Architecture of wide ( top ) and narrow ( bottom ) channels. (A) Electrostatic surface of the wide channel with yellow line indicating the 28-Å route connecting the exterior and the central chamber. (B) Details of residues lining the wide channel, showing only one set of residues from one dimer. (C) Electrostatic surface along the 33-Å length of the narrow channel. (D) Details of residues lining the narrow channel. Bound glycerol (GOL) and magnesium (Mg) molecules are shown in stick and sphere, respectively.