Structural Basis for the Major Role of O-Phosphoseryl-tRNA Kinase in the UGA-Specific Encoding of Selenocysteine

SummaryThe 21st amino acid, selenocysteine (Sec), is assigned to the codon UGA and is biosynthesized on the selenocysteine-specific tRNA (tRNASec) with the corresponding anticodon. In archaea/eukarya, tRNASec is ligated with serine by seryl-tRNA synthetase (SerRS), the seryl moiety is phosphorylated by O-phosphoseryl-tRNA kinase (PSTK), and the phosphate group is replaced with selenol by Sep-tRNA:Sec-tRNA synthase. PSTK selectively phosphorylates seryl-tRNASec, while SerRS serylates both tRNASer and tRNASec. In this study, we determined the crystal structures of the archaeal tRNASec•PSTK complex. PSTK consists of two independent linker-connected domains, the N-terminal catalytic domain (NTD) and the C-terminal domain (CTD). The D-arm•CTD binding occurs independently of and much more strongly than the acceptor-arm•NTD binding. PSTK thereby distinguishes the characteristic D arm with the maximal stem and the minimal loop of tRNASec from the canonical D arm of tRNASer, without interacting with the anticodon. This mechanism is essential for the UGA-specific encoding of selenocysteine.

Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (498 K)Download as PowerPoint slideHighlights► PSTK is essential for the specific encoding of selenocysteine (Sec) by the UGA codon ► PSTK consists of two independent domains (NTD and CTD) separated by a linker ► CTD tightly/specifically binds the maximal D stem and the minimal D loop of tRNASec ► CTD guarantees that NTD replaces Ser with Sec on tRNASec, but not on tRNASer