Fig. 1 Topology of aquaporin. The general design principle of aquaporins is a tandem repeat of two three-helix bundles (TM1–3 and TM4–6) that are related by a pseudo two-fold symmetry axis (marked blue) parallel to the plane of the membrane. Such a design has not yet been seen in other membrane proteins. Another novel feature unique to aquaporins is that two pore helices (orange) form an additional transmembrane span. Furthermore, TM2 and 5 are not long enough to completely traverse the membrane. Not surprisingly, these helices are sequestered from the lipid environment by confinement to the packing interface close to the four-fold axis of the tetramer. The functionally important signature sequence of the aquaporin family, the ‘NPA-motif’, forms the N-terminal end of each of the two pore helices in the middle of the membrane.