Figure 1.
Selenocysteine biosynthesis and selenoprotein translation pathways.
Selenoproteins incorporate the amino acid Selenocysteine (Sec) which is coded by the codon UGA, normally a stop codon. The recoding of UGA as a Sec codon is mediated by a structural element on the 3′ Untranslated Region (UTR) of selenoprotein mRNAs, the SElenoCysteine Insertion Sequence (SECIS). This is recognised by the SECIS Binding Protein 2 (SBP2), which binds to both the SECIS element and the ribosome. SBP2, in turn, recruits the Sec-specific Elongation Factor EFsec, and the selenocysteine transfer RNA, tRNASec. SBP2 and tRNASec form a complex with the tRNA Selenocysteine associated protein, secp43. Sec is synthesized from serine in a multi-step reaction: Ser-tRNA[Sec] is phosphorylated by A Phosphoseryl tRNA Kinase (PSTK) and converted to Sec-tRNA[Sec] by Sec synthetase (SecS). Secp43 is also known to be involved in the conversion from seryl to selenocysteyl but its exact role is unclear. Finally, Selenophosphate Synthetase 2 (SPS2), catalyses the formation of mono-selenophosphate, the donor compound of Selenium necessary for the synthesis of selenocysteine, from either selenite (SeO3) or from an unstable selenide compound depicted as (Se2−). The exact role of SPS1 is still not clear. This figure was partially adapted from [7].
Figure 2.
Alignment of insect SelH proteins.
The black arrow shows the position of the selenocysteine (U) residue (cysteine in D. willistoni). Here, as in the other alignments, only insect species encoding SelH are shown.
Figure 3.
Alignment of insect SelK and SelK cysteine paralogs.
SelK has been duplicated, producing a Cys-paralog, in species of the melanogaster group. These paralogs are shown with a “C” after the name of the species. The black arrow shows the position of the selenocysteine (U) residue (cysteine in D. Willistoni SelK and in the SelK Cys-paralogs).
Figure 4.
Alignment of insect SPS1 and SPS2 proteins.
The black arrow shows the position of the selenocysteine residue in SPS2 and Arganine or Cysteine in SPS1. In A. mellifera, we use “?” to denote the codon TGA. Although we believe that in this case, TGA is being readthrough to incorporate Arginine (R), it actually aligns with the Sec-incorporating codon in SPS2.
Figure 5.
Phylogenetic tree for the insect SPS1 and SPS2 proteins.
This tree was built from the alignment of all insects sequences in Figure 4. Note that the D. willistoni sequence (in magenta) clusters with the other SPS1 sequences. This is also the case of the sequence from A. mellifera (in blue), in spite of the fact that the in-frame UGA codon in this sequence aligns with the Sec codon in the insect SPS2 sequences.
Figure 6.
Alignment of insect tRNASec sequences.
The black arrow points to the position of the TCA anticodon.
Table 1.
tRNASec predictions and their scores in each species.
Figure 7.
Alignment of insect SBP2 proteins.
Alignment of insect SBP2 proteins. The conserved region containing the insertion in D. willistoni is bound by a magenta box.
Table 2.
A summary of the results for each selenoprotein and selenoprotein factor in all completely sequenced insect genomes.
Figure 8.
Selenoprotein extinction in arthropoda.
Species whose genomes do not code for selenoprotein genes are shown in red. Sec-encoding species are shown in green with the number of selenoproteins found in each genome in parentheses next to its name. Species for which the available data was inconclusive are shown in white. The phylogenetic relationships have been taken from the ncbi's Taxonomy database (http://www.ncbi.nlm.nih.gov/Taxonomy/) and the Tree of Life project (http://www.tolweb.org/tree/). The Drosphilidae tree was taken from the Drosophila Sequencing Consortium wiki (http://rana.lbl.gov/drosophila/caf1.html).