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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Arabidopsis Branched-Chain Aminotransferase 3 Functions in Both Amino Acid and Glucosinolate Biosynthesis^1,[W],[OA]

Institut Molekulare Botanik, Universität Ulm, 89069 Ulm, Germany (T.K., J.S., S.B.); and Max Planck Institut für Chemische Ökologie, Biochemie, 07745 Jena, Germany (M.R., J.G.)

In Arabidopsis thaliana, transamination steps in the leucine biosynthetic and catabolic pathways and the methionine (Met) chain elongation cycle of aliphatic glucosinolate formation are catalyzed by branched-chain aminotransferases (BCATs) that are encoded by a small gene family of six members. One member of this family, the plastid-located BCAT3, was shown to participate in both amino acid and glucosinolate metabolism. In vitro activity tests with the recombinant protein identified highest activities with the 2-oxo acids of leucine, isoleucine, and valine, but also revealed substantial conversion of intermediates of the Met chain elongation pathway. Metabolite profiling of bcat3-1 single and bcat3-1/bcat4-2 double knockout mutants showed significant alterations in the profiles of both amino acids and glucosinolates. The changes in glucosinolate proportions suggest that BCAT3 most likely catalyzes the terminal steps in the chain elongation process leading to short-chain glucosinolates: the conversion of 5-methylthiopentyl-2-oxo and 6-methylthiohexyl-2-oxo acids to their respective Met derivatives, homomethionine and dihomo-methionine, respectively. The enzyme can also at least partially compensate for the loss of BCAT4, which catalyzes the initial step of Met chain elongation by converting Met to 4-methylthio-2-oxobutanoate. Our results show the interdependence of amino acid and glucosinolate metabolism and demonstrate that a single enzyme plays a role in both processes.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Stefan Binder (stefan.binder{at}uni-ulm.de).

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.111609

^* Corresponding author; e-mail stefan.binder{at}uni-ulm.de.

Received October 24, 2007; accepted December 12, 2007; published December 27, 2007.