complex I (NADH: ubiquinone oxidoreductase), generating ROS which can trigger cell death in extreme circumstances [9,10]. In this opinion article we propose that chloroplast redox stress is linked by a CTM malate transport system that brings about mROS signaling to modulate energy metabolism in the cell under variable environmental conditions. We further propose the term 'malate circulation' to describe this form of redistribution of reducing equivalents in the cell. ROS in Mitochondria Chloroplast ROS production occurs when electron carriers in the photosynthetic electron transport chain (pETC) become over-reduced. This might happen when the supply of reducing equivalents is greater than their consumption by the cell. Mitochondria provide a means to alleviate

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“…Essential elements of this model include the formation of a hetero-hexameric ring complex at the inner envelope membrane and a stromal localization of the ATPase domains of its subunits, as depicted in several previous publications (refs. 13 – 16 and Fig. 1 A ).…”

Section: Discussionmentioning

confidence: 85%

Chloroplast import motor subunits FtsHi1 and FtsHi2 are located on opposite sides of the inner envelope membrane

Chang,

Chen,

2023

Proc. Natl. Acad. Sci. U.S.A.

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Protein import into chloroplasts is powered by ATP hydrolysis in the stroma. Establishing the identity and functional mechanism of the stromal ATPase motor that drives import is critical for understanding chloroplast biogenesis. Recently, a complex consisting of Ycf2, FtsHi1, FtsHi2, FtsHi4, FtsHi5, FtsH12, and malate dehydrogenase was shown to be important for chloroplast protein import, and it has been proposed to act as the motor driving protein translocation across the chloroplast envelope into the stroma. To gain further mechanistic understanding of how the motor functions, we performed membrane association and topology analyses on two of its subunits, FtsHi1 and FtsHi2. We isolated cDNA clones encoding FtsHi1 and FtsHi2 preproteins to perform in vitro import experiments in order to determine the exact size of each mature protein. We also generated antibodies against the C-termini of the proteins, i.e., where their ATPase domains reside. Protease treatments and alkaline and high-salt extractions of chloroplasts with imported and endogenous proteins revealed that FtsHi1 is an integral membrane protein with its C-terminal portion located in the intermembrane space of the envelope, not the stroma, whereas FtsHi2 is a soluble protein in the stroma. We further complemented an FtsHi1 -knockout mutant with a C-terminally tagged FtsHi1 and obtained identical results for topological analyses. Our data indicate that the model of a single membrane–anchored pulling motor at the stromal side of the inner membrane needs to be revised and suggest that the Ycf2–FtsHi complex may have additional functions.

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Moonlighting NAD⁺ Malate Dehydrogenase Is Essential for Chloroplast Biogenesis

Cited by 4 publications

References 6 publications

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

Malate Circulation: Linking Chloroplast Metabolism to Mitochondrial ROS

Chloroplast import motor subunits FtsHi1 and FtsHi2 are located on opposite sides of the inner envelope membrane

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Moonlighting NAD+ Malate Dehydrogenase Is Essential for Chloroplast Biogenesis

Cited by 4 publications

References 6 publications

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

Comprehensive Proteomic Analysis of Lysine Ubiquitination in Seedling Leaves of Nicotiana tabacum

Malate Circulation: Linking Chloroplast Metabolism to Mitochondrial ROS

Chloroplast import motor subunits FtsHi1 and FtsHi2 are located on opposite sides of the inner envelope membrane

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Moonlighting NAD⁺ Malate Dehydrogenase Is Essential for Chloroplast Biogenesis