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Evolutionary conservation of biogenesis of β-barrel membrane proteins

Abstract

The outer membranes of mitochondria and chloroplasts are distinguished by the presence of β-barrel membrane proteins1,2. The outer membrane of Gram-negative bacteria also harbours β-barrel proteins3. In mitochondria these proteins fulfil a variety of functions such as transport of small molecules (porin/VDAC), translocation of proteins (Tom40) and regulation of mitochondrial morphology (Mdm10)4,5,6,7. These proteins are encoded by the nucleus, synthesized in the cytosol, targeted to mitochondria as chaperone-bound species, recognized by the translocase of the outer membrane, and then inserted into the outer membrane where they assemble into functional oligomers8,9,10,11. Whereas some knowledge has been accumulated on the pathways of insertion of proteins that span cellular membranes with α-helical segments, very little is known about how β-barrel proteins are integrated into lipid bilayers and assembled into oligomeric structures12. Here we describe a protein complex that is essential for the topogenesis of mitochondrial outer membrane β-barrel proteins (TOB). We present evidence that important elements of the topogenesis of β-barrel membrane proteins have been conserved during the evolution of mitochondria from endosymbiotic bacterial ancestors13.

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Figure 1: Tob55 is essential for the biogenesis of β-barrel proteins.
Figure 2: Tob55 contains β-stranded segments, is present as an oligomeric assembly, and forms channels in lipid bilayers.
Figure 3: Electron microscopy of native and recombinant refolded TOB complexes using cryo-negative staining with ammonium molybdate.
Figure 4: The TOB complex engages β-barrel proteins after their interaction with the TOM complex.

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Acknowledgements

We thank U. Gärtner and P. Heckmeyer for technical assistance; L. Peters, T. Silberzahn and A. Weinzierl for their help in some experiments; S. Schmitt for outer membranes of N. crassa mitochondria; and C. Bornhövd for help in subcellular yeast fractionation. We are grateful to W. Baumeister for support and access to the electron microscope facility. We thank E. Weyher-Stingl for help with the CD measurements, and R. Benz and S. Nussberger for support with the electrophysiological experiments. This work was supported by the Deutsche Forschungsgemeinschaft (D.R.), Sonderforschungsbereich 594, the Bundesministerium für Bildung und Forschung (MITOP), the Fonds der Chemischen Industrie (W.N.), and predoctoral fellowships from the Boehringer Ingelheim Fonds (S.A.P. and T.W.).

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Correspondence to Walter Neupert.

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Supplementary information

41586_2003_BFnature02208_MOESM1_ESM.pdf

Supplementary Figures and Figure Legends: These four figures and accompanying legends provide additional information on the identification of Tob55, alignments of Tob55 genes and cDNAs from various organisms including bacterial species, the subcellular localization of Tob55 and on the presence of the Mas37 protein in the TOB complex of yeast. (PDF 577 kb)

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Paschen, S., Waizenegger, T., Stan, T. et al. Evolutionary conservation of biogenesis of β-barrel membrane proteins. Nature 426, 862–866 (2003). https://doi.org/10.1038/nature02208

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