Abstract
Rice has five plant-type PEPCs, four of which (Osppc1, 2a, 2b, and 3) are cytosolic, while one (Osppc4) is targeted to the plastid. To clarify physiological conditions where they function, we investigated their kinetic properties using recombinant proteins. We found that Vmax of Osppc1, a major PEPC in roots, was much higher than previously reported values for plant-type PEPCs, although its activity was severely inhibited by glutamate and aspartate. Since the root is a major site of ammonium assimilation in rice, it is likely that the high Vmax value of Osppc1 could be to meet a great demand for organic acids in the roots. By contrast, Osppc4, a major PEPC in the leaves, showed low sensitivities to glutamate and aspartate. Like C4-type PEPCs, Vmax of Osppc4 was not substantially affected by glucose 6-phosphate (G6P), a general activator for PEPC, while its Km value for PEP was largely lowered by G6P. Further studies for other plant-type PEPCs are in progress and the data shall be discussed.
