Rébeillé Fabrice, Jabrin Samuel, Bligny Richard, Loizeau Karen, Gambonnet Bernadette, Van Wilder Valérie, Douce Roland, Ravanel Stéphane
Centre National de la Recherche Scientifique/Commissariat à l'Energie Atomique/Institut National de la Recherche Agronomique-Université Joseph Fourier Grenoble I, Département Réponse et Dynamique Cellulaires, Grenoble, France.
Proc Natl Acad Sci U S A. 2006 Oct 17;103(42):15687-92. doi: 10.1073/pnas.0606195103. Epub 2006 Oct 9.
Despite recent progress in elucidating the regulation of methionine (Met) synthesis, little is known about the catabolism of this amino acid in plants. In this article, we present several lines of evidence indicating that the cleavage of Met catalyzed by Met gamma-lyase is the first step in this process. First, we cloned an Arabidopsis cDNA coding a functional Met gamma-lyase (AtMGL), a cytosolic enzyme catalyzing the conversion of Met into methanethiol, alpha-ketobutyrate, and ammonia. AtMGL is present in all of the Arabidopsis organs and tissues analyzed, except in quiescent dry mature seeds, thus suggesting that AtMGL is involved in the regulation of Met homeostasis in various situations. Also, we demonstrated that the expression of AtMGL was induced in Arabidopsis cells in response to high Met levels, probably to bypass the elevated Km of the enzyme for Met. Second, [13C]-NMR profiling of Arabidopsis cells fed with [13C]Met allowed us to identify labeled S-adenosylmethionine, S-methylmethionine, S-methylcysteine (SMC), and isoleucine (Ile). The unexpected production of SMC and Ile was directly associated to the function of Met gamma-lyase. Indeed, we showed that part of the methanethiol produced during Met cleavage could react with an activated form of serine to produce SMC. The second product of Met cleavage, alpha-ketobutyrate, entered the pathway of Ile synthesis in plastids. Together, these data indicate that Met catabolism in Arabidopsis cells is initiated by a gamma-cleavage process and can result in the formation of the essential amino acid Ile and a potential storage form for sulfide or methyl groups, SMC.
尽管最近在阐明甲硫氨酸（Met）合成的调控方面取得了进展，但对于这种氨基酸在植物中的分解代谢却知之甚少。在本文中，我们提供了几条证据表明，由甲硫氨酸γ-裂解酶催化的Met裂解是这一过程的第一步。首先，我们克隆了一个编码功能性甲硫氨酸γ-裂解酶（AtMGL）的拟南芥cDNA，AtMGL是一种胞质酶，催化Met转化为甲硫醇、α-酮丁酸和氨。AtMGL存在于所有分析过的拟南芥器官和组织中，但静止的干燥成熟种子除外，这表明AtMGL在各种情况下都参与了Met稳态的调节。此外，我们证明，AtMGL的表达在拟南芥细胞中受高Met水平诱导，可能是为了绕过该酶对Met升高的Km值。其次，用[13C]Met喂养的拟南芥细胞的[13C]-NMR分析使我们能够鉴定出标记的S-腺苷甲硫氨酸、S-甲基甲硫氨酸、S-甲基半胱氨酸（SMC）和异亮氨酸（Ile）。SMC和Ile的意外产生与甲硫氨酸γ-裂解酶的功能直接相关。事实上，我们表明，Met裂解过程中产生的部分甲硫醇可以与一种活化形式的丝氨酸反应生成SMC。Met裂解的第二个产物α-酮丁酸进入质体中Ile的合成途径。总之，这些数据表明，拟南芥细胞中的Met分解代谢是由γ-裂解过程启动的，并且可以导致必需氨基酸Ile的形成以及硫化物或甲基的潜在储存形式SMC的形成。
