Fig. 3.

The key of mechanosensitive AQPs could be in the cytoplasmic extreme of the single file region of the water pathway. a Model of the AQP4 monomer (PDB 3GD8) showing the water pathway in blue. The key regions of the single file path are indicated on the right (SF, selectivity filter; NPA, NPA filter; CE, cytoplasmic extreme of the single file region). Transmembrane segments and loops are shown in light and dark green, respectively. The channel (blue) allows the passage of waters in single file between the SF and the CE. The SF or Ar/R filter (His201 from TM5 and Arg216 from loop E, shown in orange) restricts the transit to only two water molecules at a time by a geometric hinderance and offers the possibility to form H-bonds with the protein, ordering the water molecules. Both Asn97 from loop B and Asn213 from loop E are shown in coral at the center of the membrane. This is the NPA filter, where the passage of protons is prevented and the water dipole moment reversal prompted. Both Gly93 and Gly94 from the GxxxG motif of loop B (shown in yellow) are located at the CE of the single file region (also called the cytoplasmic mouth) and contribute with H-bonds to the ordering of water molecules. Next, His95 (shown in ochre) can occlude the pore by a gating mechanism mediated by cytosolic acidification. Visualization made by means of VMD (Humphrey et al., 1996). Far and near clipping planes have been defined on behalf of clarity. b Logo of the loop B sequence alignment from mechanosensitive aquaporins AQY1, AQP1, AQP4, BvTIP1;2, and VvTIP2;1. Arbitrary positions 6, 7, and 8 show the key Gly and His residues that play a key role in ordering water molecules at the cytoplasmic extreme of the single file, just before the NPA. The image was created with WebLogo (Schneider and Stephens, 1990; Crooks et al., 2004)