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Plant and Cell Physiology Advance Access published online on December 13, 2006
Plant and Cell Physiology, doi:10.1093/pcp/pcl055
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© The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Functional Characterization of a Methionine {gamma}-Lyase in Arabidopsis and Its Implication in an Alternative to the Reverse Transsulfuration Pathway

Aymeric Goyer1,3, Eva Collakova2, Yair Shachar-Hill2 and Andrew D. Hanson1

1Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611
2Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824

Correspondence to: Aymeric Goyer, Oregon State University/USDA-ARS, 24106 N. Bunn Road, Prosser, WA 99350-8694,Telephone: (509) 786 9354, FAX: (509) 786 9370, e-mail: agoyer{at}pars.ars.usda.gov


  Abstract

Methionine {gamma}-lyase (MGL) catalyzes the degradation of L-methionine to {alpha}-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 fresh weight), 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 transsulfuration pathway (methionine -> homocysteine -> cystathionine -> cysteine) in which methanethiol is an intermediate, as hypothesized by Schwenn et al. (J.D. Schwenn, U. Schriek, H.H. Kiltz [1983] Planta 158: 540-549).

Keywords: Methionine - Cell wall - cysteine - Arabidopsis thaliana

Present address: 3Oregon State University/USDA-ARS, Prosser, Washington 99350


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