title: Structural insights into RNA-dependent eukaryal and archaeal selenocysteine formation.
abstract: The micronutrient selenium is present in proteins as selenocysteine (Sec). In eukaryotes and archaea, Sec is formed in a tRNA-dependent conversion of O-phosphoserine (Sep) by O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS). Here, we present the crystal structure of Methanococcus maripaludis SepSecS complexed with PLP at 2.5 A resolution. SepSecS, a member of the Fold Type I PLP enzyme family, forms an (alpha2)2 homotetramer through its N-terminal extension. The active site lies on the dimer interface with each monomer contributing essential residues. In contrast to other Fold Type I PLP enzymes, Asn247 in SepSecS replaces the conserved Asp in binding the pyridinium nitrogen of PLP. A structural comparison with Escherichia coli selenocysteine lyase allowed construction of a model of Sep binding to the SepSecS catalytic site. Mutations of three conserved active site arginines (Arg72, Arg94, Arg307), protruding from the neighboring subunit, led to loss of in vivo and in vitro activity. The lack of active site cysteines demonstrates that a perselenide is not involved in SepSecS-catalyzed Sec formation; instead, the conserved arginines may facilitate the selenation reaction. Structural phylogeny shows that SepSecS evolved early in the history of PLP enzymes, and indicates that tRNA-dependent Sec formation is a primordial process.
authors.affiliation: Department of Biological Information, Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama-shi, Kanagawa 226-8501, Japan.
authors.forename: Yuhei
authors.initials: Y
authors.lastname: Araiso
authors.forename: Sotiria
authors.initials: S
authors.lastname: Palioura
authors.forename: Ryuichiro
authors.initials: R
authors.lastname: Ishitani
authors.forename: R Lynn
authors.initials: RL
authors.lastname: Sherrer
authors.forename: Patrick
authors.initials: P
authors.lastname: O'Donoghue
authors.forename: Jing
authors.initials: J
authors.lastname: Yuan
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Oshikane
authors.forename: Naoshi
authors.initials: N
authors.lastname: Domae
authors.forename: Julian
authors.initials: J
authors.lastname: Defranco
authors.forename: Dieter
authors.initials: D
authors.lastname: Söll
authors.forename: Osamu
authors.initials: O
authors.lastname: Nureki
mesh_terms: D000595:Amino Acid Sequence; D019843:Archaeal Proteins / Q000737:chemistry* / Q000145:classification / Q000235:genetics; D019611:Archaeoglobus fulgidus / Q000201:enzymology; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology; D006801:Humans; D017017:Methanococcus / Q000201:enzymology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D010768:Phosphoserine / Q000737:chemistry; D010802:Phylogeny; D017279:Selenocysteine / Q000378:metabolism*; D016415:Sequence Alignment; D014166:Transferases / Q000737:chemistry* / Q000145:classification / Q000235:genetics
chemical_list: D019843:Archaeal Proteins; D017279:Selenocysteine; D010768:Phosphoserine; D014166:Transferases; C060600:selenium transferase