Functional selectivity for glycerol of the nodulin 26 subfamily of plant membrane intrinsic proteins.

The nodulin-like intrinsic protein (NIP) subfamily of water and solute channels in plants is named for nodulin 26 of legume nodules. Two NIPs, soybean nodulin 26 and Lotus japonicus LIMP2, show a distinct functional profile with a low intrinsic osmotic water permeability (P(f)) and the ability to flux uncharged polyols such as glycerol. NIPs have a conserved signature sequence within the 'aromatic/arginine' region that forms the selectivity filter for major intrinsic proteins. This sequence is a hybrid of glyceroporin and aquaporin residues as well as exhibiting substitutions unique to the NIP subfamily. Site-directed mutagenesis of a conserved tryptophan in helix 2 of LIMP2 shows that this is a major determinant of glycerol selectivity.