PMC

Lipids surrounding an aquaporin-0. (a) Space-filling model of the lipid bilayer surrounding an aquaporin-0 tetramer; the protein is shown in yellow and the lipids in two shades of blue. (b) The nine lipids surrounding an aquaporin-0 monomer; the protein is shown as ribbon diagram and the lipids as ball-and-stick models (adapted from [●●]).

Membrane junctions formed by aquaporins-0 and 4. (a) Structure of the aquaporin-0 mediated membrane junction visualized by electron crystallography of double-layered 2D crystals (pdb accession code: 2B6O) [,●●]. The paired tetramers are exactly in register. (b) Paired aquaporin-0 tetramers as seen in loosely packed 3D crystals (pdb accession code: 2C32) []. The paired tetramers are rotated by 24° with respect to each other. (c) Structure of the aquaporin-4 mediated membrane junction visualized by electron crystallography of double-layered 2D crystals (pdb accession code: 2D57) [●●].

Structural basis of vectorial proton translocation by bacteriorhodopsin. (a) The bR structure determined by Henderson and co-workers, which was the first atomic model produced by electron crystallography (pdb accession code: 1BRD) []. (b) 3Å resolution structure of bR determined by Fujiyoshi and co-workers (pdb accession code: 2AT9). (c,d) Superposition of wild-type bacteriorhodopsin (green; pdb accession code: 1FBB) and the D96G, F171C, F219L triple mutant (red; pdb accession code: 1FBK), which always shows the full extent of the light-driven conformational change, illustrating differences in the cytoplasmic portions of helices F and G [].

Open and closed states of the acetylcholine receptor. (a) Atomic model of the acetylcholine receptor in the closed conformation based on a 4Å density map []. (b) Binding of acetylcholine opens the channel by initiating rotational movements (arrows) of the inner β-sheets of the α subunits in the ligand-binding domain. These movements are communicated to the inner (M2) helices lining the pore and break apart the gate – a hydrophobic girdle in the middle of the membrane – so that ions can flow through. A tryptophan side chain in the ligand-binding domain identifies the acetylcholine-binding region; a valine side chain links the inner sheet to the inner helix; leucine and valine side chains on the inner helices make the gate (pink patch); the locations of the membrane surfaces are indicated by broken lines; the relevant moving parts are in blue. Figure reprinted by permission of Federation of the European Biochemical Societies from “Structure and action of the nicotinic acetylcholine receptor explored by electron microscopy” by Nigel Unwin, FEBS Letters, 555, 91–95, 2003.