Structure / sequence comparisons. (a) Extracellular view of the selectivity filter of an average conformation of the simulated AqpM, (b) the crystal structure of AqpZ [PDB-ID: 1RC2(A chain)] and (c) GlpF [PDB-ID: 1LDI]. Structures were aligned with STAMP  (as shown in (d) below). The protein structures are represented as transparent ribbons. Residues comprising the selectivity filter of each aquaporin are colored by element (cyan = carbon, red = oxygen, blue = nitrogen) and surrounded by a mesh surface. The red sphere in the center of each pore represents the oxygen atom of a water molecule in vdW representation. (d) Sequence alignment produced using STAMP  of an average structure of AqpM, the crystal structure of AqpZ [PDB-ID: 1RC2(A chain)], b-Aqp1 [PDB-ID: 1J4N], h-Aqp4 [PDB-ID: 3GD8] and GlpF [PDB-ID: 1LDI]. Horizontal black lines labeled TM1 to TM6 indicate the consensus transmembrane helical regions of the five aquaporins. HB and HE correspond to the half-helices from the re-entrant loops B and E, respectively. NPA motifs are highlighted in green, residues belonging to the selectivity filter (positions H2, H5, LE1 and LE2) are highlighted in blue and hydrophobic residues that line the pore below the SF are highlighted in red.