title: Crystal structure of an mRNA-binding fragment of Moorella thermoacetica elongation factor SelB.
abstract: SelB is an elongation factor needed for the co-translational incorporation of selenocysteine. Selenocysteine is coded by a UGA stop codon in combination with a specific downstream mRNA hairpin. In bacteria, the C-terminal part of SelB recognizes this hairpin, while the N-terminal part binds GTP and tRNA in analogy with elongation factor Tu (EF-Tu). We present the crystal structure of a C-terminal fragment of SelB (SelB-C) from Moorella thermoacetica at 2.12 A resolution, solved by a combination of selenium and yttrium multiwavelength anomalous dispersion. This 264 amino acid fragment contains the entire C-terminal extension beginning after the EF-Tu-homologous domains. SelB-C consists of four similar winged-helix domains arranged into the shape of an L. This is the first example of winged-helix domains involved in RNA binding. The location of conserved basic amino acids, together with data from the literature, define the position of the mRNA-binding site. Steric requirements indicate that a conformational change may occur upon ribosome interaction. Structural observations and data in the literature suggest that this change happens upon mRNA binding.
authors.affiliation: Department of Molecular Biophysics, Lund University, PO Box 124, S-221 00 Lund, Sweden. maria@mrc-lmb.cam.ac.uk
authors.forename: M
authors.initials: M
authors.lastname: Selmer
authors.forename: X-D
authors.initials: XD
authors.lastname: Su
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D003013:Clostridium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D010445:Peptide Elongation Factors / Q000737:chemistry*; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D012329:RNA, Bacterial / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D012343:RNA, Transfer / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013431:Sulfates / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D010445:Peptide Elongation Factors; D012329:RNA, Bacterial; D012333:RNA, Messenger; C061878:SelB protein, Bacteria; D013431:Sulfates; D012343:RNA, Transfer