title: 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-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.
authors.affiliation: Structural Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA 92037, USA.
authors.forename: Chloe
authors.initials: C
authors.lastname: Zubieta
authors.forename: Jeannine R
authors.initials: JR
authors.lastname: Ross
authors.forename: Paul
authors.initials: P
authors.lastname: Koscheski
authors.forename: Yue
authors.initials: Y
authors.lastname: Yang
authors.forename: Eran
authors.initials: E
authors.lastname: Pichersky
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Noel
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology / Q000235:genetics; D002110:Caffeine / Q000096:biosynthesis; D020134:Catalytic Domain / Q000235:genetics; D031663:Clarkia / Q000201:enzymology* / Q000235:genetics; D018360:Crystallography, X-Ray; D007210:Indoleacetic Acids / Q000378:metabolism; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D007210:Indoleacetic Acids; D011994:Recombinant Proteins; D002110:Caffeine; C030737:indoleacetic acid; D008780:Methyltransferases; C110994:salicylic acid carboxyl methyltransferase