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J. Biol. Chem., Vol. 281, Issue 26, 17989-17998, June 30, 2006

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OEP37 Is a New Member of the Chloroplast Outer Membrane Ion Channels^*

Tom Alexander Goetze, Katrin Philippar, Irina Ilkavets, Jürgen Soll, and Richard Wagner¹

From the Biophysik, Universität Osnabrück, FB Biologie/Chemie, Barbarastrasse 13, D-49076 Osnabrück, and Botanik, Universität München, Department Biologie I, Menzinger Strasse 67, 80638 München, Germany

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_K^– = 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.

Received for publication, January 24, 2006 , and in revised form, April 7, 2006.

^* This work was supported by Deutsche Forschungsgemeinschaft Grant SPP1108 (to R. W. and J. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 49-541-969-2851; Fax: 49-541-969-2243; E-mail: wagner{at}uos.de.

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