Biosynthesis of selenocysteine on its tRNA in eukaryotes

Abstract

Selenocysteine (Sec) is co‐translationally inserted into protein in response to UGA codons and is the 21 st amino acid in the genetic code. However, the means by which Sec is synthesized in eukaryotes is not known. Herein, comparative genomics and experimental analyses revealed that the mammalian Sec synthase (SecS) is the previously identified pyridoxal phosphate‐containing protein known as the soluble liver antigen. SecS required selenophosphate and O ‐phosphoseryl‐tRNA [Ser]Sec as substrates to generate selenocysteyl‐tRNA [Ser]Sec . Moreover, it was found that Sec was synthesized on the tRNA scaffold from selenide, ATP and serine using tRNA [Ser]Sec , seryl‐tRNA synthetase, O ‐phosphoseryl‐tRNA [Ser]Sec kinase, selenophosphate synthetase and SecS. By identifying the pathway of Sec biosynthesis in mammals, this study not only functionally characterized SecS, but also assigned the function of the O ‐phosphoseryl‐tRNA [Ser]Sec kinase. In addition, we found that selenophosphate synthetase 2 could synthesize monoselenophosphate in vitro, but selenophosphate synthetase 1 could not. Conservation of the overall pathway of Sec biosynthesis suggests that this pathway is also active in other eukaryotes and archaea that synthesize selenoproteins. This research was supported by the Intramural Research Program of NIH/NCI/CCR and by Grants GM065204, GM061603 and CA080946 to V.N.G. and DK47320 and DK52963 to M.J.B. and CA‐41108 to R.S.G.