Zubieta Chloe, Ross Jeannine R, Koscheski Paul, Yang Yue, Pichersky Eran, Noel Joseph P
Structural Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA 92037, USA.
Plant Cell. 2003 Aug;15(8):1704-16. doi: 10.1105/tpc.014548.
Recently, a novel family of methyltransferases was identified in plants. Some members of this newly discovered and recently characterized methyltransferase family catalyze the formation of small-molecule methyl esters using S-adenosyl-L-Met (SAM) as a methyl donor and carboxylic acid-bearing substrates as methyl acceptors. These enzymes include SAMT (SAM:salicylic acid carboxyl methyltransferase), BAMT (SAM:benzoic acid carboxyl methyltransferase), and JMT (SAM:jasmonic acid carboxyl methyltransferase). Moreover, other members of this family of plant methyltransferases have been found to catalyze the N-methylation of caffeine precursors. The 3.0-A crystal structure of Clarkia breweri SAMT in complex with the substrate salicylic acid and the demethylated product S-adenosyl-L-homocysteine reveals a protein structure that possesses a helical active site capping domain and a unique dimerization interface. In addition, the chemical determinants responsible for the selection of salicylic acid demonstrate the structural basis for facile variations of substrate selectivity among functionally characterized plant carboxyl-directed and nitrogen-directed methyltransferases and a growing set of related proteins that have yet to be examined biochemically. Using the three-dimensional structure of SAMT as a guide, we examined the substrate specificity of SAMT by site-directed mutagenesis and activity assays against 12 carboxyl-containing small molecules. Moreover, the utility of structural information for the functional characterization of this large family of plant methyltransferases was demonstrated by the discovery of an Arabidopsis methyltransferase that is specific for the carboxyl-bearing phytohormone indole-3-acetic acid.
最近，在植物中发现了一个新的甲基转移酶家族。这个新发现且最近得到表征的甲基转移酶家族的一些成员，以S-腺苷-L-甲硫氨酸（SAM）作为甲基供体，以含羧酸的底物作为甲基受体，催化小分子甲酯的形成。这些酶包括水杨酸羧基甲基转移酶（SAMT）、苯甲酸羧基甲基转移酶（BAMT）和茉莉酸羧基甲基转移酶（JMT）。此外，已发现该植物甲基转移酶家族的其他成员催化咖啡因前体的N-甲基化。与底物水杨酸和去甲基化产物S-腺苷-L-高半胱氨酸复合的布鲁氏克拉克ia SAMT的3.0埃晶体结构揭示了一种具有螺旋活性位点封端结构域和独特二聚化界面的蛋白质结构。此外，负责选择水杨酸的化学决定因素证明了在功能表征的植物羧基导向和氮导向甲基转移酶以及一组尚未进行生化研究的相关蛋白质中，底物选择性易于变化的结构基础。以SAMT的三维结构为指导，我们通过定点诱变和针对12种含羧基小分子的活性测定来研究SAMT的底物特异性。此外，通过发现一种对含羧基植物激素吲哚-3-乙酸具有特异性的拟南芥甲基转移酶，证明了结构信息对于这一大型植物甲基转移酶家族功能表征的实用性。
