Structural Basis for Substrate Recognition in the Salicylic Acid Carboxyl Methyltransferase Family

Abstract

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-Å 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.

Proteomic Analysis of a Sea-Ice Diatom: Salinity Acclimation Provides New Insight into the Dimethylsulfoniopropionate Production Pathway

“…Substrates for the third SAMmt protein spot (Fc207357, relative protein abundance elevated 2.8-fold and transcripts elevated 10-to 15-fold in high-salinity/DMSP conditions) have not been characterized. It has similarity to higher-plant small molecule methyltransferases, containing all three of the required SAM-binding domains identified through crystal structures of salicylic acid and caffeine methyltransferases (Zubieta et al, 2003;McCarthy and McCarthy, 2007) but shows considerable divergence in substrate-binding regions. The last step in DMSP synthesis is the oxidative decarboxylation of 4-dimethylsulfonio-2-hydroxybutyrate to form DMSP.…”

Biosynthesis and emission of insect-induced methyl salicylate and methyl benzoate from rice

“…A homology model of OsBSMT1 was built by using the experimentally-determined structure of C. breweri SAMT (CbSAMT) [48] as the template. This model was then used as a target for an in silico docking experiment to rank the affinities for the different potent substrates and to analyze their binding mode.…”

The Fast and Transient Transcriptional Network of Gravity and Mechanical Stimulation in the Arabidopsis Root Apex

“…AtPEN1 is known to be involved in sterol biosynthesis, and its fast up-regulation during gravitropism might be important for the synthesis of brassinosteroids, which are involved in gravitropic signal transduction in maize roots (Kim et al, 2000). SAMT enzymes methylate secondary metabolites, and some of them, such as methylsalicylate and methyljasmonate, are known to be in several signal transduction pathways that lead to systemic and local defense responses (Shulaev et al, 1997;Zubieta et al, 2003;Hendricks et al, 2004). At least one SAMT isoform in Arabidopsis has high affinity for IAA, catalyzing the methylation of IAA (Zubieta et al, 2003).…”

“…SAMT enzymes methylate secondary metabolites, and some of them, such as methylsalicylate and methyljasmonate, are known to be in several signal transduction pathways that lead to systemic and local defense responses (Shulaev et al, 1997;Zubieta et al, 2003;Hendricks et al, 2004). At least one SAMT isoform in Arabidopsis has high affinity for IAA, catalyzing the methylation of IAA (Zubieta et al, 2003). The specific functions/targets of this SAMT and the other fast up-regulated genes are unknown.…”