Point symmetry types in membrane protein structures. Structures are shown as cartoon helices, viewed down onto the membrane. Different colors are used to indicate symmetric elements, i.e., independent chains or (*)internal repeats. Non-symmetric elements are in gray. EmrE is an asymmetric homo-dimer (PDB entry: 3B5D), while other transporters are asymmetric as well as pseudo-symmetric (e.g., NCX, 3V5U). Two-fold screw axis (22)-pseudo-symmetry is seen in the Mrp antiporter-like subunits of complex I (see Figure 4). Presenilin is an aspartate protease (4HYG). ModB2 is from the homodimeric molybdate type I ABC importer (2ONK). MalFG is from the heterodimeric MalFGK2 type I ABC importer (2R6G). BsYetJ is a pH-dependent Ca2+ channel from the TM Bax inhibitor motif (TMBIM) family (4PGW). FLAP, or five-lipooxygenase-activating protein, is a member of the family of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG, 2Q7M). M-PPase is a membrane pyrophosphatase (4AV3). M2 is a H+ channel from influenza A (3LBW, TM domains only). TrkH is a K+ channel from the superfamily of K+ transporters (SKT, 3PJZ). TehA is a SLAC anion channel homolog (3M73 chain A). Connexin-26 is a gap junction (2ZW3); each hemi-channel exhibits C6 symmetry. α-hemolysin is a pore-forming toxin (7AHL). The c8-ring is a F-type ATP synthase membrane rotor (2XND). Symmetry axes were defined using SymD v1.3 (52) and figures were made with Pymol v1.7 (Schrödinger Ltd).