title: Structural Analysis of Glycine Sarcosine N-methyltransferase from Methanohalophilus portucalensis Reveals Mechanistic Insights into the Regulation of Methyltransferase Activity.
abstract: Methyltransferases play crucial roles in many cellular processes, and various regulatory mechanisms have evolved to control their activities. For methyltransferases involved in biosynthetic pathways, regulation via feedback inhibition is a commonly employed strategy to prevent excessive accumulation of the pathways' end products. To date, no biosynthetic methyltransferases have been characterized by X-ray crystallography in complex with their corresponding end product. Here, we report the crystal structures of the glycine sarcosine N-methyltransferase from the halophilic archaeon Methanohalophilus portucalensis (MpGSMT), which represents the first structural elucidation of the GSMT methyltransferase family. As the first enzyme in the biosynthetic pathway of the osmoprotectant betaine, MpGSMT catalyzes N-methylation of glycine and sarcosine, and its activity is feedback-inhibited by the end product betaine. A structural analysis revealed that, despite the simultaneous presence of both substrate (sarcosine) and cofactor (S-adenosyl-L-homocysteine; SAH), the enzyme was likely crystallized in an inactive conformation, as additional structural changes are required to complete the active site assembly. Consistent with this interpretation, the bound SAH can be replaced by the methyl donor S-adenosyl-L-methionine without triggering the methylation reaction. Furthermore, the observed conformational state was found to harbor a betaine-binding site, suggesting that betaine may inhibit MpGSMT activity by trapping the enzyme in an inactive form. This work implicates a structural basis by which feedback inhibition of biosynthetic methyltransferases may be achieved.
authors.affiliation: Institute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei 100, Taiwan.
authors.forename: Yi-Ru
authors.initials: YR
authors.lastname: Lee
authors.affiliation: Institute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei 100, Taiwan.
authors.forename: Te-Sheng
authors.initials: TS
authors.lastname: Lin
authors.affiliation: Department of Life Sciences, National Chung Hsing University, Taichung 402, Taiwan.
authors.forename: Shu-Jung
authors.initials: SJ
authors.lastname: Lai
authors.affiliation: Institute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei 100, Taiwan.
authors.forename: Mu-Sen
authors.initials: MS
authors.lastname: Liu
authors.affiliation: Department of Life Sciences, National Chung Hsing University, Taichung 402, Taiwan.
authors.forename: Mei-Chin
authors.initials: MC
authors.lastname: Lai
authors.affiliation: Institute of Biochemistry and Molecular Biology, College of Medicine, National Taiwan University, Taipei 100, Taiwan.|Institute of Biochemistry, National Chung Hsing University, Taichung 402, Taiwan.
authors.forename: Nei-Li
authors.initials: NL
authors.lastname: Chan
mesh_terms: D019843:Archaeal Proteins / Q000737:chemistry / Q000378:metabolism; D001622:Betaine / Q000378:metabolism; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D025461:Feedback, Physiological; D019848:Gene Expression Regulation, Archaeal; D015971:Gene Expression Regulation, Enzymologic; D005998:Glycine / Q000378:metabolism; D050938:Glycine N-Methyltransferase / Q000737:chemistry* / Q000378:metabolism*; D017019:Methanosarcinaceae / Q000737:chemistry / Q000201:enzymology*; D008745:Methylation; D008958:Models, Molecular; D017433:Protein Structure, Secondary; D012521:Sarcosine / Q000378:metabolism
chemical_list: D019843:Archaeal Proteins; D001622:Betaine; D050938:Glycine N-Methyltransferase; D005998:Glycine; D012521:Sarcosine