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 ( ⌳ ‫؍‬ 1.3 nS in 1 M KCl), and slightly cation-selective ion channel in reconstituted proteoliposomes. The highest open probability ( P open Ϸ 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 ␤ 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. INTRODUCTIONCarbon dioxide reduction and its assimilation into carbohydrates, amino acids, fatty acids, and terpenoid compounds (Douce and Joyard, 1990) take place in chloroplasts. Annually, ‫ف‬ 120 ϫ 10 9 tons of CO 2 is converted into organic substances by chloroplasts of higher land plants. In addition, chloroplasts are the sole site of nitrite and sulfate reduction (Anderson, 1990;Crawford, 1995) and their assimilation in organic compounds. Chloroplasts are surrounded by a pair of double membranes, the inner and the outer envelopes, which delimit spatially and temporally the chloroplastic compartment from the cytoplasm. Both outer and inner envelope membranes differ in structure, function, and biochemical properties but also cooperate, for example, in the synthesis of lipids or in protein translocation. The various biosynthetic functions of the chloroplasts require the existence of different and effective transport mechanisms of solutes across the envelope membranes to provide the cell with carbohydrates, organic nitrogen, and sulfur compounds. On the other hand, chloroplasts take up inorganic cations (K ϩ , Na ϩ , Mg 2 ϩ , and Ca 2 ϩ ), anions (nitrite, sulfate, and phosphate), and a variety of organic biosynthetic pathway intermediates, such as phosphoenolpyruvate, dicarboxylic acids, acetate, amino acids, and ATP to fulfill their biosynthetic tasks.The outer envelope membrane is assumed to be freely permeable for most small molecular weight solutes (Flügge and Benz, 1984), like the outer membranes of mitochondria and Gram-negative bacteria (reviewed in Benz, 1994;Nikaido, 1994). The permeability of the outer membranes of mitochondria and Gram-negative bacteria is caused by poreforming proteins, designated porins, which share structural and functional features. Similar to these systems, the osmotic barrier against the cytosol is formed by the inner envelope membrane, containing specific carrier proteins of which some have been iden...

Chloroplast envelope membranes: a dynamic interface between plastids and the cytosol

“…This is probably not as simple since substrate-specific gated pore-forming proteins were characterized in the outer envelope membrane (Pohlmeyer et al 1997(Pohlmeyer et al , 1998Bölter et al 1999;Goetze et al 2006;Hemmler et al 2006). Altogether, the combined proteomic and in silico approaches suggest that a series of known or putative transport systems are likely to be localized in the chloroplast envelope (Seigneurin-Berny et al 1999;Ferro et al 2002;Weber et al 2005).…”

Chloroplast membrane transport: Interplay of prokaryotic and eukaryotic traits

“…In addition, OEP24 appears to be related to the major mitochondrial solute channel, the voltage-gated anionselective channel protein VDAC (Pohlmeyer et al, 1998). OEP24 displays similar specificity characteristics and it can complement a yeast VDAC mutant strain (Rö hl et al, 1999).…”

“…OEP16 is a cation-selective channel with a selectivity for amino acids and amines (Pohlmeyer et al, 1997), whereas OEP21 is an intrinsic-rectifying anion-selective channel specific for inorganic phosphates, triosephosphates, and 3-phosphoglycerate (Bö lter et al, 1999). OEP24 also allows the passage of triosephosphates, ATP, and inorganic phosphates as well as both cation and anions (Pohlmeyer et al, 1998). Very little is known so far about the characteristics of OEP7 or OEP37.…”

Intracellular localization of VDAC proteins in plants

“…Numerous substrate-specific transporters in the plastid inner envelopes again reflect this metabolic interdependence (Flu¨gge 1998). Recently, evidence has accumulated that the chloroplast outer envelope contains different solute-selective ion channels (Pohlmeyer et al 1998a(Pohlmeyer et al , 1998bBo¨lter et al 1999;Bo¨lter and Soll 2001). These data indicated that the outer envelope functions as a selective molecular filter rather than an unspecific sieve that runs by size-exclusion only.…”