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Annu Rev Microbiol. 2017 Sep 8;71:539-556. doi: 10.1146/annurev-micro-090816-093754.

Outer Membrane Biogenesis.

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Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544; email:
Department of Chemistry and Chemical Biology and.
Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138.
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115.


The hallmark of gram-negative bacteria and organelles such as mitochondria and chloroplasts is the presence of an outer membrane. In bacteria such as Escherichia coli, the outer membrane is a unique asymmetric lipid bilayer with lipopolysaccharide in the outer leaflet. Integral transmembrane proteins assume a β-barrel structure, and their assembly is catalyzed by the heteropentameric Bam complex containing the outer membrane protein BamA and four lipoproteins, BamB-E. How the Bam complex assembles a great diversity of outer membrane proteins into a membrane without an obvious energy source is a particularly challenging problem, because folding intermediates are predicted to be unstable in either an aqueous or a hydrophobic environment. Two models have been put forward: the budding model, based largely on structural data, and the BamA assisted model, based on genetic and biochemical studies. Here we offer a critical discussion of the pros and cons of each.


LptD; envelope biogenesis; lateral gate; outer membrane protein; protein folding

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