Size matters: a view of selenocysteine incorporation from the ribosome

Abstract

This review focuses on the known factors required for selenocysteine (Sec) incorporation in eukaryotes and highlights recent findings that have compelled us to propose a new model for the mechanism of Sec incorporation. In light of this data we also review the controversial aspects of the previous model specifically regarding the proposed interaction between SBP2 and eEFSec. In addition, the relevance of two recently discovered factors in the recoding of Sec are reviewed. The role of the ribosome in this process is emphasized along with a detailed analysis of kinkturn structures present in the ribosome and the L7Ae RNA-binding motif present in SBP2 and other proteins.

Figure 1

Comparison of the consensus kink-turn motif (left) and the GPX4 SECIS element (right). The SECIS core motif is boxed, emphasizing the SBP2- and rpL30-binding site. The conserved AAR motif is also noted within the apical bulge.

Figure 2

Alignment of the core L7Ae binding motif in the human versions of the 15.5-kDa snRNA-binding protein, rpL30 and SBP2. Residues shaded in black represent identities, those shaded in gray represent conservative substitutions. The secondary-structure motifs, derived from the 15.5-kDa co-crystal analysis [24] are illustrated above the sequence. Red arrows indicate alanine substitutions in SBP2 that inactivated SECIS element-binding activity. Green arrows indicate alanine substitutions in SBP2 that had a minimal affect on SECIS element binding [27].

Figure 3

Contrasting models for Sec incorporation. (A) A common model for Sec incorporation. An SBP2-bound SECIS element recruits the eEFSec/Sec-tRNA^[Ser]Sec complex. After association with the ribosome, SBP2 is exchanged for rpL30 resulting from a kinked SECIS element. This conformational change in the SECIS element is hypothesized to trigger the release of the Sec-tRNA^[Ser]Sec and GTP hydrolysis. (B) A model of Sec incorporation based on the topics discussed in this review. SBP2 is pre-bound to the ribosome. Upon arrival at a UGA codon, ribosome stalling allows for a distant SECIS element to find and interact with SBP2. Movement of SBP2 may trigger conformational changes in the ribosomal A-site that favor the binding of eEFSec/Sec-tRNA^[Ser]Sec and the subsequent insertion of Sec. After incorporation, rpL30 may function to displace SBP2 off the SECIS element and back to its original position on the ribosome.