Improving the glycosyltransferase activity of Agrobacterium tumefaciens glycogen synthase by fusion of N-terminal starch binding domains (SBDs).

Glycogen and starch, the major storage carbohydrate in most living organisms, result mainly from the action of starch or glycogen synthases (SS or GS, respectively, EC 2.4.1.21). SSIII from Arabidopsis thaliana is an SS isoform with a particular modular organization: the C-terminal highly conserved glycosyltransferase domain is preceded by a unique specific region (SSIII-SD) which contains three in tandem starch binding domains (SBDs, named D1, D2 and D3) characteristic of polysaccharide degrading enzymes. N-terminal SBDs have a probed regulatory role in SSIII activity, showing starch binding ability and modulating the catalytic properties of the enzyme. On the other hand, GS from Agrobacterium tumefaciens has a simple primary structure organization, characterized only by the highly conserved glycosyltransferase domain and lacking SBDs. To further investigate the functional role of A. thaliana SSIII-SD, three chimeric proteins were constructed combining the SBDs from A. thaliana with the GS from A. tumefaciens. Recombinant proteins were expressed in and purified to homogeneity from Escherichia coli cells in order to be kinetically characterized. Furthermore, we tested the ability to restore in vivo glycogen biosynthesis in transformed E. coli glgA(-) cells, deficient in GS. Results show that the D3-GS chimeric enzyme showed increased capacity of glycogen synthesis in vivo with minor changes in its kinetics parameters compared to GS.

An enzyme-coupled continuous spectrophotometric assay for glycogen synthases.

The metabolic pathways leading to the synthesis of bacterial glycogen involve the action of several enzymes, among which glycogen synthase (GS) catalyzes the elongation of the α-1,4-glucan. GS from Agrobacterium tumefaciens uses preferentially ADPGlc, although UDPGlc can also be used as glycosyl donor with less efficiency. We present here a continuous spectrophotometric assay for the determination of GS activity using ADP- or UDPGlc. When ADPGlc was used as the substrate, the production of ADP is coupled to NADH oxidation via pyruvate kinase (PK) and lactate dehydrogenase (LDH). With UDPGlc as substrate, UDP was converted to ADP via adenylate kinase and subsequent coupling to PK and LDH reactions. Using this assay, we determined the kinetic parameters of GS and compared them with those obtained with the classical radiochemical method. For this purpose, we improved the expression procedure of A. tumefaciens GS using Escherichia coli BL21(DE3)-RIL cells. This assay allows the continuous monitoring of glycosyltransferase activity using ADPGlc or UDPGlc as sugar-nucleotide donors.

Heat shock protein 70 induction and its urinary excretion in a model of acetaminophen nephrotoxicity.

Acetaminophen (APAP) is an analgesic-antipyretic drug widely used in children. In the present study, we used an in vivo model of APAP-induced nephrotoxicity in male Wistar rats. We analyzed whether toxic doses of APAP could induce heat shock protein 70 (HSP70) in the kidney and whether HSP70 could be detected in urine. Renal function and histological evaluation of the kidneys were performed at different times after APAP administration (1,000 mg/kg body weight i.p.). Cellular injury was assessed by Triton X-100 solubilization of Na(+)/K(+) ATPase. Renal and hepatic glutathione levels were also measured. Urinary N-acetyl-beta-D glucosaminidase (NAG) excretion increased 4 h after intoxication. At this time, urea and creatinine were at control levels and a slight degree of histological alteration was detected. Kidney microscopic evaluation, Na(+)/K(+) ATPase solubility, creatinine, and urea levels and NAG excretion did not differ from those of controls 48 h after APAP administration. HSP70 was detected in urine obtained from 4 to 24 h after APAP administration. HSP70 abundance in renal cortex was increased at early time points and 48 h after APAP administration. Urinary HSP70 excretion would be a marker of its renal induction combined with the loss of tubule integrity. NAG would be a suitable early biomarker of APAP-induced nephrotoxicity.

The starch-binding capacity of the noncatalytic SBD2 region and the interaction between the N- and C-terminal domains are involved in the modulation of the activity of starch synthase III from Arabidopsis thaliana.

Starch synthase III from Arabidopsis thaliana contains an N-terminal region, including three in-tandem starch-binding domains, followed by a C-terminal catalytic domain. We have reported previously that starch-binding domains may be involved in the regulation of starch synthase III function. In this work, we analyzed the existence of protein interactions between both domains using pull-down assays, far western blotting and co-expression of the full and truncated starch-binding domains with the catalytic domain. Pull-down assays and co-purification analysis showed that the D(316-344) and D(495-535) regions in the D2 and D3 domains, respectively, but not the individual starch-binding domains, are involved in the interaction with the catalytic domain. We also determined that the residues W366 and Y394 in the D2 domain are important in starch binding. Moreover, the co-purified catalytic domain plus site-directed mutants of the D123 protein lacking these aromatic residues showed that W366 was key to the apparent affinity for the polysaccharide substrate of starch synthase III, whereas either of these amino acid residues altered ADP-glucose kinetics. In addition, the analysis of full-length and truncated proteins showed an almost complete restoration of the apparent affinity for the substrates and V(max) of starch synthase III. The results presented here suggest that the interaction of the N-terminal starch-binding domains, particularly the D(316-344) and D(495-535) regions, with the catalytic domains, as well as the full integrity of the starch-binding capacity of the D2 domain, are involved in the modulation of starch synthase III activity.

Role of the N-terminal starch-binding domains in the kinetic properties of starch synthase III from Arabidopsis thaliana.

Starch synthase III (SSIII), one of the SS isoforms involved in plant starch synthesis, has been reported to play a regulatory role in the synthesis of transient starch. SSIII from Arabidopsis thaliana contains 1025 amino acid residues and has an N-terminal transit peptide for chloroplast localization which is followed by three repeated starch-binding domains (SBDs; SSIII residues 22-591) and a C-terminal catalytic domain (residues 592-1025) similar to bacterial glycogen synthase. In this work, we constructed recombinant full-length and truncated isoforms of SSIII, lacking one, two, or three SBDs, and recombinant proteins, containing three, two, or one SBD, to investigate the role of these domains in enzyme activity. Results revealed that SSIII uses preferentially ADPGlc, although UDPGlc can also be used as a sugar donor substrate. When ADPGlc was used, the presence of the SBDs confers particular properties to each isoform, increasing the apparent affinity and the V max for the oligosaccharide acceptor substrate. However, no substantial changes in the kinetic parameters for glycogen were observed when UDPGlc was the donor substrate. Under glycogen saturating conditions, the presence of SBDs increases progressively the apparent affinity and V max for ADPGlc but not for UDPGlc. Adsorption assays showed that the N-terminal region of SSIII, containing three, two, or one SBD module have increased capacity to bind starch depending on the number of SBD modules, with the D23 protein (containing the second and third SBD module) being the one that makes the greatest contribution to binding. The results presented here suggest that the N-terminal SBDs have a regulatory role, showing a starch binding capacity and modulating the catalytic properties of SSIII.

Functional and structural characterization of the catalytic domain of the starch synthase III from Arabidopsis thaliana.

Glycogen and starch are the major energy storage compounds in most living organisms. The metabolic pathways leading to their synthesis involve the action of several enzymes, among which glycogen synthase (GS) or starch synthase (SS) catalyze the elongation of the alpha-1,4-glucan backbone. At least five SS isoforms were described in Arabidopsis thaliana; it has been reported that the isoform III (SSIII) has a regulatory function on the synthesis of transient plant starch. The catalytic C-terminal domain of A. thaliana SSIII (SSIII-CD) was cloned and expressed. SSIII-CD fully complements the production of glycogen by an Agrobacterium tumefaciens glycogen synthase null mutant, suggesting that this truncated isoform restores in vivo the novo synthesis of bacterial glycogen. In vitro studies revealed that recombinant SSIII-CD uses with more efficiency rabbit muscle glycogen than amylopectin as primer and display a high apparent affinity for ADP-Glc. Fold class assignment methods followed by homology modeling predict a high global similarity to A. tumefaciens GS showing a fully conservation of the ADP-binding residues. On the other hand, this comparison revealed important divergences of the polysaccharide binding domain between AtGS and SSIII-CD.