Figure 1. Model of β-barrel membrane protein biogenesis in mitochondria and bacteria. β-barrel precursor proteins are synthesized in the cytosol of bacteria and eukaryotic cells. Mitochondrial β-barrel precursors are imported by the TOM–SAM supercomplex composed of the translocase of the outer membrane and the sorting and assembly machinery. Chaperones help to shield the precursors on the intermembrane space side of the supercomplex. In bacteria, β-barrel proteins are transported across the inner membrane by the Sec translocase. Periplasmic chaperones are required to transfer the precursors to the bacterial β-barrel assembly machinery (BAM) in the outer membrane. The SAM and BAM complexes mediate β-barrel folding and membrane insertion into the outer membranes. Figure modified from reference 2 and from video related to reference 6.