OEP21 of 177 aas. Voltage-dependent rectifying anion channel that facilitates the translocation between chloroplast and cytoplasm of phosphorylated carbohydrates such as triosephosphate, 3-phosphoglycerate and inorganic phosphate (Pi), depending on the ATP to triosephosphate ratio in the plastidial intermembrane space.  In high triosephosphate/ATP conditions (e.g. photosynthesis), export of triosphophate from the chloroplast occurs (outward rectifying channels), but in high ATP/triosephosphate conditions (e.g. dark phase), import of phosphosolutes (inward rectifying channels) occurs (Bölter et al. 1999). The channel is formed by eight beta-strands with a wider pore vestibule of dvest approximately 2.4 nm at the intermembrane site and a narrower filter pore of drestr approximately 1 nm. The Oep21 pore contains two high affinity sites for ATP, one located at a relative transmembrane electrical distance delta = 0.56 and the second close to the vestibule at the intermembrane site. The ATP-dependent current block and reduction in anion selectivity of the Oep21 channel is relieved by the competitive binding of phosphorylated metabolic intermediates like 3-phosphoglycerate and glycerinaldehyde 3-phosphate (Hemmler et al. 2006).