Heterotetramer. Expressed at very low levels in leaves, inflorescences, fruits, and roots. In leaves, mainly observed in starch-producing tissues including the mesophyll and the vascular companions cells. In flowers, detected in the stamens and pistil, as well as in the receptacle. Also expressed in the embryo. Belongs to the bacterial/plant glucose-1-phosphate adenylyltransferase family. No detectable activity.