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CELL BIOLOGY AND SIGNAL TRANSDUCTION

The Omp85-Related Chloroplast Outer Envelope Protein OEP80 Is Essential for Viability in Arabidopsis^1,[W],[OA]

Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom (R.P., J.B., P.J.); and Department of Plant Sciences, University of California, Davis, California 95616 (S.-C.H., K.I.)

β-Barrel proteins of the Omp85 (Outer membrane protein, 85 kD) superfamily exist in the outer membranes of Gram-negative bacteria, mitochondria, and chloroplasts. Prominent Omp85 proteins in bacteria and mitochondria mediate biogenesis of other β-barrel proteins and are indispensable for viability. In Arabidopsis (Arabidopsis thaliana) chloroplasts, there are two distinct types of Omp85-related protein: Toc75 (Translocon at the outer envelope membrane of chloroplasts, 75 kD) and OEP80 (Outer Envelope Protein, 80 kD). Toc75 functions as a preprotein translocation channel during chloroplast import, but the role of OEP80 remains elusive. We characterized three T-DNA mutants of the Arabidopsis OEP80 (AtOEP80) gene. Selectable markers associated with the oep80-1 and oep80-2 insertions segregated abnormally, suggesting embryo lethality of the homozygous genotypes. Indeed, no homozygotes were identified among >100 individuals, and heterozygotes of both mutants produced approximately 25% aborted seeds upon self-pollination. Embryo arrest occurred at a relatively late stage (globular embryo proper) as revealed by analysis using Nomarski optics microscopy. This is substantially later than arrest caused by loss of the principal Toc75 isoform, atToc75-III (two-cell stage), suggesting a more specialized role for AtOEP80. Surprisingly, the oep80-3 T-DNA (located in exon 1 between the first and second ATG codons of the open reading frame) did not cause any detectable developmental defects or affect the size of the AtOEP80 protein in chloroplasts. This indicates that the N-terminal region of AtOEP80 is not essential for the targeting, biogenesis, or functionality of the protein, in contrast with atToc75-III, which requires a bipartite targeting sequence.

The authors responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) are: Kentaro Inoue (kinoue{at}ucdavis.edu) and Paul Jarvis (rpj3{at}le.ac.uk).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.122754

^* Corresponding author; e-mail rpj3{at}le.ac.uk.

Received May 12, 2008; accepted July 7, 2008; published July 11, 2008.

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Plant Physiol. 2008 148: 1-2. [Full Text]