Ribbon diagram of the structure of an AQP1 subunit (sagittal section). The model is based on Protein Data Bank coordinates 1H6I of human red cell AQP1 (8). The amino-terminus is blue and the carboxy-terminus is red. Six membrane-spanning helices surround two hemipores (loops with short helices — cyan and orange) that meet in the center of the bilayer. The white arrow illustrates the aqueous channel through the protein.