Decameric SelA•tRNA Sec Ring Structure Reveals Mechanism of Bacterial Selenocysteine Formation

The 21st amino acid, selenocysteine (Sec), occurs in the active site of many redox enzymes. Its cognate transfer RNA (tRNA) is first loaded with Ser by seryl-tRNA synthetase and the Ser-tRNA Sec is then converted to Sec-tRNA Sec . Itoh et al. (p. 75 ) determined the crystal structures of the selenocysteine synthase, SelA, that is responsible for this conversion in bacteria, alone and in complex with tRNA. The decameric SelA complex binds to 10 tRNA Sec molecules. The structures, together with biochemistry, show how SelA discriminates tRNA Sec from tRNA Ser , give insight into the mechanism of catalysis, and show that decamerization is essential for function. Structural and biochemical data reveal how selenocysteine is produced from serine on transfer RNA. The 21st amino acid, selenocysteine (Sec), is synthesized on its cognate transfer RNA (tRNA Sec ). In bacteria, SelA synthesizes Sec from Ser-tRNA Sec , whereas in archaea and eukaryotes SepSecS forms Sec from phosphoserine (Sep) acylated to tRNA Sec . We determined the crystal structures of Aquifex aeolicus SelA complexes, which revealed a ring-shaped homodecamer that binds 10 tRNA Sec molecules, each interacting with four SelA subunits. The SelA N-terminal domain binds the tRNA Sec -specific D-arm structure, thereby discriminating Ser-tRNA Sec from Ser-tRNA Ser . A large cleft is created between two subunits and accommodates the 3′-terminal region of Ser-tRNA Sec . The SelA structures together with in vivo and in vitro enzyme assays show decamerization to be essential for SelA function. SelA catalyzes pyridoxal 5′-phosphate–dependent Sec formation involving Arg residues nonhomologous to those in SepSecS. Different protein architecture and substrate coordination of the bacterial enzyme provide structural evidence for independent evolution of the two Sec synthesis systems present in nature.