Ravanel Stéphane, Block Maryse A, Rippert Pascal, Jabrin Samuel, Curien Gilles, Rébeillé Fabrice, Douce Roland
Département de Réponse et Dynamique Cellulaires, CEA-Grenoble, 17 Rue des Martyrs, 38054 Grenoble Cedex 9, France.
J Biol Chem. 2004 May 21;279(21):22548-57. doi: 10.1074/jbc.M313250200. Epub 2004 Mar 15.
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.
蛋氨酸（Met）和S-腺苷甲硫氨酸（AdoMet）代谢在植物细胞中的亚细胞分布揭示了胞质溶胶和细胞器之间复杂的分配情况。在本研究中，我们发现拟南芥含有三种不依赖维生素B12的蛋氨酸合酶（MS）功能异构体，该酶催化以5-甲基四氢叶酸作为甲基供体将同型半胱氨酸甲基化为蛋氨酸。一种MS异构体存在于叶绿体中，很可能是为了甲基化在该细胞器中从头合成的同型半胱氨酸。因此，叶绿体是自主的，是植物细胞中从头合成蛋氨酸的唯一部位。另外的MS异构体存在于胞质溶胶中，很可能参与从活化甲基循环过程中产生的同型半胱氨酸再生蛋氨酸。虽然蛋氨酸合成可以在叶绿体中发生，但没有证据表明除了胞质溶胶外，其他任何地方能合成AdoMet。根据这一推测，我们表明AdoMet通过载体介导的易化扩散过程转运到叶绿体中。这种载体能够催化AdoMet单向转运进入叶绿体，以及胞质溶胶中的AdoMet与叶绿体中的AdoMet或S-腺苷高半胱氨酸之间的交换。该载体的明显功能是维持叶绿体中的甲基化反应和其他依赖AdoMet的功能，并且可能去除由甲基转移酶活性在基质中产生的S-腺苷高半胱氨酸。因此，叶绿体AdoMet载体充当了胞质溶胶和叶绿体一碳代谢之间的联系。
