1.B.29 The Plastid Outer Envelope Porin of 21 kDa (OEP21) Family
OEP21 is a 177 aa, chloroplastic, 8-stranded, β-barrel type, homooligomeric, voltage-dependent outer envelope protein that selectively allows exchange of triosephosphates, 3-phosphoglycerate and inorganic phosphate between the plant cell cytoplasm and the chloroplast. The anion selectivity and asymmetric transport properties of OEP21 are modulated by the ratio of ATP to organic and inorganic phosphates in the intermembrane space. ATP binding causes a switch from inwardly to outwardly rectifying, and the channel becomes slightly cation-selective. Conditions that lead to export of triose-P from chloroplasts (i.e., under conditions of photosynthesis), give rise to outward rectifying OEP21 channels, while a larger ATP/triose-P ratio under conditions of dark metabolism, leads to inward rectifying OEP21 channels. Therefore, OEP21 is an ATP-regulated solute-selective porin.
Oep21 is a channel formed by eight β-strands with a wide pore vestibule of approximately 2.4 nm at the intermembrane site and a narrower filter pore of approximately 1 nm (Hemmler et al., 2006). The Oep21 pore contains two high affinity sites for ATP, one located at a relative transmembrane electrical distance of 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 glyceraldehyde 3-phosphate. Deletion of a C-terminal putative FX4K binding motif in Oep21 decreased the capability of the channel to tune its ion selectivity by about 50%, whereas current block remained unchanged (Hemmler et al., 2006).
OEP21 has at least two homologues in Arabidopsis thaliana. One of them (AAG40412) is 167 aas long and is 54% identical to OEP21, while the other (AAF80623) is 203 aas long with a C-terminal extension relative to OEP21 and exhhibits 46% identity with OEP21.
The generalized transport reaction catalyzed by OEP21 is:
Solutes (anion selective; cytoplasm) solutes (chloroplast).