The allosterically unregulated isoform of ADP-glucose pyrophosphorylase from barley endosperm is the most likely source of ADP-glucose incorporated into endosperm starch

We present the results of studies of an unmodified version of the recombina nt major barley (Hordeum vulgare) endosperm ADP-glucose pyrophoshorylase (A CPase) expressed in insect cells, which corroborate previous data that this isoform of the enzynne acts independently of the allosteric regulators 3-p hosphoglycerate and inorganic phosphate. We also present a characterization of the individual subunits expressed separately in insect cells, showing t hat the SS AGPase is active in the presence of 3-phosphoglycerate and is in hibited by inorganic phosphate. As a step toward the elucidation of the rol e of the two ACPase isoforms in barley, the temporal and spatial expression profile of the four barley ACPase transcripts encoding these isoforms were studied. The results show that the steady-state level of beps and bepl, th e transcripts encoding the major endosperm isoform, correlated positively w ith the rate of endosperm starch accumulation. In contrast, blps and blpl, the transcripts encoding the major leaf isoform, were constitutively expres sed at a very low steady-state level throughout the barley plant. The impli cations of these findings for the evolution of plant AGPases are discussed.