Three-dimensional structure of an aquaporin subunit monomer (a ribbon model of NtAQP1, a PIP1 protein from tobacco). The structure shows six tilted membrane-spanning helices (I-VI) and two pore-forming domains made up of two short α helices entering the membrane from the extracellular and intracellular surfaces (arrows). The two NPA boxes are indicated in green. Amino- and carboxy-terminal domains are oriented to the cytoplasmic side of the membrane. The figure was generated using MODELLER7v7 and Swiss-Pdb Viewer.