Utilization of Selenocysteine by a Cysteinyl-tRNA Synthetase from Phaseolus aureus

Abstract

An L-cysteinyl-tRNA synthetase (EC 6.1.1.16) from Phaseolus aureus has been purified approximately 200-fold. The enzyme uses selenocysteine as substrate in the ATP-PPi exchange assay; other cysteine analogs were inactive. The molecular weight as determined by Sephadex G-200 column chromatography is about 61,000; sodium dodecyl sulfate and 8 Murea acrylamide gel electrophoresis indicate that the enzyme is a dimer conssting of two identical monomers of molecular weight 30,000.A method for the preparation of selenocysteine from selenocystine is described.digests from several organisms (2, 10, 15, 23), but it was reported absent, despite the presence of selenomethionine, from proteins of other organisms grown with radioactive selenite or selenate (13, 21). Such findings suggest that different organisms vary in their ability to activate selenocysteine and incorporate it into their proteins. Therefore, to examine the activation of selenocysteine, a method for its preparation from selenocystine and its use as substrate by a purified cysteinyl-tRNA synthetase from Phaseolus aureus was undertaken and will be described here. MATERIALS AND METHODSThe ability of methionyl-and cysteinyl-tRNA synthetases to use selenium isologs as substrates has been suggested as the cause of selenium toxicity. According to this hypothesis, both the sulfur and selenium amino acids are activated and incorporated into proteins; these altered proteins, with an excess of selenoamino acids, function abnormally (7,29). On the other hand, tolerance of selenium, notably in selenium-accumulator plants, is thought to have arisen from evolutionary modifications to these aminoacyl-tRNA synthetases so that the selenium isologs, no longer activated, are excluded from proteins; instead, the selenium isologs are converted to and accumulate as nonprotein selenoamino acids (7,24,28,31). Altered aminoacyl-tRNA synthetases have been described in other plants which synthesize large amounts of nonprotein amino acids. Modified prolyl-tRNA synthetases, for example, can be isolated from Polygonatum and Convallaria species that synthesize azetidine-2-carboxylic acid; this nonprotein amino acid analog of proline is excluded from the proteins of these plants but not from the proteins of organisms poisoned by the analog (7,8,22,25 Buffer B, for dissolving the 45 to 65% (NH4)2SO4 fraction, and for equilibration of Sephadex and DE-52 columns, contained 0.05 M tris and the other components of buffer A; the pH was brought to 7.7 with HCl.Buffer C, pH 7.7, for hydroxylapatite and cellulose phosphate columns, contained 50 ml of 0.02 M KH2PO4, 500 ml of 0.02 K2HPO4, 25 mm 2-mercaptoethanol, and 10% (w/v) glycerol.Buffer D, pH 7.7, for gradient elution of hydroxylapatite and cellulose phosphate columns, contained 70 ml of 0.4 M KH2PO4 and 500 ml of 0.4 M K2HPO4, 25 mm 2-mercaptoethanol, andPreparation of Crude Enzyme Extract and Preliminary Fractionation. All operations were carried out at 3 to 4 C. Phaseolus aureus seed, milled to a powder, was homogenized for 1 min in ...

How an obscure archaeal gene inspired the discovery of selenocysteine biosynthesis in humans

“…A SecRS has apparently never evolved (and perhaps never will) because typical protein substrate recognition, which depends on distinct differences between the shape and charge of cognate versus noncognate substrates, cannot differentiate Cys from Sec (53,54). The indirect route for Sec-tRNA formation evolved, therefore, to ensure that critical Sec codons were read with an exceedingly high fidelity by eliminating or minimizing the possibility that Cys-tRNA Sec would be formed.…”

Cysteinyl-tRNA Synthetase from Astragalus Species

“…The activation of selenocysteine by cysteinyl-tRNA synthetases is less well documented, but in two instances selenocysteine-dependent ATP-PPi exchange, catalyzed by a purified cysteinyltRNA synthetase, equaled the cysteinyl-tRNA exchange (2,10); in another instance, selenocysteine was used in the aminoacylation of tRNA, but the Se analog appeared to be significantly less effective as substrate than was cysteine (I 1 Chemicals. All chemicals were purchased from the sources described previously (2).…”

Amino acid modifications on tRNA<xref xml:base="fn" rid="fn1">†</xref>

“…Why Sec-tRNA is only formed via indirect paths remains unknown but it may be due to selective pressures to maintain translational fidelity. CysRSs from E. coli [95] and Phaseolus aureus [96] have the ability to aminoacylate tRNA Cys with Sec. Therefore, given the fact that misincorporation of Sec in place of Cys can be detrimental to protein function [97], the levels of free Sec are likely well regulated and kept low to prevent misacylation of tRNA Cys with Sec.…”