Barley HvNIP2;1 aquaporin permeates water, metalloids, saccharides, and ion pairs due to structural plasticity and diversification
Aquaporins can facilitate the passive movement of water and small polar molecules and some ions. The barley Nodulin 26-like Intrinsic Protein (HvNIP2;1) embedded in liposomes and examined through stopped-flow light scattering spectrophotometry and Xenopus oocyte swelling assays was found to permeate water, boric and germanic acids, sucrose and L-arabinose but not D-glucose or D-fructose. Other saccharides, such as neutral (D-mannose, D-galactose, D-xylose, D-mannoheptaose) and charged (N-acetyl D-glucosamine, D-glucosamine, D-glucuronic acid) aldoses, disaccharides (lactose, cellobiose, gentiobiose, trehalose), trisaccharide raffinose, and urea, glycerol, and acyclic polyols were permeated to a much lower extent. Apparent permeation of hydrated KCl and MgSO4 ion pairs was observed, while CH3COONa and NaNO3 permeated at significantly lower rates. Experiments with boric acid and sucrose revealed no apparent interaction between solutes when permeated together, and AgNO3 blocked the permeation of all solutes. Full-scale steered molecular dynamics simulations of HvNIP2;1 and spinach SoPIP2;1 revealed possible rectification for water, boric acid, and sucrose transport, and defined key residues interacting with permeants. In a biological context, the simulated sucrose rectification could mediate its apoplastic-to-intracellular transport but not the reverse, thus, constituting a novel element of plant saccharide-transporting machinery. Phylogenomic analyses of 164 Viridiplantae and 2,993 Archaean, bacterial, fungal, and Metazoan aquaporins rationalised solute poly-selectivity in NIP3 sub-clade entries and suggested that they diversified from other sub-clades to acquire a unique specificity of saccharide transporters. Solute specificity definition in NIP aquaporins could inspire developing plants for sustained food production. Significance StatementAquaporins are fundamental to water and solute movements in nearly all living organisms. Solute selectivity inspections of the HvNIP2;1 aquaporin revealed that it transported water, hydroxylated metalloids boric and germanic acids, sucrose, L-arabinose, KCl, and MgSO4 ion pairs, but not D-glucose or D-fructose and to lesser extent urea, and acyclic polyols. This poly-selective transport by HvNIP2;1 classified in the NIP3 sub-clade aquaporins may afford nutritional and protective roles during plant development and in response to abiotic stresses. It is anticipated that the solute specificity definition of HvNIP2;1 inspires protein engineering and in silico mining to develop plants, which when exposed to suboptimal soil conditions of high soil metalloids, would overcome toxicity for sustained food production.