Ar/R selectivity filters of OsTIP4;2 and OsNIP2;1. Ar/R selectivity filter of water-specific OsTIP4;2 (red; left) and OsNIP2;1 (pink; right) homology models. Transmembrane regions of both the models were first superposed individually on glycerol transporter GlpF (blue) and only the residues forming the ar/R tetrad from the superposed structures are shown in ball-and-stick model. Residue names in one letter code are given for OsTIP4;2 in red, for OsNIP2;1 in pink and for GlpF in blue. The transmembrane segments and the loop regions to which these residues belong are indicated. The projection shown for each filter is viewed perpendicular to the membrane plane from the extracellular side.