Small and polar residues at the helix-helix interfaces of SoPIP2;1. Small and polar residues occurring in helix-helix interfaces of spinach plant aquaporin SoPIP2;1 are displayed. The helix pairs TM1–TM3 (left), TM2–TM5 (middle) and TM4–TM6 (right) are shown. The backbone is drawn in ribbon representation and the interfacial residues are depicted as space-filling models. Residue numbers of interfacial residues correspond to the PDB structure 1Z98.