complex The SAM Complex is essential for sorting and assembling beta-barrel proteins from the intermembrane space side into the outer membrane. The SAM complex consists of three subunits: The β-barrel protein Sam50 and two peripheral subunits Sam35 and Sam37. Sam50 belongs to the conserved Omp85 protein family which can be characterized by a 16-stranded β-barrel and by a different number of polypeptide transport-associated (POTRA) domains. Sam50 exposes a single POTRA domain towards the intermembrane space. Sam35 caps the Sam50 β-barrel, stabilizing the core of the protein translocase. Sam50 and Sam35 are responsible for the binding of precursors of β-barrel proteins, which contain conserved β-signal that is formed by the last β-strand. The β-barrel of Sam50 is the functional domain that inserts and folds substrate proteins into the outer membrane. Sam35 binds to Sam50 and closely interacts with Sam37, in which Sam37 does not bind to Sam50. Sam37 and Sam35 have a conformation similar to glutathione-S-transferase, except they do not possess residues required for enzymatic activity. Sam37 accommodates the release of the folded β-barrel proteins from the SAM complex. Voltage-dependent anion ion channel or VDAC VDAC (voltage-dependent anion ion channel) is important for the exchange of small hydrophilic ions and metabolites with the cytosol, which is driven by the gradient concentration across the outer membrane. VDAC is the most abundant protein in the outer membrane. Like Tom40, VDAC has a β-barrel structure with antiparallel β-strands that can facilitate the passage of β-barrel membrane proteins. VDAC has a pore size of 2-4 nm for small hydrophilic molecules. VDAC plays a crucial role in facilitating energy metabolism by transporting ADP and ATP in and out of the outer membrane. VDAC also accommodates the passage of NADH and many anionic metabolites. VDAC operation is voltage-dependent in which it closes at high voltage and can partially open towards slightly reduced anion selectivity. α-Helical outer membrane proteins The Mitochondrial import complex (MIM) The import pathways of α-helical membrane anchors or signal-anchored proteins are carried out mainly by outer membrane proteins. Precursors of the polytopic or multi-spanning proteins can be recognized by Tom70, but cannot be passed through the Tom40 channel. Tom70 transfers the precursor proteins to the MIM Complex. The MIM complex constitutes the major inserts for alpha-helical proteins into the target membrane. The MIM Complex consists of several copies of Mim1 and one or two copies of Mim2. Both subunits are necessary for stabilizing partner