engleski

Structural studies of amino acid:[carrier protein] ligase in the complex with carrier protein

Aminoacyl-tRNA synthetases (aaRS) are enzymes responsible for attachment of amino acids to the cognate tRNA molecules, which makes them essential for ribosomal protein biosynthesis. Recent investigations discovered numerous aaRS-like proteins in a wide range of organisms although aaRSs are evolutionary highly conserved. We have recently identified and characterised bacterial homologues of atypical arcaeal seryl-tRNA synthetases (aSerRS). These aSerRS homologues do not aminoacylate tRNA. Instead, they covalently attach amino acid to phosphopantetheine prosthetic arm of the cognate carrier protein (CP) and function as amino acid:[carrier protein] ligases. In comparison to SerRSs, they show different amino acid specificity, activating Gly (or Ala) instead of Ser. The crystal structure of glycine:[carrier protein] ligase (Gly:CP ligase) Bll0957 from Bradyrhizobium japonicum shows remarkable similarity to catalytic domain of aSerRS as well as the active site topology. Serine-ordering loop responsible for Ser binding into the active site of aSerRS is replaced by a loop-helix motif located further away from the active site. To understand the binding mode of CP to aSerRS homologue, the crystal structure of Gly:CP ligase in a complex with its cognate carrier protein was recently solved. The carrier protein binds to the helix of the idiosyncratic loop-helix motif of Gly:CP ligase mostly through hydrophobic interactions. Only two Gly:CP ligase residues (Arg220 and Gln231) form hydrogen bonds with the CP. The phosphopantetheine group of carrier protein enters into the active site of the Gly:CP ligase through a wide tunnel 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 into how these aSerRS homologues recognize fundamentally different macromolecular substrate.

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

DOI: 10.1111/j.1742-4658.2011.08138.x