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Published on July 11, 2008; 10.1104/pp.108.122754

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Received May 12, 2008
Accepted July 7, 2008

The Omp85-Related Chloroplast Outer Envelope Protein, OEP80, is Essential for Viability in Arabidopsis

Ramesh Patel , Shih-chi Hsu , Jocelyn Bedard , Kentaro Inoue , and Paul Jarvis *

Department of Biology, University of Leicester, Leicester LE1 7RH, United Kingdom; and Department of Plant Sciences, University of California, Davis, CA 95616

* Corresponding author; email: rpj3{at}le.ac.uk.

Beta-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 {beta}-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 amongst >100 individuals, and heterozygotes of both mutants produced ~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.






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