The chloroplast outer envelope protein OEP37 is a member of the growing ␤-barrel protein family of the outer chloroplast membrane. The reconstituted recombinant protein OEP37 from pea forms a rectifying high conductance channel with a main conductance () of ⌳ ‫؍‬ 500 picosiemens (symmetrical 250 mM KCl). The OEP37 channel is cation-selective (P K ؉/P Cl ؊ ‫؍‬ 14:1) with a voltage-dependent open probability maximal at V mem ‫؍‬ 0 mV. The channel pore reveals an hourglass-shaped form with different diameters for the vestibule and restriction zone. The diameters of the vestibule at the high conductance side were estimated by d ‫؍‬ 3.0 nm and the restriction zone by d ‫؍‬ 1.5 nm. The OEP37 channel displayed a nanomolar affinity for the precursor of the chloroplast inner membrane protein Tic32, which is imported into the chloroplast through a yet unknown pathway. Pre-proteins imported through the usual Toc pathway and synthetic control peptides, however, did not show a comparable block of the OEP37 channel. In addition to the electrophysiological characterization, we studied the gene expression of OEP37 in the model plant Arabidopsis thaliana. Here, transcripts of AtOEP37 are ubiquitously expressed throughout plant development and accumulate in early germinating seedlings as well as in late embryogenesis. The plastid intrinsic protein could be detected in isolated chloroplasts of cotyledons and rosette leaves. However, the knock-out mutant oep37-1 shows that the proper function of this single copy gene is not essential for development of the mature plant. Moreover, import of Tic32 into chloroplasts of oep37-1 was not impaired when compared with wild type. Thus, OEP37 may constitute a novel peptide-sensitive ion channel in the outer envelope of plastids with function during embryogenesis and germination.The plastid organelle family conducts vital biosynthetic functions in every plant cell. Chloroplasts carry out photosynthesis, which converts atmospheric carbon dioxide to carbohydrates. Furthermore, plastids are the site for fatty acid, amino acid, and porphyrin biosynthesis as well as sulfate and nitrate reduction. Thus, all biosynthetic pathway products are steadily exchanged with the surrounding cell by the assistance of specific transport proteins, localized in the plastid envelope membranes. Although the inner envelope carriers, e.g. the triose phosphatephosphate translocator, the dicarboxylic acid translocator, or the hexose phosphate carrier, show a distinct substrate specificity, until now it remained elusive to what extent transport through outer membrane channels was regulated (for review of plastid envelope transporters compare with Ref. 1).The outer envelope membrane has long been assumed to be freely permeable for small molecular mass solutes of up to 10 kDa (2). Correspondingly, it is believed that the osmotic barrier against the cytosol is formed exclusively by the inner envelope. In Gram-negative bacteria, however, several different types of high conductance channels exist in the outer membrane (3) as ...