Probable adenylate translocator that mediates transport of ADP-glucose into endosperm storage plastids during starch synthesis. Transports cytosolic ADP-glucose to amyloplast stroma by counter-exchange with ADP. Inhibited by mersalyl. Dually targeted to mitochondria and plastids. The N-terminal extension acts as plastidic transit peptide. Dual localization of BT1 does not seem to be due to alternative transcription start sites, translation initiation sites or alternative exon splicing (PubMed:21330298). Highly expressed in silks and endosperm of developing kernels. Expressed at intermediate levels in tassels and lower levels in stems and leaves. Severely reduced starch content of endosperm resulting in kernels with a collapsed angular appearance at maturity. Belongs to the mitochondrial carrier (TC 2.A.29) family.