title: The human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation.
abstract: 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-TPsiC arm (where Psi 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.
authors.affiliation: Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, CT 06520, USA.
authors.forename: Sotiria
authors.initials: S
authors.lastname: Palioura
authors.forename: R Lynn
authors.initials: RL
authors.lastname: Sherrer
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Steitz
authors.forename: Dieter
authors.initials: D
authors.lastname: Söll
authors.forename: Miljan
authors.initials: M
authors.lastname: Simonovic
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000737:chemistry* / Q000378:metabolism*; D001483:Base Sequence; D055162:Biocatalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D006801:Humans; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010710:Phosphates / Q000737:chemistry / Q000378:metabolism; D010768:Phosphoserine / Q000737:chemistry / Q000378:metabolism; D011487:Protein Conformation; D055503:Protein Multimerization; D017433:Protein Structure, Secondary; D012345:RNA, Transfer, Amino Acid-Specific / Q000737:chemistry* / Q000378:metabolism*; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D017279:Selenocysteine / Q000096:biosynthesis* / Q000235:genetics
chemical_list: D010710:Phosphates; D012345:RNA, Transfer, Amino Acid-Specific; D012346:RNA, Transfer, Amino Acyl; C112705:selenocysteinyl-tRNA; C036928:tRNA, selenocysteine-; D017279:Selenocysteine; D010768:Phosphoserine; D000604:Amino Acyl-tRNA Synthetases; C542370:O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase, human; C035638:thiophosphoric acid