FIG. 1.
Selenoprotein synthesis. The process is initiated by the charging of serine (Ser) onto a dedicated tRNA (tRNA[ser]Sec) to generate Ser-tRNASec. The seryl residue of Ser-tRNASec is enzymatically phosphorylated, and then is converted to Sec-tRNASec using monoselenophosphate as a donor of Se. The Sec-tRNASec is used to transfer Sec into nascent selenoproteins co-translationally through a mechanism that requires several dedicated cis elements present in the selenoprotein mRNA (SECIS element) and protein factors that act in trans including SBP2 and EFsec and others. This results in recoding UGA from a stop codon to a Sec-insertion codon and the resulting protein contains the Sec amino acid, which is utilized by selenoproteins for various biological processes. Sec-tRNASec, selenocysteyl-tRNASec; Se, selenium; SBP2, SECIS-binding protein 2; SECIS, selenocysteine insertion sequence; EFSec, selenocysteine-specific translation elongation factor.