Chloroplastic glutamine synthetase is activated by direct binding of aluminium

Abstract

Acidification of soils may release water soluble, toxic aluminium species from clay minerals. Al interferes with a wide range of physical and cellular processes. Glutamine synthetase (GS, EC 6.3.1.2) is the key enzyme of primary N assimilation, as well as ammonia reassimilation and detoxification. Plant GS requires two magnesium ions per subunit for activity, which makes GS a potential target of metal stress. The objective of this investigation was to prove that Al from an organic metal complex is able to activate GS, and Al becomes bound to the polypeptide structure of the GS molecule. Aluminium(III)-nitrilotriacetic acid complex (Al(III)NTA) activated the GS prepared from wheat (Triticum aestivum L.) leaves, as Al(3+) did in vivo, but could not functionally substitute magnesium ions, which were also necessary for the activity in the in vitro GS assay. GS2 was isolated by non-denaturing polyacrylamide gel electrophoresis, and the Al and Mg content of the enzyme was determined by inductively coupled plasma atomic emission spectroscopy. The GS octamer remained intact and contained Mg(2+) bound to its specific sites after the electrophoretic separation. Al was detected in the Al(III)NTA-treated sample bound to the structure of the enzyme protein, potentially occupying one of the specific metal-binding sites of the subunits. Our results indicate that the activatory effect of the Al(III)NTA complex is because of specific binding of aluminium to the polypeptide chain of GS2, however presence of magnesium at least on one of the metal-binding sites is essential to the active state of the enzyme.

Can Adverse Effects of Acidity and Aluminum Toxicity Be Alleviated by Appropriate Rootstock Selection in Cucumber?

“…In addition to the reduction in K and Ca uptake, Al can also decrease Mg uptake in aerial parts (Rengel and Robinson, 1989), as we can confirm with our current data. The competition between these two cations (Al and Mg) was demonstrated for membrane transporters and metal binding sites in enzymatic reactions (Rengel and Robinson, 1989; Pécsváradi et al, 2009). …”

Reduced light and moderate water deficiency sustain nitrogen assimilation and sucrose degradation at low temperature in durum wheat

“…The activity of the GS enzyme was determined in vitro with a modified version of the 'synthetase reaction', by measuring the formation of γ-glutamyl monohydroxamate (Pécsváradi et al, 2009). Five hundred mg of leaf samples were finely ground in a mortar in 2 mL of extraction buffer [0.2 M TRIS-HCl (pH 7.5) containing 9.6% glycerol, 2 mM EDTA, 3 mM DTT, 1 mM GSH], after which the extracts were centrifuged at 10 000 g for 10 min.…”

Coadaptation of Plants to Multiple Stresses in Acidic Soils

“…In fact, Al toxicity can even increase, rather than decrease, with increasing pH (Kinraide et al, 1992), which is one possible reason why NH 4 + can decrease Al toxicity. Aluminum activated the chloroplastic glutamine synthetase in wheat (Pécsváradi et al, 2009) and enhanced the expression of the gene encoding glutamine synthetase in rice . Thus, it is necessary to reevaluate the effects of NH 4 + on wheat Al tolerance using low NH 4 + concentrations.…”