Crystal structures of the human elongation factor eEFSec suggest a non-canonical mechanism for selenocysteine incorporation.

Malgorzata Dobosz-Bartoszek,Dieter Söll,Srinivas Chakravarthy,Zbyszek Otwinowski,Miljan Simonović,Mark H Pinkerton,Paul R Copeland

doi:10.1038/ncomms12941

Abstract

Selenocysteine is the only proteinogenic amino acid encoded by a recoded in-frame UGA codon that does not operate as the canonical opal stop codon. A specialized translation elongation factor, eEFSec in eukaryotes and SelB in prokaryotes, promotes selenocysteine incorporation into selenoproteins by a still poorly understood mechanism. Our structural and biochemical results reveal that four domains of human eEFSec fold into a chalice-like structure that has similar binding affinities for GDP, GTP and other guanine nucleotides. Surprisingly, unlike in eEF1A and EF-Tu, the guanine nucleotide exchange does not cause a major conformational change in domain 1 of eEFSec, but instead induces a swing of domain 4. We propose that eEFSec employs a non-canonical mechanism involving the distinct C-terminal domain 4 for the release of the selenocysteinyl-tRNA during decoding on the ribosome.

Highlights

Selenocysteine is the only proteinogenic amino acid encoded by a recoded in-frame UGA codon that does not operate as the canonical opal stop codon
The B90° rotation of domain 1 (D1) relative to domains 2 (D2) and 3 (D3)[13,14] disrupts the interactions between the aa-tRNA and the aminoacyl-recognition pocket, which leads to dissociation of EF-Tu:GDP from the aa-tRNA and the ribosome
The cotranslational insertion of Sec into a nascent selenoprotein is promoted by a specialized elongation factor, eEFSec in eukaryotes[19,20] and SelB in prokaryotes21. eEFSec is a translational GTPase that binds SectRNASec with high affinity and stringent specificity, and plays a pivotal role during decoding[19] by delivering Sec-tRNASec to the site of translation in response to a particular in-frame UGA codon

Summary

Introduction

Selenocysteine is the only proteinogenic amino acid encoded by a recoded in-frame UGA codon that does not operate as the canonical opal stop codon. The most surprising observation is that D1 assumes a similar orientation relative to D2 and D3 in both functional states of eEFSec. Even more unexpected was the finding that the GTP-to-GDP exchange induces B26° swing of the C-terminal domain D4 away from the predicted tRNA-binding site (Fig. 2a), which results in 415 Å translation of the entire domain.

Results

Conclusion