The Human SepSecS-tRNA~(Sec) Complex Reveals the Mechanism of Selenocysteine Formation

摘要

Selenocysteine is the only genetically encoded amino acid in humans whose biosynthesis occurs on its cognate transfer RNA (tRNA). O-Phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS) catalyzes the final step of selenocysteine formation by a poorly understood tRNA-dependent mechanism. The crystal structure of human tRNA~(Sec) in complex with SepSecS, phosphoserine, and thiophosphate, together with in vivo and in vitro enzyme assays, supports a pyridoxal phosphate-dependent mechanism of Sec-tRNA~(Sec) formation. Two tRNA(Sec) molecules, with a fold distinct from other canonical tRNAs, bind to each SepSecS tetramer through their 13-base pair acceptor-TΨC arm (where Ψ indicates pseudouridine). The tRNA binding is likely to induce a conformational change in the enzyme's active site that allows a phosphoserine covalently attached to tRNA~(Sec), but not free phosphoserine, to be oriented properly for the reaction to occur.