Several gene products are involved in co-translational insertion of 
selenocysteine by the tRNA(Sec). In addition, a stem-loop structure in the mRNAs 
coding for selenoproteins is essential to mediate the selection of the proper 
selenocysteine UGA codon. Interestingly, in eukaryotic selenoprotein mRNAs, this 
stem-loop structure, the selenocysteine insertion sequence (SECIS) element, 
resides in the 3'-untranslated region, far downstream of the UGA codon. In view 
of unravelling the underlying complex mechanism, we have attempted to detect 
RNA-binding proteins with specificity for the SECIS element. Using mobility 
shift assays, we could show that a protein, present in different types of 
mammalian cell extracts, possesses the capacity of binding the SECIS element of 
the selenoprotein glutathione peroxidase (GPx) mRNA. We have termed this protein 
SBP, for Secis Binding Protein. Competition experiments attested that the 
binding is highly specific and UV cross-linking indicated that the protein has 
an apparent molecular weight in the range of 60-65 kDa. Finally, some data 
suggest that the SECIS elements in the mRNAs of GPx and another selenoprotein, 
type I iodothyronine 5' deiodinase, recognize the same SBP protein. This 
constitutes the first report of the existence of a 3' UTR binding protein 
possibly involved in the eukaryotic selenocysteine insertion mechanism.