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Integral membrane proteins account for ∼20–30% of fully sequenced proteomes (Krogh et al., 2001). To date, two major structural architectures can be distinguished: the α-helical and the β-barrel membrane proteins. The former are located primarily in cell membranes of eukaryotic cells and bacterial inner membranes, while the latter are found exclusively in the outer membranes of Gram-negative bacteria and in the outer membranes of mitochondria and chloroplasts (Cavalier-Smith, 2000; Elofsson and von Heijne, 2007; Gray et al., 1999). The β-barrel outer membrane proteins (OMPs) are crucial for the life of bacteria, serving a variety of diverse roles, such as passive nutrient uptake and active transport of large molecules, protein secretion, enzymatic activity or adhesion to host cells (Schulz, 2003; Wimley, 2003; Schulz, 2000; Gromiha and Suwa, 2007). Source text: Tsirigos et al., 2011.