Selenoproteins are a group of proteins which contain a residue of selenocysteine (Sec), known as the 21st amino acid. This unusual amino acid is a cysteine analog with selenium instead of sulfur in the radical chain. Interestingly, Sec is cotranslationally inserted into the emergent polypeptide in response to UGA codons (Mariotti et al, 2010) which usually acts as a STOP codon. This dual function of UGA codon has been ignored for many years, with the subsequent misinterpretation of UGA codon as Sec codon and terminator.

The inclusion of the Sec and the synthesis of a selenoprotein require a sequence in the 3'-UTR region called SElenoCysteine Insertion Sequence (SECIS). The eukaryotic SECIS element has a length of approximately 60 nucleotides and it adopts a stem-loop structure in the mRNA.

From an evolutionary point of view, the maintenance of a selenoprotein is a trade-off between its efficacy and the handicap of synthethising it. Some selenocysteines mutate to cysteine but keep their functionality. Thus, some members of selenoprotein families are analogs containing cysteine.

The first comprehensive analysis of all the selenoprotein families in mammals was published very recently (Mariotti et al., 2012). This survey of several mammalian genomes gives us a wide view of the families present in mammals and their changes along evolution and separation of clades.

There is great number and a vast variety of selenoproteins. To date, 45 selenoprotein families have been described (Mariotti et al, 2012). Bony fishes have the largest variety with 41 different families, whereas mammals have 28 families all together. According to previous studies a single mammalian species can have up to 25 selenoproteins in its genome.

Perhaps due to their complexity, a lot about selenoproteins is still unknown. Several families do not have an assigned function, and the distribution of selenoproteins among the evolutionary tree is incomplete. Moreover, new proteins involved in UGA recoding have been identified, although their function is not yet known.

Nowadays a lot of new organisms are being genotyped, but the distribution of selenoporoteins in their genomes is unknown. Expanding the number of genomes with annotated selenoproteins would provide a better understanding of their evolution. From this we could derive their importance, and be closer to inferring their function and their implication in disease.

Synthesis of selenoproteins

The common feature of selenoproteins is the fact that they contain at least one residue of selenocysteine. Selenocysteine is an aminoacid which is analogous to cyestine but contains an atom of selenium. This aminoacid is coded by the codon UGA, which would usually code for a STOP.

The UGA codon duality is circumvented by the presence of conserved cis- and trans- acting elements dedicated to the decoding of UGA as a SeCys residue. (Allmang et al, 2006; Lobanov et al, 2009; Castellano et al, 2009).

The basic cis-acting elements for the synthesis of a selenoprotein is the SECIS element, as well as the in-frame UGA codon. On the other hand, the basic trans-acting elements are SPS1,SPS2, SecS, Pstk, eEFsec, SBP2 and SeCys/tRNA[Ser]Sec. All these factors are globally known as the selenoprotein translation machinery.

Interestingly, Sec is the only aminoacid synthesized directly on the tRNA, and isn’t produced from a cysteine but from a serine. tRNA Pstk phosphorylates the tRNA[Ser]Sec , which will allow the next reaction. Selenophosphate synthetase 2 SPS2 prepares the Selenium to be incorporated, and selenocysteine synthase (SecS) binds the atom to the serine, completing the synthesis of the tRNA. This tRNA is not recognized by usual elongation factors, and is instead bound to the specific factor eEFSec.

tRNA Selenocysteine 1 associated protein 1 (Secp43) is also involved in the synthesis of the tRNA[Ser]Sec and selenoproteins (Reeves et al, 2009).

Image: Machinery involved in synthesis of selenoproteins (Bellinger F.P., et al; 2009).

After the trans-acting elements have participated in the tRNA synthesis, the SECIS element acts in Cys to recode the UGA codon. SECIS recruits SECIS Biding Protein 2 (SBP2). SBP2 will in turn recruit and bind eEFSec, the specific elongation factor. Finally to achieve the incorporation of the selenocysteine, the protein has to select the tRNA[Ser]Sec, which is taken to the UGA codon.

We can distinguish two major groups of selenoproteins depending on their function. The first group is formed by housekeeping genes and the second group contains stress activated proteins.

Selenoprotein families


Glutathione peroxidases are the largest and best-known selenoprotein family. They are expressed in all tissues and protect cells from oxidation. They catalyze a reaction which forms glutathione disulfide from two molecules of reduced glutathione.

This family is formed for 8 GPx homologs (GPx1-GPx8). GPx1-4 and GPx6 are selenoproteins in humans and in macaque, whereas GPx5, GPx7 and GPx8 are cysteine-containing homologues.

Mammalians Sec-containing GPx genes are highly conserved.

According to different studies of the evolution of GPx, has been discovered that GPx7 and GPx8 evolved from a GPx4-like selenoprotein ancestor. What’s more, GPx5 and GPx6 had been the latest to evolve; they have appeared due to a duplication of GPx3. Interestingly a fast displacement of Sec to Cys must have happened in GPx5. (Mariotti M., et al; 2012)


It is related to the translation machinery and regulates selenoprotein expression, but its exact fuction is still unknown. It is not a seleprotein.


Iodothyronine deiodinases are expressed in all tissues. Their main function is to control activation and inactivation of thyroid hormones. Tetraiodothyronin (T4) is activated by removing a iodine to convert it to T3. However, other reactions lead to reverse T3 and to T2, which are inactive forms. All three DI isoforms contain at least one selenocyesteine. In the case of DI2 contains three Sec in humans.


In humans we find two isoforms. SPS2 contains a Sec, and SPS1 is a Cysteine homologue. The function of SPS1 is not clear yet (falta ref paragraph dalt). As mentioned above, SPS2 provides an activated form of selenium ready to incorporate in the Serine.


Secis Binding Protein recognizes the SECIS element and recruits eEFSec and other factors in order to recode the UGA codon.


Studies in mouse suggest that this selenoprotein may have redox function and may be involved in the quality control of protein folding. This gene is localized on chromosome 1p31, a genetic locus commonly mutated or deleted in human cancers.


Overexpression in neurons is protective again UV effects. Several papers report that its overexpression (in neurons and other cells) promotes formation of mitochondria. The exact function is not known. Human SelH contains Sec in its active site.


Its function is not known. There is no relevant information about the expression in different tissues. It is a selenoprotein.


Also called selG, glycine-rich selenoprotein. Localized in the endoplasmic reticulum and highly expressed in heart. It might have antioxidant functions and has been related to prostate cancer and impaired immune responses (consultat a NCBI Gene i Protein).


It is expressed in several tissues and its location is perinuclear. Its function remains to be elucidated. SelM contains a Cys residue in humans.


SelN is expressed in the membrane of the endoplasmic reticulum of several tissues. Different mutations in this gene lead to different congenital muscular disease, named in general SEPN1-related myopathies. Its function is not yet clear, but it is thought to have a role in myogenesis. Animal models and cellular studies have provided advances towards unveiling its function (Castets et al, 2012).


It is a selenoprotein, but its function is not known.


SelP is found in plasma and binds to heparin. It is thought to have antioxidant effects in the extracellular space. It contains multiple selenocysteines.


It is expressed in a variety of tissues and its distribution is both perinuclear and cytosolic. Its function is to reduce oxidized methionines. It is part of a bigger family of proteins involved in the same process (Methionine sulfoxide reductase, Msr).


VIMP has been associetd to the regulation of inflammatory response, and has been associated with cardiovascular disease in diabetes (Cox et al, 2013). It is possibly involved in the regulation of certain cytokines. VIMP also contains only one Sec.


SelT, SelW, SelH, and Rdx12 have been proposed as members of a novel thioredoxin-like family. (Dikiy et al, 2008) genomics and molecular insights into the functions of selenoproteins of a novel thioredoxin-like family (Rdx family). In humans, the first three are selenoproteins.(Rdx has a Sec homologue in fishes only).


Also known as FAM213A. It has been validated, but it’s function is not known. SelU1 is expressed in bone, brain, liver, and kidney. Chimpanzees only have SelU2 variant, while in humans, 3 variants have been predicted.


They are involved in a sequence of reactions which ultimately leads to the formation of reduced disulfide bonds. TRs reduce thioredoxin, taking its electrons in order to give them to a protein and transform it from the S2 (oxidized) form to the SH2 (reduced) form. This process is an antioxidant mechanism present in all cells. TR1 is cytosolic, TR2 mitochondrial and TR3 is testes specific.


It originated as a duplication of SelW. It is also a selenoprotein but its function is not known.


It is highly expressed in skeletal and cardiac muscle. It may be involved in antioxidation processes, as most selenoproteins. Its specific function is still unknown.


After the formation of selenophosphate (reaction which prepares the selenium for incorporation), SecS binds it to the Serine carried by the seryl-tRNA.


Catalyses the reaction from seryl-tRNA Ser(Sec) to the phosphoseryl form using an ATP molecule. It is part of the cellular machinery which allows selenoprotein synthesis. It is not a selenorpotein.Ptsk has lost its Sec and contains a Cys.


This enzyme, Methionine Sulfoxide Reducatse A, is not part of a family. It catalyses the reaction from methionine sulfoxide to methionine. This process is also part of the cellular protection against oxidative stress. It does not contain a Sec.

Image: Evolution of the vertebrate selenoproteome (Mariotti M., et al; 2012)

Chlorocebus sabaeus

The Chlorocebus sabaeus, or green monkey, is an Old World monkey which inhabits the deltas, savannas and forests from Senegal and Guinea-Bissau to Ghana and Burkina-Faso (central western part of Africa). It has also been described in some African islands and was introduced in part of the Caribbean islands during the slave trade in the XVIII century.

It is currently included in the IUCN red list of threatened species, but its population is now considered stable.

Of the already analysed primates the macaque (Macaca mulatta), which is also inside the family Cercopitechidae), is the most closely related. This is why we have used both human and macaque sequences as queries for selenoprotein search based in t-BLASTn alignments.

The green monkey and the macaque are part of the Cercopitechidae family, whereas humans and apes are part of the Hominidae family. Both lineages are part of the Catarrhini group. According to an estimation from (Zalmout et al, 2010), the separation of monkeys and apes occurred around 24 to 29 milion years ago.

  • Phylum: Chordata
  • Class: Mammalia
  • Order: Primates
  • Family: Cercopitechidae
  • Genre: Chlorocebus

Image: Phylogenetic tree of primates