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Research Project: Disease Defense Responses Signaling in Maize

Location: Chemistry Research

Title: Metabolic reconstructions identify plant 3-methylglutaconyl-CoA hydratase that is crucial for branched-chain amino acid catabolism in mitochondria

Author
item Latimer, Scott
item Li, Yubing
item Nguyen, Thuong
item Soubeyrand, Eric
item Fatihi, Abdelhak
item Elowsky, Christian
item Block, Anna
item Pichersky, Eran
item Basset, Gilles

Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/24/2018
Publication Date: N/A
Citation: N/A

Interpretive Summary: Scientists at the chemistry group, ARS CMAVE, Gainesville Florida in collaboration with a scientist at University of Florida, University of Michigna, University of Nebraska, and the Institut Jean-Pierre Bourgin in France, branched-chain amino acids (BCAAs) leucine, isoleucine, and valine are essential nutrients for mammals. In plants, they act as an alternative energy source when carbohydrates become limiting. Yet, the actual architecture of the degradation pathways of BCAAs is not well understood. In this study, gene network modeling in Arabidopsis and rice, and plant-prokaryote comparative genomics are used to detect candidates for a missing enzyme of leucine catabolism in plants. We show that this enzyme is a mitochondrial 3-methyleglutaconyl-CoA hydratase that displays kinetic features similar to those of its prokaryotic homolog. Furthermore when Arabidopsis knockout mutants in this gene were subjected to dark-induced carbon starvation, their rosette leaves displayed accelerated senescence when compared to the wild type, and this phenotype was paralleled by a marked increase in the accumulation of free leucine, isoleucine and valine. The seeds of the mutant showed a similar accumulation of all three BCAAs. These data demonstrate that 3-methylglutaconyl-CoA hydratase is not only involved in the degradation of leucine, but also to that of isoleucine and valine.

Technical Abstract: The proteinogenic branched-chain amino acids (BCAAs) leucine, isoleucine, and valine are essential nutrients for mammals. In plants, they double as alternative energy sources when carbohydrates become limiting, the catabolism of BCAAs providing electrons to the respiratory chain and intermediates to the tricarboxylic cycle. Yet, the actual architecture of the degradation pathways of BCAAs is not well understood. In this study, gene network modeling in Arabidopsis and rice, and plant- prokaryote comparative genomics detected candidates for 3-methylglutaconyl-CoA hydratase (4.2.1.18), one of the missing enzymes of the catabolism of leucine in plants. Alignments of these protein candidates sampled from various spermatophytes revealed non- homologous N-terminal extensions that are lacking in their bacterial counterparts, and GFP-fusion experiments demonstrated that the Arabidopsis protein, product of gene At4g16800, is targeted to mitochondria. Recombinant At4g16800 catalyzed the dehydration of 3-hydroxymethylglutaryl-CoA into 3-methylglutaconyl-CoA, and displayed kinetic features similar to those of its prokaryotic homolog. When at4g16800 knockout plants were subjected to dark- induced carbon starvation, their rosette leaves displayed accelerated senescence as compared to the wild type, and this phenotype was paralleled by a marked increase in the accumulation of free leucine, isoleucine and valine. The seeds of the at4g16800 mutant showed a similar accumulation of all three BCAAs. These data demonstrate that 3- methylglutaconyl-CoA hydratase is not solely involved in the degradation of leucine, but is also a significant contributor to that isoleucine and valine. Furthermore, evidence is shown that unlike the situation observed in Trypanosomatidae, leucine catabolism does not significantly contribute to the formation of the terpenoid precursor mevalonate.