Fig. 8 .

Some aquaporins (AQPs) contain a structure called the NPA (asparagine-proline-alanine) motif. The center portion of this motif has an hourglass-shaped structure, which penetrates through the lipid bilayer. Substance transport occurs here. AQP1 only allows water to pass through. At this hourglass-shaped portion, the diameter is narrowest at approximately 2.8 Å, which is the size in which one water molecule can just pass through. Water molecules can traverse in both directions at this site. Reprinted with the permission from Reference²⁵⁵⁾ Figure 1 . (Badaut J, Lasbennes F, Magistretti PJ, Regli L: Aquaporins in brain: distribution, physiology, and pathophysiology. J Cereb Blood Flow Metab 22: 367–378, 2002).