This protein plays a role in synthesis of starch. It catalyzes the synthesis of the activated glycosyl donor, ADP-glucose from Glc-1-P and ATP. alpha-D-glucose 1-phosphate + ATP + H(+) = ADP-alpha-D-glucose + diphosphate Activated by 3'phosphoglycerate, inhibited by orthophosphate. Allosteric regulation. Glycan biosynthesis; starch biosynthesis. Heterotetramer. Leaves and seeds. It is present in young cotyledons at 14 days after fertilization (daf) when cells are still rapidly dividing. Levels steadily accumulate until the end of the cell expansion phase (35-40 daf) and with the beginning of the seeds desiccation phase at 50 daf, the levels decrease to very low levels. Belongs to the bacterial/plant glucose-1-phosphate adenylyltransferase family.