A novel RNA binding protein, SBP2, is required for the translation of mammalian selenoprotein mRNAs

In eukaryotes, the decoding of the UGA codon as selenocysteine (Sec) requir es a Sec insertion sequence (SECIS) element in the 3' untranslated region o f the mRNA, We purified a SECIS binding protein, SBP2, and obtained a cDNA clone that encodes this activity. SBP2 is a novel protein containing a puta tive RNA binding domain found in ribosomal proteins and a yeast suppressor of translation termination. By UV cross-linking and immunoprecipitation, we show that SBP2 specifically binds selenoprotein mRNAs both in vitro and in vivo. Using Se-75-labeled Sec-tRNA(Sec), we developed an in vitro system f or analyzing Sec incorporation in which the translation of a selenoprotein mRNA was both SBP2 and SECIS element dependent. Immunodepletion of SBP2 fro m the lysates abolished Sec insertion, which was restored when recombinant SBP2 was added to the reaction. These results establish that SBP2 is essent ial for the co-translational insertion of Sec into selenoproteins. We hypot hesize that the binding activity of SBP2 may be involved in preventing term ination at the UGA/Sec codon.