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).



This family belongs to the .

 

References:

Bölter, B. and J. Soll. (2001). Ion channels in the outer membranes of chloroplasts and mitochondria: open doors or regulated gates? EMBO J. 20: 1-6.

Bölter, B., J. Soll, K. Hill, R. Hemmler, and R. Wagner. (1999). A rectifying ATP-regulated solute channel in the chloroplastic outer envelope from pea. EMBO. J. 18: 5505-5516.

Hemmler, R., T. Becker, E. Schleiff, B. Bölter, T. Stahl, J. Soll, T.A. Götze, S. Braams, and R. Wagner. (2006). Molecular properties of Oep21, an ATP-regulated anion-selective solute channel from the outer chloroplast membrane. J. Biol. Chem. 281: 12020-12029.

Examples:

TC#NameOrganismal TypeExample
1.B.29.1.1

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).

plants

OEP21 of Pisum sativum

 
1.B.29.1.2

OEP21 homologue

Plants

OEP21 homologue of Coccomyxa subellipsoidea

 
1.B.29.1.3

OEP21 homologue

Green algae

OEP21 homologue of Chlorella variabilis

 
1.B.29.1.4

Oep21 of 167 aas is a voltage-dependent rectifying anion channel that facilitates the translocation between the 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 chloroplasts occurs (outward rectifying channels), but in high ATP/triosephosphate conditions (e.g., the dark phase), import of phosphosolutes occurs (inward rectifying channels).

Oep21 of Arabidopsis thaliana (Mouse-ear cress)

 
Examples:

TC#NameOrganismal TypeExample
1.B.29.2.1

OEP21 homologue

Chlorophyta

OEP21 homologue of Chlamydomonas reinhardtii

 
1.B.29.2.2

OEP21 homologu of 169 aas

Green algae

OEP21 homologue of Volvox carteri