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...