Crystal structure of the full-length bacterial selenocysteine-specific elongation factor SelB.

Abstract

Selenocysteine (Sec), the 21st amino acid in translation, uses its specific tRNA (tRNASec) to recognize the UGA codon. The Sec-specific elongation factor SelB brings the selenocysteinyl-tRNASec (Sec-tRNASec) to the ribosome, dependent on both an in-frame UGA and a Sec-insertion sequence (SECIS) in the mRNA. The bacterial SelB binds mRNA through its C-terminal region, for which crystal structures have been reported. In this study, we determined the crystal structure of the full-length SelB from the bacterium Aquifex aeolicus, in complex with a GTP analog, at 3.2-Å resolution. SelB consists of three EF-Tu-like domains (D1–3), followed by four winged-helix domains (WHD1–4). The spacer region, connecting the N- and C-terminal halves, fixes the position of WHD1 relative to D3. The binding site for the Sec moiety of Sec-tRNASec is located on the interface between D1 and D2, where a cysteine molecule from the crystallization solution is coordinated by Arg residues, which may mimic Sec binding. The Sec-binding site is smaller and more exposed than the corresponding site of EF-Tu. Complex models of Sec-tRNASec, SECIS RNA, and the 70S ribosome suggest that the unique secondary structure of tRNASec allows SelB to specifically recognize tRNASec and characteristically place it at the ribosomal A-site.