Structure of the mammalian ribosome as it decodes the selenocysteine UGA codon

Abstract

The elongation of eukaryotic selenoproteins relies on a poorly understood process of interpreting in-frame UGA stop codons as selenocysteine (Sec). We used cryo-electron microscopy to visualize Sec UGA recoding in mammals. A complex between the noncoding Sec-insertion sequence (SECIS), SECIS-binding protein 2 (SBP2), and 40 S ribosomal subunit enables Sec-specific elongation factor eEFSec to deliver Sec. eEFSec and SBP2 do not interact directly but rather deploy their carboxyl-terminal domains to engage with the opposite ends of the SECIS. By using its Lys-rich and carboxyl-terminal segments, the ribosomal protein eS31 simultaneously interacts with Sec-specific transfer RNA (tRNA Sec ) and SBP2, which further stabilizes the assembly. eEFSec is indiscriminate toward l -serine and facilitates its misincorporation at Sec UGA codons. Our results support a fundamentally distinct mechanism of Sec UGA recoding in eukaryotes from that in bacteria.

High-Resolution Ribosome Profiling Reveals Gene-Specific Details of UGA Re-Coding in Selenoprotein Biosynthesis

“…Conversely, the fraction of sRPF from ribosomes with the UGA/Sec codon in an empty A-site is about two-fold increased across all selenoproteins in the SECISBP2 -mutant cells ( Figure 5 c). Since there is no shortage of available selenium (Se) in the culture, this finding likely does not stem from a lack of charged tRNA Sec but may be related to a role of SECISBP2 in EEFSEC:Sec-tRNA Sec recruitment [ 13 ]. This effect is again particularly evident in the case of SELENOO ( Figure 5 d).…”

“…In eukaryotes, these functions are divided between the elongation factor EEFSEC/mSelB [ 9 , 10 ] and the SECIS-binding protein 2 ( SECISBP2 ) [ 11 ]. The structural details of these interactions have been resolved in greater detail in recent cryo-EM structures of the bacterial ribosome in complex with mRNA, Sec-tRNA Sec , and SelB [ 12 ] and the mammalian ribosome with mRNA, tRNA Sec , EEFSEC, and SECISBP2 [ 13 ].…”

eIF3 Interacts with Selenoprotein mRNAs

“…The SECIS binding protein 2 (SECISBP2) interacts with the SECIS RNA and enables the recruitment of components of the selenocysteine insertion machinery to the mRNP [ 8 , 12 , 13 ]; amongst them, the specialized elongation factor eEFSec is in complex with the selenocysteinyl-tRNA Sec [ 14 , 15 ]. The structure of the mammalian ribosome as it decodes the selenocysteine UGA codon was recently obtained by cryo-electron microscopy [ 16 ]. SBP2-SECIS forms a docking site on the ribosome for eEFSec/GTP/Sec-tRNASec that is stabilized by ribosomal protein eS31 [ 16 ].…”

“…The structure of the mammalian ribosome as it decodes the selenocysteine UGA codon was recently obtained by cryo-electron microscopy [ 16 ]. SBP2-SECIS forms a docking site on the ribosome for eEFSec/GTP/Sec-tRNASec that is stabilized by ribosomal protein eS31 [ 16 ]. Other SECIS-RNA-binding proteins, such as the ribosomal protein L30, translation initiation factor 4A3 (eIF4A3), and nucleolin, have been identified as modulators of selenoprotein synthesis [ 17 , 18 , 19 ].…”

Seizures, ataxia and parvalbumin-expressing interneurons respond to selenium supply in Selenop-deficient mice

“…The human and mouse genomes contain 25 and 24 genes encoding selenoproteins, respectively [ 2 ]. Co-translational incorporation of Sec requires several cis - and trans -acting factors, including selenocysteine insertion sequence (SECIS)-binding protein 2 (SECISBP2; [ 3 ]) and a specific elongation factor, EEFSEC [ 4 ], which cooperate to recruit Sec-tRNA Sec to the ribosome [ 1 , 5 ].…”