title: Metal-binding site in a class I tRNA synthetase localized to a cysteine cluster inserted into nucleotide-binding fold.
abstract: The 10 class I aminoacyl-tRNA synthetases share a common N-terminal nucleotide-binding fold. Idiosyncratic polypeptide insertions into this fold introduce residues important for activity, including those that interact with the tRNA acceptor helix. The class I Escherichia coli methionyl-tRNA synthetase (L-methionine:tRNA(Met) ligase, EC 6.1.1.10), a 676-amino acid homodimer, was shown previously by others to contain zinc and to have an activity dependent on its presence. We show here by atomic absorption spectroscopy and zinc titrations the presence of 1 mol of zinc per polypeptide. Replacement of zinc with cobalt yields an active enzyme with a visible absorption spectrum characteristic of tetrahedral coordination to sulfur ligands and an intense metal-to-sulfur charge-transfer band at 340 nm. Mapping of the metal-binding site by zinc blotting of recombinant and proteolytic fragments localized the site to a polypeptide insertion between two strands and a beta-sheet in the N-terminal nucleotide-binding fold that contains the catalytic site. Beginning at Cys-145, this insertion contains a Cys-Xaa2-Cys-Xaa9-Cys-Xaa2-Cys motif. Site-directed substitution of these cysteines with serines yielded proteins that were stable but generally devoid of activity. With this result there is now at least one example of a class I and of a class II E. coli tRNA synthetase with a metal-binding domain important for activity inserted into the catalytic domain.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.
authors.forename: J A
authors.initials: JA
authors.lastname: Landro
authors.forename: P
authors.initials: P
authors.lastname: Schimmel
mesh_terms: D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry / Q000145:classification / Q000378:metabolism*; D001665:Binding Sites; D003545:Cysteine; D004795:Enzyme Stability; D004926:Escherichia coli / Q000201:enzymology; D007700:Kinetics; D008718:Methionine-tRNA Ligase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009711:Nucleotides / Q000378:metabolism; D010663:Phenylmercury Compounds / Q000378:metabolism; D017510:Protein Folding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism; D013053:Spectrophotometry; D013054:Spectrophotometry, Atomic; D015032:Zinc / Q000032:analysis / Q000378:metabolism*
chemical_list: D009711:Nucleotides; D010663:Phenylmercury Compounds; D011994:Recombinant Proteins; C002910:4-hydroxymercuribenzenesulfonate; D000604:Amino Acyl-tRNA Synthetases; D008718:Methionine-tRNA Ligase; D015032:Zinc; D003545:Cysteine