title: Chloroplast Omp85 proteins change orientation during evolution.
abstract: 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 polypeptide-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 domain 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 unexpected finding explains many experimental observations and requires a reevaluation of current models of OMP biogenesis and TOC complex function.
authors.affiliation: Cluster of Excellence Frankfurt, Goethe University, D-60438 Frankfurt, Germany.
authors.forename: Maik S
authors.initials: MS
authors.lastname: Sommer
authors.forename: Bertram
authors.initials: B
authors.lastname: Daum
authors.forename: Lucia E
authors.initials: LE
authors.lastname: Gross
authors.forename: Benjamin L M
authors.initials: BL
authors.lastname: Weis
authors.forename: Oliver
authors.initials: O
authors.lastname: Mirus
authors.forename: Lars
authors.initials: L
authors.lastname: Abram
authors.forename: Uwe-G
authors.initials: UG
authors.lastname: Maier
authors.forename: Werner
authors.initials: W
authors.lastname: Kühlbrandt
authors.forename: Enrico
authors.initials: E
authors.lastname: Schleiff
mesh_terms: D017360:Arabidopsis / Q000737:chemistry / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism*; D002736:Chloroplasts / Q000737:chemistry*; D003593:Cytoplasm; D019143:Evolution, Molecular; D007425:Intracellular Membranes / Q000737:chemistry; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism*; D011498:Protein Precursors / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D021381:Protein Transport
chemical_list: D029681:Arabidopsis Proteins; D008565:Membrane Proteins; C532780:OEP80 protein, Arabidopsis; D011498:Protein Precursors; C491244:TOC75 protein, Arabidopsis; C568082:Toc75-III protein, Arabidopsis