Plastidial NAD-Dependent Malate Dehydrogenase: A Moonlighting Protein Involved in Early Chloroplast Development through Its Interaction with an FtsH12-FtsHi Protease Complex

Abstract

Malate dehydrogenases (MDHs) convert malate to oxaloacetate using NAD(H) or NADP(H) as a cofactor. mutants lacking plastidial NAD-dependent MDH () are embryo-lethal, and constitutive silencing (1) causes a pale, dwarfed phenotype. The reason for these severe phenotypes is unknown. Here, we rescued the embryo lethality of via embryo-specific expression of pdNAD-MDH. Rescued seedlings developed white leaves with aberrant chloroplasts and failed to reproduce. Inducible silencing of pdNAD-MDH at the rosette stage also resulted in white newly emerging leaves. These data suggest that pdNAD-MDH is important for early plastid development, which is consistent with the reductions in major plastidial galactolipid, carotenoid, and protochlorophyllide levels in1 seedlings. Surprisingly, the targeting of other NAD-dependent MDH isoforms to the plastid did not complement the embryo lethality of , while expression of enzymatically inactive pdNAD-MDH did. These complemented plants grew indistinguishably from the wild type. Both active and inactive forms of pdNAD-MDH interact with a heteromeric AAA-ATPase complex at the inner membrane of the chloroplast envelope. Silencing the expression of FtsH12, a key member of this complex, resulted in a phenotype that strongly resembles1. We propose that pdNAD-MDH is essential for chloroplast development due to its moonlighting role in stabilizing FtsH12, distinct from its enzymatic function.

On the move: Redox –dependent protein relocation

“…Alteration in nuclear DNA methylation (Virdi et al, 2015) pdNAD-MDH NAD-dependent malate dehydrogenase Plastid Activity independent stabilisation of FtsH12 component of inner envelope membrane protease AAA-ATPase complex. Essential for viability (Schreier et al, 2018) AROGENATE DEHYDRATASE2/5…”

“…Firstly, as an enzyme it is 78 responsible for the synthesis of ascorbic acid, and as a chaperone it is essential for the 79 assembly of respiratory complex I (Schimmeyer et al, 2016). Similarly plastid NAD 80 dependent malate dehydrogenase has a non-enzymatic function stabilising the FtsH12 81 component of the inner envelope AAA ATPase (Schreier et al, 2018). 82…”

A Force-Generating Machine in the Plant’s Powerhouse: A Pulling AAA-ATPase Motor Drives Protein Translocation into Chloroplasts

“…Except for FtsH12, these proteins have lost their proteolytic activity (therefore, they are called FtsHi for FtsH-inactive) and even for the essential FtsH12 subunit, the proteolytic activity proved to be dispensable. The NAD-MDH subunit presumably plays a structural role in the motor because its catalytic activity is dispensable and it cannot be functionally replaced by unrelated malate dehydrogenases, even if they exhibit the same enzymatic activity (Schreier et al, 2018).…”

Moonlighting NAD+ Malate Dehydrogenase Is Essential for Chloroplast Biogenesis

“…A classic example of a moonlighting protein is the enzyme lactate dehydrogenase, which was adapted to serve as a crystallin protein in the eye lens of some vertebrates (Wistow et al, 1987). Schreier et al (2018) now provide a striking example of a moonlighting function for plastidial NAD + -dependent malate dehydrogenase (pdNAD-MDH) in Arabidopsis. The authors report that pdNAD-MDH is essential for plastid development, not as a functional metabolic enzyme but, rather, as a protein constituent of a large AAA + (ATPase Associated with various cellular Activities) protease complex localized to the chloroplast envelope inner membrane and essential for protein processing during plastid biogenesis.…”

“…In order to understand the essential function of this enzyme, Schreier et al (2018) adopted several approaches.…”