Inorganic phosphate and glucose-6-phosphate antiporter. May transport cytoplasmic glucose-6-phosphate into the lumen of the endoplasmic reticulum and translocate inorganic phosphate into the opposite direction. Independent of a lumenal glucose-6-phosphatase. May not play a role in homeostatic regulation of blood glucose levels. Inhibited by vanadate but not by chlorogenic acid. Expressed in numerous tissues, with highest expression in pancreas, kidney, bone marrow, spleen, liver, small intestine, as well as in fetal brain, liver and spleen. Belongs to the major facilitator superfamily. Organophosphate:Pi antiporter (OPA) (TC 2.A.1.4) family.