Ar/R selectivity filters of PtPIP2;10 and PtSIP1;1. Ar/R selectivity filters of non-XIP Populus MIPs that are not found in their counterparts from Arabidopsis, rice and maize. Transmembrane regions of the MIP models from Populus were superposed on the experimentally determined structures of glycerol transporter GlpF (PDB ID: 1FX8) and the water-transporting spinach aquaporin SoPIP2;1 (PDB ID: 1Z98). Populus MIP residues are shown in stick representation with nitrogen and oxygen atoms in blue and red colors respectively. PtMIP residues are displayed in one letter code and their corresponding positions in the selectivity filter are indicated. For comparison purpose, the ar/R filters of GlpF and SoPIP2;1 are also shown in blue and pink respectively. (A) Out of 54 PIPs from four plants, PtPIP2;10 is the only PIP in which the Phe at H2 position is substituted by an Asn, making it more hydrophilic. (B) Residues forming the ar/R filter of PtSIP1;1 are very hydrophobic. Even the normally conserved Arg at LE2 position is substituted by a hydrophobic Phe. This will be one of the most hydrophobic constrictions in known MIPs.