The outer membrane protects Gram-negative bacteria against a harsh environment. At the same time, the embedded proteins fulfill a number of tasks that are crucial to the bacterial cell, such as solute and protein translocation as well as signal transduction. Unlike membrane proteins from all other sources, integral outer membrane proteins do not consist of transmembrane alpha-helices, but instead fold into antiparallel beta-barrels. Over recent years, atomic structures of several outer membrane proteins, belonging to six families, were determined: They include the OmpA membrane domain, the OmpX protein, phospholipase A, general porins (OmpF, PhoE), substrate-specific porins (LamB, ScrY), and the TonB-dependent siderophore transporters FhuA and FepA. These crystallographic studies have yielded invaluable insight into and have decisively advanced the understanding of the functions of these intriguing proteins. Our review is aimed at discussing their common principles and peculiarities as well as open questions associated with them.
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