A High-Conductance Solute Channel in the Chloroplastic Outer Envelope from Pea

Abstract

The pea chloroplastic outer envelope protein OEP24 can function as a general solute channel. OEP24 is present in chloroplasts, etioplasts, and non-green root plastids. The heterologously expressed protein forms a voltage-dependent, high-conductance (Lambda = 1.3 nS in 1 M KCl), and slightly cation-selective ion channel in reconstituted proteoliposomes. The highest open probability (P open approximately 0. 8) is at 0 mV, which is consistent with the absence of a transmembrane potential across the chloroplastic outer envelope. The OEP24 channels allow the flux of triosephosphate, dicarboxylic acids, positively or negatively charged amino acids, sugars, ATP, and Pi. Structure prediction algorithms and circular dichroism spectra indicate that OEP24 contains seven amphiphilic beta strands. The primary structure of OEP24 shows no homologies to mitochondrial or bacterial porins on a primary sequence basis, and OEP24 is functionally not inhibited by cadaverine, which is a potent inhibitor of bacterial porins. We conclude that OEP24 represents a new type of solute channel in the plastidic outer envelope.

A Transit Peptide–Like Sorting Signal at the C Terminus Directs the Bienertia sinuspersici Preprotein Receptor Toc159 to the Chloroplast Outer Membrane

“…Although both Toc75 and its paralog, outer envelope protein 80 kD (OEP80), are targeted with the assistance of some proteinaceous components on the chloroplast surface, the targeting of OEP80 is independent of the general import pathway, in agreement with the absence of an N-terminal cleavable cTP (Inoue and Potter, 2004;Huang et al, 2011). In fact, all other identified b-barrel proteins on the chloroplast outer membrane, including OEP21 (Bölter et al, 1999), OEP24 (Pohlmeyer et al, 1998), and OEP37 (Goetze et al, 2006), do not have a predicted cTP and are spontaneously integrated into the membrane. In addition to b-barrel proteins, chloroplasts have evolved a-helical integral proteins in the outer membrane, in spite of the endosymbiotic origin from a bacterial progenitor.…”

Protein import machineries in endosymbiotic organelles

“…In the case of ß-barrel proteins, represented by the solute transporter family (e. g., OEP21, OEP24), it is even less clear how they are integrated into the membrane. Experimental findings so far indicate that insertion is a spontaneous process, independent of a translocation machinery and energy [114,115]. A notable exception in the family of ß-barrel proteins is Toc75.…”

The Predicted Candidates of Arabidopsis Plastid Inner Envelope Membrane Proteins and Their Expression Profiles,

“…However, the protein composition of outer membranes is much simpler than that of inner membrane. The lipid to protein ratio of outer membrane is about 3 times higher than that of inner membrane and the inner envelope membrane, which contains various metabolite translocators, is the main permeability barrier for solutes, although there is growing evidence suggesting that the outer envelope may contain regulated ion channels (Cline et al, 1981;Block et al, 1983;Douce and Joyard, 1990;Pohlmeyer et al, 1998;Flugge, 2000;Bolter and Soll, 2001). Thus, we consider it likely that AtPEM candidates represent the majority of integral plastid envelope proteins.…”