Structure of a tRNA-dependent kinase essential for selenocysteine decoding

Compared to bacteria, archaea and eukaryotes employ an additional enzyme for the biosynthesis of selenocysteine (Sec), the 21st natural amino acid (aa). An essential RNA-dependent kinase, O-phosphoseryl-tRNASec kinase (PSTK), converts seryl-tRNASec to O-phosphoseryl-tRNASec, the immediate precursor of selenocysteinyl-tRNASec. The sequence of Methanocaldococcus jannaschii PSTK (MjPSTK) suggests an N-terminal kinase domain (177 aa) followed by a presumed tRNA binding region (75 aa). The structures of MjPSTK complexed with ADP and AMPPNP revealed that this enzyme belongs to the P-loop kinase class, and that the kinase domain is closely related to gluconate kinase and adenylate kinase. ATP is bound by the P-loop domain (residues 11-18). Formed by antiparallel dimerization of two PSTK monomers, the enzyme structure shows a deep groove with positive electrostatic potential. Located in this groove is the enzyme's active site, which biochemical and genetic data suggest is composed of Asp-41, Arg-44, Glu-55, Tyr-82, Tyr-83, Met-86, and Met-132. Based on structural comparison with Escherichia coli adenylate kinase a docking model was generated that assigns these amino acids to the recognition of the terminal A76-Ser moieties of Ser-tRNASec. The geometry and electrostatic environment of the groove in MjPSTK are perfectly complementary to the unusually long acceptor helix of tRNASec.