Chloroplasts and mitochondria are unique endosymbiotic cellular organelles surrounded by two membranes. Essential metabolic networking between these compartments and their hosting cells requires the exchange of a large number of biochemical pathway intermediates in a directed and coordinated fashion across their inner and outer envelope membranes. Here, we describe the identification and functional characterization of a highly specific, regulated solute channel in the outer envelope of chloroplasts, named OEP40. Loss of OEP40 function in Arabidopsis thaliana results in early flowering under cold temperature. The reconstituted recombinant OEP40 protein forms a high conductance ␤-barrel ion channel with subconductant states in planar lipid bilayers. The OEP40 channel is slightly cation-selective P K؉ /P Cl؊ ≈ 4:1 and rectifying (ı ᠬ/ı ᠭ Х 2) with a slope conductance of Ḡ max Х 690 picosiemens. The OEP40 channel has a restriction zone diameter of Х1.4 nm and is permeable for glucose, glucose 1-phosphate and glucose 6-phosphate, but not for maltose. Moreover, channel properties are regulated by trehalose 6-phosphate, which cannot permeate. Altogether, our results indicate that OEP40 is a "glucose-gate" in the outer envelope membrane of chloroplasts, facilitating selective metabolite exchange between chloroplasts and the surrounding cell.The plant cell is highly compartmentalized, containing at least seven different types of organelles that are involved in metabolism and cellular maintenance. In many cases, metabolic pathways and other cellular processes are not confined to one organelle but require the cooperation of several of them, e.g. in protein secretion or photorespiration. Metabolic networking between compartments thus requires directed and coordinated exchange of biochemical pathway intermediates across organellar membranes (1). Chloroplasts and mitochondria are unique endosymbiotic cellular organelles, which like their prokaryotic ancestors are surrounded by two membranes. The inner membrane of both organelles is derived from the bacterial plasma membrane. Surprisingly, also the outer membrane largely originated from and still resembles the outer membrane of the Gram-negative-like bacterial endosymbiont (2). In the inner chloroplast envelope membrane (IE), 5 numerous metabolite transporter proteins were identified and characterized to large detail with respect to their physiological impact and molecular mechanisms (3). These transporters are hydrophobic, polytopic, and mainly ␣-helical membrane proteins facilitating the exchange of metabolic precursors, intermediates, and final products between plastids and the cytoplasm. In contrast, the characteristic channels of the outer membranes in bacteria, chloroplasts as well as mitochondria, span the membrane in the form of ␤-strands that are organized to form a barrel-like pore structure (4). Proteins sharing this three-dimensional arrangement were implied in a variety of functions, including protein import (5). In addition, solute pores like the voltage-dependen...

BackgroundMetabolite, ion and protein translocation into chloroplasts occurs across two membranes, the inner and the outer envelope. Solute and metabolite channels fulfill very important functions in integrating the organelles into the metabolic network of the cell. However so far only a few have been identified. Here we describe the identification and the characterization of the outer envelope protein of 23 kDa, Oep23 from garden pea.ResultsOep23 is found in the entire plant lineage from green algae to flowering plants. It is expressed in all organs and developmental states tested so far. The reconstituted recombinant protein Oep23 from pea forms a high conductance ion channel with a maximal conductance in the fully open state of 466 ± 14pS at a holding potential of +100 mV (in 250 mM KCl). The Oep23 channel is cation selective (PK+ : PCl- = 15 : 1) with a voltage dependent open probability of maximal Vmem = 0 mV.ConclusionThe data indicate that the Oep23 activity represents a single channel unit and does not assemble into a multiple pore complex like bacterial type porins or mitochondrial voltage dependent anion channel. Thus, Oep23 represents a new member of ion channels in the outer envelope of chloroplasts involved in solute exchange.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0445-1) contains supplementary material, which is available to authorized users.