Preparation of Selenocysteine-Containing Forms of Human SELENOK and SELENOS

Selenoprotein S Interacts with the Replication and Transcription Complex of SARS-CoV-2 by Binding nsp7

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) replicates and evades detection using ER membranes and their associated protein machinery. Among these hijacked human proteins is selenoprotein S (selenos). This selenoprotein takes part in the protein quality control, signaling, and the regulation of cytokine secretion. While the role of selenos in the viral life cycle is not yet known, it has been reported to interact with SARS-CoV-2 nonstructural protein 7 (nsp7), a viral protein essential for the replication of the virus. We set to study whether selenos and nsp7 interact directly and if they can still bind when nsp7 is bound to the replication and transcription complex of the virus. Using biochemical assays, we show that selenos binds directly to nsp7. In addition, we found that selenos can bind to nsp7 when it is in a complex with the coronavirus's minimal replication and transcription complex, comprised of nsp7, nsp8, and the RNA-dependent RNA polymerase nsp12. In addition, through crosslinking experiments, we mapped the interaction sites of selenos and nsp7 in the replication complex and showed that the hydrophobic segment of selenos is essential for binding to nsp7. This arrangement leaves an extended helix and the intrinsically disordered segment of selenos-including the reactive selenocysteine-exposed and free to potentially recruit additional proteins to the replication and transcription complex.

Impact of Selenium on Biomarkers and Clinical Aspects Related to Ageing. A Review

Selenium (Se) is an essential dietary trace element that plays an important role in the prevention of inflammation, cardiovascular diseases, infections, and cancer. Selenoproteins contain selenocysteine in the active center and include, i.a., the enzymes thioredoxin reductases (TXNRD1–3), glutathione peroxidases (GPX1–4 and GPX6) and methionine sulfoxide reductase, involved in immune functions, metabolic homeostasis, and antioxidant defense. Ageing is an inevitable process, which, i.a., involves an imbalance between antioxidative defense and reactive oxygen species (ROS), changes in protein and mitochondrial renewal, telomere attrition, cellular senescence, epigenetic alterations, and stem cell exhaustion. These conditions are associated with mild to moderate inflammation, which always accompanies the process of ageing and age-related diseases. In older individuals, Se, by being a component in protective enzymes, operates by decreasing ROS-mediated inflammation, removing misfolded proteins, decreasing DNA damage, and promoting telomere length. Se-dependent GPX1–4 and TXNRD1–3 directly suppress oxidative stress. Selenoprotein H in the cell nucleus protects DNA, and selenoproteins residing in the endoplasmic reticulum (ER) assist in the removal of misfolded proteins and protection against ER stress. In this review, we highlight the role of adequate Se status for human ageing and prevention of age-related diseases, and further its proposed role in preservation of telomere length in middle-aged and elderly individuals.

Molecular Mechanisms by Which Selenoprotein K Regulates Immunity and Cancer

Many of the 25 members of the selenoprotein family function as enzymes that utilize their selenocysteine (Sec) residues to catalyze redox-based reactions. However, some selenoproteins likely do not exert enzymatic activity by themselves and selenoprotein K (SELENOK) is one such selenoprotein family member that uses its Sec residue in an alternative manner. SELENOK is an endoplasmic reticulum (ER) transmembrane protein that has been shown to be important for ER stress and for calcium-dependent signaling. Molecular mechanisms for the latter have recently been elucidated using knockout mice and genetically manipulated cell lines. These studies have shown that SELENOK interacts with an enzyme in the ER membrane, DHHC6 (letters represent the amino acids aspartic acid, histidine, histidine, and cysteine in the catalytic domain), and the SELENOK/DHHC6 complex catalyzes the transfer of acyl groups such as palmitate to cysteine residues in target proteins, i.e., palmitoylation. One protein palmitoylated by SELENOK/DHHC6 is the calcium channel protein, the inositol 1,4,5-trisphosphate receptor (IP3R), which is acylated as a means for stabilizing the tetrameric calcium channel in the ER membrane. Factors that lower SELENOK levels or function impair IP3R-driven calcium flux. This role for SELENOK is important for the activation and proliferation of immune cells, and recently, a critical role for SELENOK in promoting calcium flux for the progression of melanoma has been demonstrated. This review provides a summary of these findings and their implications in terms of designing new therapeutic interventions that target SELENOK for treating cancers like melanoma.

Selenoprotein K Increases Efficiency of DHHC6 Catalyzed Protein Palmitoylation by Stabilizing the Acyl-DHHC6 Intermediate

Selenoprotein K (SELENOK) is a selenocysteine (Sec)-containing protein localized in the endoplasmic reticulum (ER) membrane where it interacts with the DHHC6 (where single letter symbols represent Asp-His-His-Cys amino acids) enzyme to promote protein acyl transferase (PAT) reactions. PAT reactions involve the DHHC enzymatic capture of palmitate via a thioester bond to cysteine (Cys) residues that form an unstable palmitoyl-DHHC intermediate, followed by transfer of palmitate to Cys residues of target proteins. How SELENOK facilitates this reaction has not been determined. Splenocyte microsomal preparations from wild-type mice versus SELENOK knockout mice were used to establish PAT assays and showed decreased PAT activity (~50%) under conditions of SELENOK deficiency. Using recombinant, soluble versions of DHHC6 along with SELENOK containing Sec₉₂, Cys₉₂, or alanine (Ala₉₂), we evaluated the stability of the acyl-DHHC6 intermediate and its capacity to transfer the palmitate residue to Cys residues on target peptides. Versions of SELENOK containing either Ala or Cys residues in place of Sec were equivalently less effective than Sec at stabilizing the acyl-DHHC6 intermediate or promoting PAT activity. These data suggest that Sec₉₂ in SELENOK serves to stabilize the palmitoyl-DHHC6 intermediate by reducing hydrolyzation of the thioester bond until transfer of the palmitoyl group to the Cys residue on the target protein can occur.