title: Methylation of sulfhydryl groups: a new function for a family of small molecule plant O-methyltransferases.
abstract: In plants, type I and II S-adenosyl-l-methionine-dependent O-methyltransferases (OMTs) catalyze most hydroxyl group methylations of small molecules. A homology-based RT-PCR strategy using Catharanthus roseus (Madagascar periwinkle) RNA previously identified six new type I plant OMT family members. We now describe the molecular and biochemical characterization of a seventh protein. It shares 56-58% identity with caffeic acid OMTs (COMTs), but it failed to methylate COMT substrates, and had no activity with flavonoids. However, the in vitro incubations revealed unusually high background levels without added substrates. A search for the responsible component revealed that the enzyme methylated dithiothreitol (DTT), the reducing agent added for enzyme stabilization. Unexpectedly, product analysis revealed that the methylation occurred on a sulfhydryl moiety, not on a hydroxyl group. Analysis of 34 compounds indicated a broad substrate range, with a preference for small hydrophobic molecules. Benzene thiol (Km 220 microm) and furfuryl thiol (Km 60 microm) were the best substrates (6-7-fold better than DTT). Small isosteric hydrophobic substrates with hydroxyl groups, like phenol and guaiacol, were also methylated, but the activities were at least 5-fold lower than with thiols. The enzyme was named C. roseus S-methyltransferase 1 (CrSMT1). Models based on the COMT crystal structure suggest that S-methylation is mechanistically identical to O-methylation. CrSMT1 so far is the only recognized example of an S-methyltransferase in this protein family. Its properties indicate that a few changes in key residues are sufficient to convert an OMT into a S-methyltransferase (SMT). Future functional investigations of plant methyltransferases should consider the possibility that the enzymes may direct methylation at sulfhydryl groups.
authors.affiliation: TU München, FG Biomolekulare Lebensmitteltechnologie, Lise-Meitner-Str. 34, D-85354 Freising, Germany.
authors.forename: Heather
authors.initials: H
authors.lastname: Coiner
authors.forename: Gudrun
authors.initials: G
authors.lastname: Schröder
authors.forename: Elke
authors.initials: E
authors.lastname: Wehinger
authors.forename: Chang-Jun
authors.initials: CJ
authors.lastname: Liu
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Noel
authors.forename: Wilfried
authors.initials: W
authors.lastname: Schwab
authors.forename: Joachim
authors.initials: J
authors.lastname: Schröder
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D030008:Catharanthus / Q000201:enzymology* / Q000235:genetics*; D003001:Cloning, Molecular; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D021241:Spectrometry, Mass, Electrospray Ionization; D013438:Sulfhydryl Compounds / Q000378:metabolism*
chemical_list: D010940:Plant Proteins; D011994:Recombinant Proteins; D013438:Sulfhydryl Compounds; D008780:Methyltransferases