A HIGH-CONDUCTANCE SOLUTE CHANNEL IN THE CHLOROPLASTIC OUTER ENVELOPEFROM PEA

The pea chloroplastic outer envelope protein OEP24 can function as a g eneral solute channel. OEP24 is present in chloroplasts, etioplasts, a nd non-green root plastids. The heterologously expressed protein forms a voltage-dependent, high-conductance (Lambda = 1.3 nS in 1 M KCl), a nd slightly cation-selective ion channel in reconstituted proteoliposo mes. The highest open probability (P-open approximate to 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 algorit hms and circular dichroism spectra indicate that OEP24 contains seven amphiphilic beta strands. The primary structure of OEP24 shows no homo logies to mitochondrial or bacterial porins on a primary sequence basi s, and OEP24 is functionally not inhibited by cadaverine, which is a p otent inhibitor of bacterial porins. We conclude that OEP24 represents a new type of solute channel in the plastidic outer envelope.