Redox Regulation of Rice Endosperm ADP‐glucose Pyrophosphorylase

Abstract

The starch regulatory enzyme ADP‐glucose pyrophosphorylase (AGPase) is subject to allosteric regulation by the activator 3‐phosphoglyceric acid (3‐PGA) and inhibitor inorganic phosphate (Pi). The plastid‐localized enzyme is also subject to fine regulation by the redox state of the cells in response to light and sugar levels. The oxidized and low activity enzyme form contains an interchain disulfide bond formed by Cys12 residues between the pair of small subunits. Although these Cys residues are not conserved in the small subunit of seed (endosperm)‐specific AGPase, we provide direct evidence for redox control of cytosolic AGPase from developing rice endosperm. Biochemical analysis of the purified recombinant heterotetrameric AGPase showed that the enzyme is three times more responsive to 3‐PGA activation in the presence of reducing agent (A0.5 values, 0.8 vs. 2.9 mM). In addition, truncation of the large subunit by removal of N‐terminal peptide, a region which contains two conserved cysteines, resulted in loss of redox response of the enzyme (A0.5 values, 4.4 vs. 4.9 mM with and without DTT). Overall, our results show that the rice endosperm AGPase is controlled by a combination of allosteric regulation and redox control, the latter through modification of the large subunit instead of the small subunit as evident in the plastid‐localized enzyme. (This work was supported by DOE, grant number DE‐FG02‐08ER20216.)