Journal Article

Functional Characterization of a Methionine γ-Lyase in Arabidopsis and its Implication in an Alternative to the Reverse Trans-sulfuration Pathway

Aymeric Goyer, Eva Collakova, Yair Shachar-Hill and Andrew D. Hanson

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 48, issue 2, pages 232-242
Published in print February 2007 | ISSN: 0032-0781
Published online February 2007 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pcl055
Functional Characterization of a Methionine γ-Lyase in Arabidopsis and its Implication in an Alternative to the Reverse Trans-sulfuration Pathway

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Methionine γ-lyase (MGL) catalyzes the degradation of l-methionine to α-ketobutyrate, methanethiol and ammonia. The Arabidopsis (Arabidopsis thaliana) genome includes a single gene (At1g64660) encoding a protein (AtMGL) with ∼35% identity to bacterial and protozoan MGLs. When overexpressed in Escherichia coli, AtMGL allowed growth on l-methionine as sole nitrogen source and conferred a high rate of methanethiol emission. The purified recombinant protein exhibited a spectrum typical of pyridoxal 5′-phosphate enzymes, and had high activity toward l-methionine, l-ethionine, l-homocysteine and seleno-l-methionine, but not l-cysteine. Quantitation of mRNA showed that the AtMGL gene is expressed in aerial organs and roots, and that its expression in leaves was increased 2.5-fold by growth on low sulfate medium. Emission of methanethiol from Arabidopsis plants supplied with 10 mM l-methionine was undetectable (<0.5 nmol min−1 g−1 FW), suggesting that AtMGL is not an important source of volatile methanethiol. Knocking out the AtMGL gene significantly increased leaf methionine content (9.2-fold) and leaf and root S-methylmethionine content (4.7- and 7-fold, respectively) under conditions of sulfate starvation, indicating that AtMGL carries a significant flux in vivo. In Arabidopsis plantlets fed l-[35S]methionine on a low sulfate medium, label was incorporated into protein-bound cysteine as well as methionine, but incorporation into cysteine was significantly (30%) less in the knockout mutant. These data indicate that plants possess an alternative to the reverse trans-sulfuration pathway (methionine→homocysteine→cystathionine→cysteine) in which methanethiol is an intermediate.

Keywords: Arabidopsis thaliana; Cysteine; Methionine

Journal Article.  6620 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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