The Branched-Chain Amino Acid Transaminase Gene Family in Arabidopsis Encodes Plastid and Mitochondrial Proteins

Abstract

Branched-chain amino acid transaminases (BCATs) play a crucial role in the metabolism of leucine, isoleucine, and valine. They catalyze the last step of the synthesis and/or the initial step of the degradation of this class of amino acids. In Arabidopsis, seven putative BCAT genes are identified by their similarity to their counterparts from other organisms. We have now cloned the respective cDNA sequences of six of these genes. The deduced amino acid sequences show between 47.5% and 84.1% identity to each other and about 30% to the homologous enzymes from yeast (Saccharomyces cerevisiae) and mammals. In addition, many amino acids in crucial positions as determined by crystallographic analyses of BCATs from Escherichia coli and human (Homo sapiens) are conserved in the AtBCATs. Complementation of a yeast ⌬bat1/⌬bat2 double knockout strain revealed that five AtBCATs can function as BCATs in vivo. Transient expression of BCAT:green fluorescent protein fusion proteins in tobacco (Nicotiana tabacum) protoplasts shows that three isoenzymes are imported into chloroplasts (AtBCAT-2, -3, and -5), whereas a single enzyme is directed into mitochondria (AtBCAT-1).

Characterization of the Branched-Chain Amino Acid Aminotransferase Enzyme Family in Tomato

“…AtBCAT2, -3, and -5 localize to chloroplasts, suggesting roles in BCAA synthesis (Diebold et al, 2002). AtBCAT4 is cytosolic (Schuster et al, 2006), and the location of AtBCAT6 is suggested to be cytosolic because of its lack of a defined target peptide sequence (Diebold et al, 2002). Complementation analysis in BCAT-deficient yeast strains confirmed the functions of AtBCAT1, -2, -3, -5, and -6 but not AtBCAT4 (Diebold et al, 2002).…”

“…AtBCAT4 is cytosolic (Schuster et al, 2006), and the location of AtBCAT6 is suggested to be cytosolic because of its lack of a defined target peptide sequence (Diebold et al, 2002). Complementation analysis in BCAT-deficient yeast strains confirmed the functions of AtBCAT1, -2, -3, -5, and -6 but not AtBCAT4 (Diebold et al, 2002). AtBCAT1 is the most likely candidate for initiating BCAA breakdown (Schuster and Binder, 2005), although AtBCAT5 has also been found in mitochondrial fractions (Binder et al, 2007).…”

“…The unigene SGN-U569828 (SlBCAT1; 45 members) has the most ESTs of all putative SlBCATs, while the unigene SGN-U569952 (SlBCAT3; 27 members) has the second highest, both far surpassing the numbers of ESTs of the other putative SlBCATs. Phylogenetic analysis of all putative SlBCATs and comparisons with Arabidopsis BCATs (Diebold et al, 2002) revealed that SlBCAT1 is most similar to the AtBCAT2 and AtBCAT1 genes from Arabidopsis. The unigene SGN-U569830 (SlBCAT2; seven members) is most similar to AtBCAT3.…”

“…AtBCAT1 localizes to mitochondria and is thought to be active primarily in catabolism. AtBCAT2, -3, and -5 localize to chloroplasts, suggesting roles in BCAA synthesis (Diebold et al, 2002). AtBCAT4 is cytosolic (Schuster et al, 2006), and the location of AtBCAT6 is suggested to be cytosolic because of its lack of a defined target peptide sequence (Diebold et al, 2002).…”

A friend in need is a friend indeed

“…mpg.de/general/ora/ora.shtml) of this clique revealed significant enrichment of nine enzymes belonging to amino acid metabolism, mainly amino acid degradation. This clique includes genes encoding catabolic enzymes of the Asp-family pathway, namely, Lys-Ketoglutarate Reductase/Saccharopine Dehydrogenase (Lys catabolism) 4,9 , Branched-chain amino acid Aminotransferase (likely functioning in Ile catabolism) 4,10 and Thr Aldolase (converting Thr into Gly). 11 In addition, this clique includes genes encoding five TFs that possess specific TFs domains, but their functions have not yet been characterized.…”