ATP:ADP antiporter that catalyzes the exchange of ATP and ADP across the endoplasmic reticulum (ER) membrane. Imports ATP from the cytosol to the ER lumen and exports ADP in the opposite direction (By similarity). Regulates ER energy metabolism and protein biogenesis. Appears to be part of a calcium-dependent ER to cytosol low energy response axis, where calcium efflux from ER to the cytosol triggers ATP import into the ER lumen to maintain sufficient ATP supply. Provides ATP to ER chaperone HSPA5 that drives protein folding and trafficking in the ER (By similarity). Can transport UTP or UDP in exchange for ATP, but the physiological relevance of this process remains to be established (By similarity). ADP(in) + ATP(out) = ADP(out) + ATP(in) The di-lysine motif confers endoplasmic reticulum localization for type I membrane proteins. Belongs to the nucleotide-sugar transporter family. SLC35B subfamily.