Seeds of conflict.

Recent studies have shown that the Arabidopsis MEDEA gene is imprinted, so that paternally and maternally inherited alleles are differentially expressed during seed development. Futhermore, a chromatin remodelling factor has been implicated as a novel trans-acting regulator of imprinting.

Chloroplast fructose-1,6-bisphosphatase: modification of non-covalent interactions promote the activation by chimeric Escherichia coli thioredoxins.

Although all thioredoxins contain a highly conserved amino acid sequence responsible for thiol/disulfide exchanges, only chloroplast thioredoxin-f is effective in the reductive stimulation of chloroplast fructose-1,6-bisphosphatase. We set out to determine whether Escherichia coli thioredoxin becomes functional when selected modulators alter the conformation of the target enzyme. Wild type and chimeric Escherichia coli thioredoxins match the chloroplast counterpart when the activation of chloroplast fructose 1,6-biphosphatase is performed in the presence of fructose 1,6-bisphosphate, Ca2+, and either trichloroacetate or 2-propanol. These modulators of enzyme activity do change the conformation of chloroplast fructose-1,6-bisphosphatase whereas bacterial thioredoxins remain unaltered. Given that fructose 1,6-bisphosphate, Ca2+, and non-physiological perturbants modify non-covalent interactions of the protein but do not participate in redox reactions, these results strongly suggest that the conformation of the target enzyme regulates the rate of thiol/disulfide exchanges catalyzed by protein disulfide oxidoreductases.

A highly sensitive rapid diagnostic test for Chagas disease that utilizes a recombinant Trypanosoma cruzi antigen.

Improved diagnostic tests for Chagas disease are urgently needed. A new lateral flow rapid test for Chagas disease is under development at PATH, in collaboration with Laboratorio Lemos of Argentina, which utilizes a recombinant antigen for detection of antibodies to Trypanosoma cruzi. To evaluate the performance of this test, 375 earlier characterized serum specimens from a region where Chagas is endemic were tested using a reference test (the Ortho T. cruzi ELISA, Johnson & Johnson), a commercially available rapid test (Chagas STAT-PAK, Chembio), and the PATH-Lemos rapid test. Compared to the composite reference tests, the PATH-Lemos rapid test demonstrated an optimal sensitivity of 99.5% and specificity of 96.8%, while the Chagas STAT-PAK demonstrated a sensitivity of 95.3% and specificity of 99.5%. These results indicate that the PATH-Lemos rapid test shows promise as an improved and reliable tool for screening and diagnosis of Chagas disease.

A procedure for the generation and the purification of Escherichia coli thioredoxins with variable N-terminal sequences.

We have developed a rapid and simple procedure for the production and the purification of Escherichia coli thioredoxins containing additional amino acid residues at the N-terminus. By the polymerase chain reaction, the complete gene encoding for E. coli thioredoxin was modified and amplified with the addition at its 5' end of a BamHI cloning site and a triplet coding for an arginine residue instead of the initiator methionine codon, whereas at the 3' end the stop codon was followed by an EcoRI cloning site. The synthetic DNA was ligated into the BamHI/EcoRI site of the vector plasmid pGEX-2T, and the novel plasmid [pFTG] was used for the transformation of E. coli cells. Following induction and cell disruption, a protein composed of Schistosoma japonicum glutathione S-transferase and E. coli thioredoxin was obtained in soluble form and purified by affinity chromatography on agarose columns bearing immobilized glutathione. This procedure yielded 50 mg of homogeneous fusion protein per liter of culture media. Digestion of the chimeric thioredoxin with bovine plasma thrombin followed by an additional chromatography on glutathione-agarose gave a protein that contained the entire sequence of E. coli thioredoxin and three additional amino acid residues [G-S-R-] at the N-terminal side. The structural characteristics and the protein disulfide oxidoreductase activity of this recombinant protein, in terms of variations of emission fluorescence and reduction of insulin disulfide bonds, respectively, were essentially identical to those of its counterpart obtained from wild-type cells by conventional techniques of proteins purification.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of electrostatic interactions on the affinity of thioredoxin for target proteins. Recognition of chloroplast fructose-1, 6-bisphosphatase by mutant Escherichia coli thioredoxins.

Chloroplast thioredoxin-f functions efficiently in the light-dependent activation of chloroplast fructose-1, 6-bisphosphatase by reducing a specific disulfide bond located at the negatively charged domain of the enzyme. Around the nucleophile cysteine of the active site (-W-C-G-P-C-), chloroplast thioredoxin-f shows lower density of negative charges than the inefficient modulator Escherichia coli thioredoxin. To examine the contribution of long range electrostatic interactions to the thiol/disulfide exchange between protein-disulfide oxidoreductases and target proteins, we constructed three variants of E. coli thioredoxin in which an acidic (Glu-30) and a neutral residue (Leu-94) were replaced by lysines. After purification to homogeneity, the reduction of the unique disulfide bond by NADPH via NADP-thioredoxin reductase proceeded at similar rates for all variants. However, the conversion of cysteine residues back to cystine depended on the target protein. Insulin and difluoresceinthiocarbamyl-insulin oxidized the sulfhydryl groups of E30K and E30K/L94K mutants more effectively than those of wild type and L94K counterparts. Moreover, the affinity of E30K, L94K, and E30K/L94K E. coli thioredoxin for chloroplast fructose-1,6-bisphosphatase (A0.5 = 9, 7, and 3 microM, respectively) increased with the number of positive charges, and was higher than wild type thioredoxin (A0.5 = 33 microM), though still lower than that of thioredoxin-f (A0.5 = 0.9 microM). We also demonstrated that shielding of electrostatic interactions with high salt concentrations not only brings the A0.5 for all bacterial variants to a limiting value of approximately 9 microM but also increases the A0.5 of chloroplast thioredoxin-f. While negatively charged chloroplast fructose-1,6-bisphosphatase (pI = 4.9) readily interacted with mutant thioredoxins, the reduction rate of rapeseed napin (pI = 11.2) diminished with the number of novel lysine residues. These findings suggest that the electrostatic interactions between thioredoxin and (some of) its target proteins controls the formation of the binary noncovalent complex needed for the subsequent thiol/disulfide exchange.

Characterization of cysteine residues involved in the reductive activation and the structural stability of rapeseed (Brassica napus) chloroplast fructose-1,6-bisphosphatase.

In higher plants, light enhances the activity of chloroplast fructose-1,6-bisphosphatase via a cascade of thiol/disulfide exchanges. We have examined the structural and functional role of seven conserved cysteine residues in the rapeseed (Brassica napus) enzyme by site-directed mutagenesis. After lysis of Escherichia coli cells, C53S and C191S variants partitioned mainly in the insoluble fraction whereas C96S, C157S, C174S, C179S, and C307S mutants were soluble. Homogeneous preparations of the latter hydrolyzed fructose 1,6-bisphosphate at similar rates in the presence of 10 mM Mg2+ but only C157S, C174S and C179S mutants were both efficient catalysts at 1 mM Mg2+ and nearly insensitive to dithiothreitol. These results demonstrate the contribution of Cys53 and Cys191 to the stability of the enzyme and the participation of Cys157, Cys174 and Cys179 in the reductive process responsive of the light-dependent regulation. Given that mutations at Cys96 and Cys307 neither destabilize the enzyme nor affect the reductive modulation, their function remains unknown.

The reductive modulation of chloroplast fructose-1,6-bisphosphatase by tributylphosphine and sodium borohydride.

The cleavage of disulfide bonds is the major modification of chloroplast fructose-1,6-bisphosphatase when the light-mediated ferredoxin-thioredoxin system enhances the activity of the enzyme. In vitro, only thiol-bearing compounds are functional in the stimulation of fructose 1,6-bisphosphate hydrolysis. This investigation was undertaken to determine the effectivity of other reductants for enhancing the catalytic capacity. In the presence of 1 mM fructose 1,6-bisphosphate and 0.1 mM Ca2+, the five-fold activation triggered by 3.5 mM tributylphosphine is further potentiated by 15% (v/v) 2-propanol. When the enzyme is incubated in the presence of 0.15 M sodium trichloroacetate in place of the cosolvent, NaH4B initially stimulates the activity but subsequently causes the inactivation of the enzyme. A model developed to analyze this dual effect suggests that the concerted action of fructose 1,6-bisphosphate, Ca2+ and trichloroacetate yields an enzyme form that is slightly activable by reduction (t0.5 = 28 min.). However, chloroplast fructose-1,6-bisphosphatase becomes highly sensitive to trichloroacetate inactivation (t0.5 = 5 min.) when NaH4B reduces fructose 1,6-bisphosphate. Hence, the thiol/disulfide exchange constitutes a particular case of reductive mechanisms that stimulate the activity of chloroplast fructose-1,6-bisphosphatase.