Specific transfer of selenium in selenoprotein biosynthesis

摘要

Seryl-tRNA synthetase (SerRS) is a class-2 aminoacyl-tRNA synthetase that catalyzes serine activation and its transfer to tRNA~(Ser). The enzyme also serylates tRNA~(Sec) and thus participates in the incorporation of selenocysteine into selenoproteins. Methanogenic archaea possess unusual SerRS evolutionarily distinct from those found in other archaea, eukaryotes, and bacteria. The core three motifs and an active site are conserved in all SerRSs, whereas methanogenic archaeal SerRS has a unique N-terminal domain, which includes a tRNA-binding site. Here, we show evidence that Methanocaldococcus jannaschii seryl-tRNA synthetase (mjSerRS) catalyzes the formation of phosphoseryl-tRNA~(Sec) (PSer-tRNA~(Sec)) from serine, tRNA~(Sec), and ATP. In vitro analyses revealed that M. jannaschii tRNA~(Sec) was charged with serine and then converted to PSer-tRNA~(Sec) by mjSerRS. Furthermore, selenocysteyl-tRNA~(Sec) (Sec-tRNA~(Sec)) was synthesized in an in vitro reaction with mjSerRS, selenocysteine synthase, and tRNA~(Sec) in the absence of seryl-tRNA kinase, which has been proposed as a key enzyme in Sec-tRNA~(Sec) synthesis in eukaryotes and archaea. Sequence analysis revealed that the N-terminal region of mjSerRS is similar to the entire sequence of M. jannaschii seryl-tRNA kinase. These results suggest that the N-terminus of mjSerRS is responsible for the phosphorylation of seryl-tRNA~(Sec).