Involved in synthesis of starch. Catalyzes the synthesis of ADP-glucose, a molecule that serves as an activated glycosyl donor for alpha-1,4-glucan synthesis. Essential for starch synthesis in seed endosperm (PubMed:17406793, Ref.13, PubMed:24747952, PubMed:27957808). Is essential for both catalytic and allosteric regulatory properties of the cytosolic heterotetramer enzyme (PubMed:24747952). alpha-D-glucose 1-phosphate + ATP + H(+) = ADP-alpha-D-glucose + diphosphate Activated by 3'phosphoglycerate, inhibited by orthophosphate. Allosteric regulation. Inhibited by inorganic phosphate (Pi). Glycan biosynthesis; starch biosynthesis. Heterotetramer composed of two small and two large subunits. Expressed in developing seeds from 10 to 20 days after flowering (DAF). Shrunken seed endosperm due to a strong reduction in starch synthesis. Belongs to the bacterial/plant glucose-1-phosphate adenylyltransferase family. Truncated N-terminus.