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Abstract
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.
Highlights
Metabolite, ion and protein translocation into chloroplasts occurs across two membranes, the inner and the outer envelope
In recent years we have described a number of outer envelope proteins (Oep) sharing several characteristics, which can be used to select them from a pool of unknown proteins for further studies
Identification of Oep23 The peptide sequence obtained in our own proteomic analysis from purified outer envelopes of pea chloroplasts matched to a number of EST pea contigs [25], which in combination with the identified sequence of the Arabidopsis homolog (At2g17695) could be used for the isolation and sequencing of a complete cDNA
Summary
Metabolite, ion and protein translocation into chloroplasts occurs across two membranes, the inner and the outer envelope. We describe the identification and the characterization of the outer envelope protein of 23 kDa, Oep from garden pea. The endosymbiotic α-proteobacterium was converted into an organelle, which we know today as mitochondrion. This was followed by a second endosymbiotic event: the cell merging with an ancestral photosynthetic cyanobacterium, which gave rise to chloroplasts.
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