engleski

Crystal structure of glycine:[carrier protein] ligase complexed with its cognate carrier protein

Aminoacyl-tRNA synthetases are enzymes essential for ribosomal translation, which catalyze attachment of amino acids to cognate tRNA. They ensure the fidelity of translation of the genetic code and are considered to be evolutionary ancient proteins. We have recently discovered and characterized homologs of atypical archaeal seryl-tRNA synthetases (aSerRS) [1, 2]. Curiously, these novel aSerRS homologs show different substrate specificity for both amino acid and macromolecular substrate and function as amino acid:[carrier protein] ligases [1]. Instead to tRNA, they covalently attach amino acid to phosphopantetheine prosthetic arm of the cognate carrier protein. In addition, their amino acid specficity is different ; they activate Gly (or Ala) instead of Ser. The structure of glycine:[carrier protein] ligase (Gly:CP ligase) Bll0957 from Bradyrhizobium japonicum is remarkably similar to the catalytic domain of aSerRS, and the structure of the active site is also preserved. aSerRS possesses additional N-terminal tRNA-binding domain which directs acceptor stem of tRNA to the active site of the second subunit. In order to shed more light on the binding mode of carrier protein to aSerRS homolog, the crystal structure of Gly:CP ligase in a complex with its cognate carrier protein was solved. One carrier protein molecule binds to each subunit of the homodimeric Bll0957. The carrier protein binds to a distinctive helix of Gly:CP ligase, which replaces serine ordering loop found in aSerRS. Carrier protein embraces the helix of Bll0957 and interactions are mostly hydrophobic. The phosphopantetheine group of carrier protein enters deep into the active site of the same subunit, from the opposite side than tRNA to aSerRS active site. The structure of Gly:CP ligase in complex with its cognate carrier protein provides insight how aa:CP ligases recognize fundamentally different macromolecular substrate.

aaRS homologs; amino acid:[carrier protein] ligase; protein-protein complex

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