Model of rat AQP4. The RasMol 2.7.2 program is available on the WWW and was used to manipulate the protein data base file, 2D57 . RasMol 2.7.2 is an updated version of RasMol 2.6 developed by Roger Sayle while at the Biomolecular Structures Group, Glaxo Wellcome Research & Development, Stevenage, Hertfordshire, UK. The left image shows both ribbon and space filling motifs. The ribbons are the six alpha helices that span the plasma membrane. The c-terminal and n-terminal domains were not part of the crystal structure but would be projecting downward from the red and blue helices, respectively. The top of the molecule shown in space filling format represents three loops that are thought to be surface exposed: Loop A (blue), Loop C (green) and Loop E (orange-red). The image to the right is the view of the top of AQP4 looking down its central axis.