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Journal Article

Chloroplast Omp85 proteins change orientation during evolution


Daum,  Bertram
Department of Structural Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Sommer, M. S., Daum, B., Gross, L. E., Weis, B. L. M., Mirus, O., Abram, L., et al. (2011). Chloroplast Omp85 proteins change orientation during evolution. Proceedings of the National Academy of Sciences of the United States of America, 108(33), 13841-13846. doi:10.1073/pnas.1108626108.

Cite as: https://hdl.handle.net/11858/00-001M-0000-0024-D63B-D
The majority of outer membrane proteins (OMPs) from Gram- negative bacteria and many of mitochondria and chloroplasts are β-barrels. Insertion and assembly of these proteins are catalyzed by the Omp85 protein family in a seemingly conserved process. All members of this family exhibit a characteristic N-terminal polypep- tide-transport–associated (POTRA) and a C-terminal 16-stranded β-barrel domain. In plants, two phylogenetically distinct and essential Omp85’s exist in the chloroplast outer membrane, namely Toc75-III and Toc75-V. Whereas Toc75-V, similar to the mitochondrial Sam50, is thought to possess the original bacterial function, its homolog, Toc75-III, evolved to the pore-forming unit of the TOC translocon for preprotein import. In all current models of OMP biogenesis and preprotein translocation, a topology of Omp85 with the POTRA do- main in the periplasm or intermembrane space is assumed. Using self- assembly GFP-based in vivo experiments and in situ topology studies by electron cryotomography, we show that the POTRA domains of both Toc75-III and Toc75-V are exposed to the cytoplasm. This un- expected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.