SelT, SelW, SelH, and Rdx12:  Genomics and Molecular Insights into the Functions of Selenoproteins of a Novel Thioredoxin-like Family

Abstract

Selenium is an essential trace element in many life forms due to its occurrence as a selenocysteine (Sec) residue in selenoproteins. The majority of mammalian selenoproteins, however, have no known function. Herein, we performed extensive sequence similarity searches to define and characterize a new protein family, designated Rdx, that includes mammalian selenoproteins SelW, SelV, SelT and SelH, bacterial SelW-like proteins and cysteine-containing proteins of unknown function in all three domains of life. An additional member of this family is a mammalian cysteine-containing protein, designated Rdx12, and its fish selenoprotein orthologue. Rdx proteins are proposed to possess a thioredoxin-like fold and a conserved CxxC or CxxU (U is Sec) motif, suggesting a redox function. We cloned and characterized three mammalian members of this family, which showed distinct expression patterns in mouse tissues and different localization patterns in cells transfected with the corresponding GFP fusion proteins. By analogy to thioredoxin, Rdx proteins can use catalytic cysteine (or Sec) to form transient mixed disulfides with substrate proteins. We employed this property to identify cellular targets of Rdx proteins using affinity columns containing mutant versions of these proteins. Rdx12 was found to interact with glutathione peroxidase 1, whereas 14-3-3 protein was identified as one of the targets of mammalian SelW, suggesting a mechanism for redox regulation of the 14-3-3 family of proteins.

Selenium biochemistry and its role for human health

“…Selenoprotein W Se-protein W (Sepw1, SelW) is a small protein with the SeCys residue as part of a Cys-X-X-SeCys redox motif localized in an exposed loop. 100 The analogy with the motif Cys-X-X-Cys of Trx, high affinity for GSH, and overexpression against oxidative stress in muscle tissues suggest an antioxidant function. 101 However, the precise molecular pathways are not yet elucidated, so the specific functions remain unknown.…”

“…112 In mouse and rat cells it localizes in Golgi, ER and possibly in the plasma membrane. 100,113 It is ubiquitously distributed, with high expression in the testes. 58 The expression of SelT is regulated by the trophic neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP).…”

“…303,304 The association between Se and inorganic Hg in exposed subjects is well established. 305 In the bloodstream, selenite reacts also with Hg 2+ to form a species with a core of HgSe and GS-moieties ligated on the surface in the form (HgSe) 100 (GS) 5 . 302 This species binds to SelP up to a proportion of 35 units per protein, so that this Se-protein is actively involved in a detoxification mechanism for Hg.…”

Cloning and expression of selenoprotein W from pearl mussels Cristaria plicata

“…Rdx is a family of thioredoxin-like proteins that all contain several common structural elements, including the overall β-α-β-β-β secondary structure core, a CxxU or CxxC motifs. An additional feature of these proteins is a conserved stretch of amino acids in the middle or C-terminal regions with the following consensus sequence: tGxFEI(V), these proteins use CxxC or Cxxu motifs as various redox functions (Aachmann et al, 2007;Dikiy et al, 2007). Comparison of the 3D structure of CpSelW and mouse SelW protein indicated that both had very similar three dimensional structures and showed an overall conservation of structure and the predicted active site CXXU motif, and consisted of one four stranded β-sheet and two extended α-helices located on one side of the β-sheet, β1 and β2 were parallel strands forming a classical β1-α1-β2 motif.…”

“…SelW is capable of binding reduced glutathione (GSH) (Jeong et al, 2002), and overexpression of SelW experiment suggested that it may be an important component of the cellular defense system against oxidative stress (Sun et al, 2001). SelW is involved in redox regulation through its interactions with 14-3-3 proteins, it is suggested that SelW is a member of the thioredoxin family (Aachmann et al, 2007;Dikiy et al, 2007). Recently, SelW gene has been cloned from humans, monkeys, rats, mice, pig, sheep (Whanger, 2009), chicken (Ou et al, 2011), and fish (Kryukov and Gladyshev, 2000).…”