Abstract
Gram-negative bacteria have a highly evolved cell wall with two membranes constituting a cell envelope consisting of an inner membrane (IM), a periplasmic space, and an outer membrane (OM). Proteins in the bacterial IM contain one or more hydrophobic α-helical transmembrane-spanning domains (TMDs). These co-translationally formed TMDs are accommodated into the IM lipid bilayer through a lateral gate in the Sec complex. While OM proteins (OMPs) reach their destination posttranslationally and contain a unique TMD known as a β-barrel. This amphipathic β-barrel closed cylindrical structure contains 8–26 TMDs that are arranged in a linear antiparallel β-sheet, with the first and last strands interacting at a junction to form this barrel-like shape. OMPs must be transported across the IM, periplasm, and peptidoglycan before they are inserted into the OM to achieve their functional form. The nascent OMPs pass the IM in an unfolded state (uOMPs) with an amino-terminal leader sequence that directs them to the Sec export machinery, which transports the uOMPs into the periplasm, where the lack of ATP is another of the various obstacles to overcome. uOMPs are transported across the periplasm, and successive folding and assembly are key for biogenesis, where chaperones and the essential β-barrel assembly machinery (BAM) complex facilitate these processes. In this chapter, I will describe the role of the BAM complex in outer membrane assembly, including the uOMP protection by periplasmic chaperones, and the structural and functional analyses that illustrate how the BAM complex inserts its substrates into the outer membrane.
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Acknowledgment
This work was supported by a Consejo Nacional de Ciencia y Tecnología (Conacyt) Grant (221130). I thank Paul Ugalde-Silva for the artistic work in Fig. 1 and Lucia Chavez-Dueñas for compiling the references with EndNote software.
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Navarro-Garcia, F. (2019). Role of the BAM Complex in Outer Membrane Assembly. In: Geiger, O. (eds) Biogenesis of Fatty Acids, Lipids and Membranes. Handbook of Hydrocarbon and Lipid Microbiology . Springer, Cham. https://doi.org/10.1007/978-3-319-50430-8_35
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