Ryuta Tobe

Identification of Proteins Interacting with Selenocysteine Lyase

Yeast two-hybrid screening of mouse cDNA libraries was performed to identify proteins interacting with selenocysteine lyase (SCL), which decomposes L-selenocysteine. Several proteins related to spermatogenesis, protein synthesis, and cell viability/apoptosis were identified as potential interactors. Major urinary proteins 1 and 2 interacted with SCL and inhibited its activity. Coimmunoprecipitation revealed interactions between SCL and each of two selenophosphate synthetase isozymes.

Selenocysteine tRNA[Ser]Sec: From Nonsense Suppressor tRNA to the Quintessential Constituent in Selenoprotein Biosynthesis

When selenocysteine (Sec) tRNA[Ser]Sec was originally discovered, it was proposed to be the first nonsense suppressor tRNA found in mammalian and avian tissues, since it exclusively decoded the nonsense codon, UGA, which normally dictates the cessation of protein synthesis. This tRNA was subsequently shown to be Sec tRNA, which inserted Sec into protein as the 21st proteinogenic amino acid. Once it was established that this tRNA was aminoacylated with serine by seryl-tRNA synthetase and served as the scaffold for Sec synthesis, Sec tRNA was appropriately named Sec tRNA[Ser]Sec. The mammalian Sec-tRNA[Ser]Sec population consists of two isoforms that differ from each other by a single 2′-O-methyl moiety on the uridine at position 34, designated Um34. The non-Um34 isoform is involved in the synthesis of a subclass of selenoproteins, called housekeeping selenoproteins, while the Um34 isoform supports synthesis of stress-related selenoproteins. These novel functions and other unique features of Sec tRNA are the subjects of this chapter, supporting the idea that this tRNA is the quintessential constituent in selenoprotein biosynthesis.

Radioactive 75 Se Labeling and Detection of Selenoproteins

The trace element selenium (Se) is incorporated into proteins as the amino acid selenocysteine (Sec), which is cotranslationally inserted into specific proteins in response to a UGA codon. Proteins containing Sec at these specific positions are called selenoproteins. Most selenoproteins function as oxidoreductases, while some serve other important functions. There are 25 known selenoprotein genes in humans and 24 in mice. The use of Sec allows selenoproteins to be detected by a convenient method involving metabolic labeling with 75Se. Labeling of cells and whole animals are used for the examination of selenoprotein expression profiles and the investigation of selenoprotein functions. In mammals, nonspecific 75Se insertion is very low, and sensitivity and specificity of selenoprotein detection approaches that of Western blotting. This method allows for the examination of selenoprotein expression and Se metabolism in model and non-model organisms. Herein, we describe experimental protocols for analyzing selenoproteins by metabolic labeling with 75Se both in vitro and in vivo. As an example, the procedure for metabolic labeling of HEK293T human embryonic kidney cells is described in detail. This approach remains a method of choice for the detection of selenoproteins in diverse settings.

Characterization of a Novel Porin-Like Protein, ExtI, from Geobacter sulfurreducens and Its Implication in the Reduction of Selenite and Tellurite

The extI gene in Geobacter sulfurreducens encodes a putative outer membrane channel porin, which resides within a cluster of extHIJKLMNOPQS genes. This cluster is highly conserved across the Geobacteraceae and includes multiple putative c-type cytochromes. In silico analyses of the ExtI sequence, together with Western blot analysis and proteinase protection assays, showed that it is an outer membrane protein. The expression level of ExtI did not respond to changes in osmolality and phosphate starvation. An extI-deficient mutant did not show any significant impact on fumarate or Fe(III) citrate reduction or sensitivity to β-lactam antibiotics, as compared with those of the wild-type strain. However, extI deficiency resulted in a decreased ability to reduce selenite and tellurite. Heme staining analysis revealed that extI deficiency affects certain heme-containing proteins in the outer and inner membranes, which may cause a decrease in the ability to reduce selenite and tellurite. Based on these observations, we discuss possible roles for ExtI in selenite and tellurite reduction in G. sulfurreducens.

Bacteria Versus Selenium: A View from the Inside Out

Bacteria and selenium (Se) are closely interlinked as the element serves both essential nutrient requirements and energy generation functions. However, Se can also behave as a powerful toxicant for bacterial homeostasis. Conversely, bacteria play a tremendous role in the cycling of Se between different environmental compartments, and bacterial metabolism has been shown to participate to all valence state transformations undergone by Se in nature. Bacteria possess an extensive molecular repertoire for Se metabolism. At the end of the 1980s, a novel mode of anaerobic respiration based on Se oxyanions was experimentally documented for the first time. Following this discovery, specific enzymes capable of reducing Se oxyanions and harvesting energy were found in a number of anaerobic bacteria. The genes involved in the expression of these enzymes have later been identified and cloned. This iterative approach undertaken outside-in led to the understanding of the molecular mechanisms of Se transformations in bacteria. Based on the extensive knowledge accumulated over the years, we now have a full(er) view from the inside out, from DNA-encoding genes to enzymes and thermodynamics. Bacterial transformations of Se for assimilatory purposes have been the object of numerous studies predating the investigation of Se respiration. Remarkable contributions related to the understating of the molecular picture underlying seleno-amino acid biosynthesis are reviewed herein. Under certain circumstances, Se is a toxicant for bacterial metabolism and bacteria have evolved strategies to counteract this toxicity, most notably by the formation of elemental Se (nano)particles. Several biotechnological applications, such as the production of functional materials and the biofortification of crop species using Se-utilizing bacteria, are presented in this chapter.

Mechanism, Structure, and Biological Role of Selenocysteine Lyase

Selenocysteine lyase is a pyridoxal 5′-phosphate-dependent enzyme catalyzing the degradation of l-selenocysteine to l-alanine and elemental selenium. It is unique in that it acts exclusively on l-selenocysteine but not on its sulfur counterpart, l-cysteine. The enzyme is proposed to function not only in the recycling of selenium via degradation of l-selenocysteine derived from selenoproteins, but also in energy metabolism linked to obesity and metabolic syndrome. Crystallographic studies have shed light on the catalytic mechanism that allows the enzyme to distinguish between l-selenocysteine and l-cysteine, which possibly contributes in uncovering the physiological role of selenocysteine lyase in mammals.

Delivery of selenium to selenophosphate synthetase for selenoprotein biosynthesis

BACKGROUNDnSelenophosphate, the key selenium donor for the synthesis of selenoprotein and selenium-modified tRNA, is produced by selenophosphate synthetase (SPS) from ATP, selenide, and H2O. Although free selenide can be used as the in vitro selenium substrate for selenophosphate synthesis, the precise physiological system that donates in vivo selenium substrate to SPS has not yet been characterized completely.nnnSCOPE OF REVIEWnIn this review, we discuss selenium metabolism with respect to the delivery of selenium to SPS in selenoprotein biosynthesis.nnnMAJOR CONCLUSIONSnGlutathione, selenocysteine lyase, cysteine desulfurase, and selenium-binding proteins are the candidates of selenium delivery system to SPS. The thioredoxin system is also implicated in the selenium delivery to SPS in Escherichia coli.nnnGENERAL SIGNIFICANCEnSelenium delivered via a protein-bound selenopersulfide intermediate emerges as a central element not only in achieving specific selenoprotein biosynthesis but also in preventing the occurrence of toxic free selenide in the cell. This article is part of a Special Issue entitled Selenium research in biochemistry and biophysics - 200 year anniversary.

A non-radioactive assay for selenophosphate synthetase activity using recombinant pyruvate pyrophosphate dikinase from thermus thermophilus HB8

Biosynthesis of selenocysteine-containing proteins requires monoselenophosphate, a selenium-donor intermediate generated by selenophosphate synthetase (Sephs). A non-radioactive assay was developed as an alternative to the standard [8-14C] AMP-quantifying assay. The product, AMP, was measured using a recombinant pyruvate pyrophosphate dikinase from Thermus thermophilus HB8. The KM and kcat for Sephs2-Sec60Cys were determined to be 26 μM and 0.352 min−1, respectively.