Ar/R selectivity filters of SoPIP2;1 and GlpF. Ar/R selectivity filter of water-specific SoPIP2;1 (green) and glycerol specific GlpF (blue). Transmembrane regions of both structures were first superposed 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 SoPIP2;1 in green 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.