Fig. 1.

Ribbon representations of six mammalian membrane protein crystal structures (top view from extracellular side; each color represents one monomer). A, bovine rhodopsin (PDB code: 2I36). B, bovine aquaporin 0 (PDB code: 1YMG) with bound water molecules shown as yellow spheres. C, rat voltage-gated K_v1.2K⁺ channel (PDB code: 2A79) with bound K⁺ ion shown as a blue sphere. D, rabbit Ca²⁺-ATPase (PDB code: 1SU4) with two bound Ca²⁺ ion shown as a green sphere. E, human LTC₄S (PDB code: 2UUH) with bound GSH shown as a sphere. The orientation of LTC₄S by Ago et al. (2007) is opposite to that reported by Martinez Molina et al. (2007). We think that the second structure, shown here, is in a correct membrane orientation that places the C and N termini inside the lumen, because of a significant homology of LTC₄S with FLAP that has this topology. F, human FLAP (PDB code: 2Q7R). Bovine rhodopsin forms a head-to-head dimer as shown in this crystal form (A). Aquaporin 0 and the K_v1.2K⁺ channel form tetramers in crystals and function as tetramers. The Ca²⁺-ATPase exists as monomer in most crystal forms. Both human LTC₄S and FLAP exit as homotrimers.