Characterization of mSelB, a novel mammalian elongation factor for selenoprotein translation

Decoding of UGA selenocysteine codons in eubacteria is mediated by the spec ialized elongation factor SelB, which conveys the charged tRNA(Sec) to the A site of the ribosome, through binding to the SECIS mRNA hairpin. In an at tempt to isolate the eukaryotic homolog of SelB, a database search in this work identified a mouse expressed sequence tag containing the complete cDNA encoding a novel protein of 583 amino acids, which we called mSelB, Severa l lines of evidence enabled us to establish that mSelB is the bona fide mam malian elongation factor for selenoprotein translation: it binds GTP, recog nizes the Sec-tRNA(Sec) in vitro and in vivo, and is required for efficient selenoprotein translation in vivo. In contrast to the eubacterial SelB, th e recombinant mSelB alone is unable to bind specifically the eukaryotic SEC IS RNA hairpin. However, complementation with HeLa cell extracts led to the formation of a SECIS-dependent complex containing mSelB and at least anoth er factor. Therefore, the role carried out by a single elongation factor in eubacterial selenoprotein translation is devoted to two or more specialize d proteins in eukaryotes.