Making an Aquaporin Water-Tight: Structural Basis of Selectivity in Plant Nodulin 26 Intrinsic Proteins

Abstract

The evolution of land plants led toan amplification and diversification of the aquaporin superfamily of membrane channels. Among the subfamilies of plant specific aquaporin-like changes are the nodulin-26 intrinsic proteins (NIPs) which are multifunctional transporters of water, ammonia, glycerol and metalloid nutrients that participate in a number of osmoregulatory and metabolic functions. NIPs share the canonical hourglass fold of the aquaporin family, but possess substitutions within the aromatic arginine (ar/R) selectivity filter. The nine proteins of the NIP subfamily in the model plant Arabidopsis thaliana can be subdivided into two ar/R pore subgroups: the NIP subgroup I, which form aquaglyceroporins that are permeated by glycerol, water and ammonia, and the NIP subgroup II, which form metalloid transporters which are lack aquaporin activity and are essentially “water tight”. These two NIP pore families differ principally by the substitution of a conserved alanine (NIP subgroup II) for a conserved tryptophan (NIP subgroup I) in the helix 2 position (H2) of the ar/R filter. Based on transport analyses and molecular dynamics simulation, a model is proposed through which the alanine substitution results in both the selectivity for critical metalloid nutrients such as boric acid while simultaneously restricting water flow through the ar/R selectivity filter. A mechanism involving two different rotameric states of the conserved arginine residue in this selectivity region is proposed to be responsible for the water-tight character of the pore. (Supported in part by NSF grant 1121465).