title: Structure and mechanism of the RNA triphosphatase component of mammalian mRNA capping enzyme.
abstract: The 5' capping of mammalian pre-mRNAs is initiated by RNA triphosphatase, a member of the cysteine phosphatase superfamily. Here we report the 1.65 A crystal structure of mouse RNA triphosphatase, which reveals a deep, positively charged active site pocket that can fit a 5' triphosphate end. Structural, biochemical and mutational results show that despite sharing an HCxxxxxR(S/T) motif, a phosphoenzyme intermediate and a core alpha/beta-fold with other cysteine phosphatases, the mechanism of phosphoanhydride cleavage by mammalian capping enzyme differs from that used by protein phosphatases to hydrolyze phosphomonoesters. The most significant difference is the absence of a carboxylate general acid catalyst in RNA triphosphatase. Residues conserved uniquely among the RNA phosphatase subfamily are important for function in cap formation and are likely to play a role in substrate recognition.
authors.affiliation: Department of Biochemistry, Molecular Biology and Cell Biology, Northwestern University, 2153 Sheridan Road, Evanston, IL 60208-3500, USA.
authors.forename: A
authors.initials: A
authors.lastname: Changela
authors.forename: C K
authors.initials: CK
authors.lastname: Ho
authors.forename: A
authors.initials: A
authors.lastname: Martins
authors.forename: S
authors.initials: S
authors.lastname: Shuman
authors.forename: A
authors.initials: A
authors.lastname: Mondragón
mesh_terms: D017766:Acid Anhydride Hydrolases / Q000737:chemistry*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D008322:Mammals; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009713:Nucleotidyltransferases / Q000737:chemistry*; D017433:Protein Structure, Secondary; D017386:Sequence Homology, Amino Acid; D000098556:mRNA Guanylyltransferases
chemical_list: D009713:Nucleotidyltransferases; D000098556:mRNA Guanylyltransferases; D017766:Acid Anhydride Hydrolases; C023090:RNA triphosphatase