title: Methionine metabolism in plants: chloroplasts are autonomous for de novo methionine synthesis and can import S-adenosylmethionine from the cytosol.
abstract: The subcellular distribution of Met and S-adenosylmethionine (AdoMet) metabolism in plant cells discloses a complex partition between the cytosol and the organelles. In the present work we show that Arabidopsis contains three functional isoforms of vitamin B(12)-independent methionine synthase (MS), the enzyme that catalyzes the methylation of homocysteine to Met with 5-methyltetrahydrofolate as methyl group donor. One MS isoform is present in chloroplasts and is most likely required to methylate homocysteine that is synthesized de novo in this compartment. Thus, chloroplasts are autonomous and are the unique site for de novo Met synthesis in plant cells. The additional MS isoforms are present in the cytosol and are most probably involved in the regeneration of Met from homocysteine produced in the course of the activated methyl cycle. Although Met synthesis can occur in chloroplasts, there is no evidence that AdoMet is synthesized anywhere but the cytosol. In accordance with this proposal, we show that AdoMet is transported into chloroplasts by a carrier-mediated facilitated diffusion process. This carrier is able to catalyze the uniport uptake of AdoMet into chloroplasts as well as the exchange between cytosolic AdoMet and chloroplastic AdoMet or S-adenosylhomocysteine. The obvious function for the carrier is to sustain methylation reactions and other AdoMet-dependent functions in chloroplasts and probably to remove S-adenosylhomocysteine generated in the stroma by methyltransferase activities. Therefore, the chloroplastic AdoMet carrier serves as a link between cytosolic and chloroplastic one-carbon metabolism.
authors.affiliation: Département de Réponse et Dynamique Cellulaires, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France. sravane1@cea.fr
authors.forename: Stéphane
authors.initials: S
authors.lastname: Ravanel
authors.forename: Maryse A
authors.initials: MA
authors.lastname: Block
authors.forename: Pascal
authors.initials: P
authors.lastname: Rippert
authors.forename: Samuel
authors.initials: S
authors.lastname: Jabrin
authors.forename: Gilles
authors.initials: G
authors.lastname: Curien
authors.forename: Fabrice
authors.initials: F
authors.lastname: Rébeillé
authors.forename: Roland
authors.initials: R
authors.lastname: Douce
mesh_terms: D017360:Arabidopsis / Q000378:metabolism; D015153:Blotting, Western; D002736:Chloroplasts / Q000378:metabolism*; D003001:Cloning, Molecular; D003600:Cytosol / Q000378:metabolism*; D018076:DNA, Complementary / Q000378:metabolism; D004058:Diffusion; D004305:Dose-Response Relationship, Drug; D004926:Escherichia coli / Q000378:metabolism; D049452:Green Fluorescent Proteins; D006710:Homocysteine / Q000737:chemistry; D015151:Immunoblotting; D007700:Kinetics; D008164:Luminescent Proteins / Q000378:metabolism; D008715:Methionine / Q000737:chemistry* / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism; D008954:Models, Biological; D008969:Molecular Sequence Data; D018532:Pisum sativum; D010802:Phylogeny; D018087:Plastids / Q000378:metabolism; D020033:Protein Isoforms; D012436:S-Adenosylmethionine / Q000737:chemistry*; D013997:Time Factors; D014805:Vitamin B 12 / Q000378:metabolism
chemical_list: D018076:DNA, Complementary; D008164:Luminescent Proteins; D020033:Protein Isoforms; D006710:Homocysteine; D049452:Green Fluorescent Proteins; D012436:S-Adenosylmethionine; D008715:Methionine; D014805:Vitamin B 12