An Unusual Aquaporin-Like Metalloid Boric Acid Channel in Arabidopsis

Abstract

The Major Instrinsic Protein/aquaporin superfamily is an ancient family with a conserved protein fold that forms a channel that mediates the bidirectional transport of water and uncharged solutes. The evolution of land plants was accompanied by a major diversification of the MIP gene family that resulted in the acquisition of new transport functions. Among these divergent plant specific MIP subfamilies are the “nodulin 26 intrinsic proteins” (NIPs), based on structural and functional homology to soybean nodulin 26. NIPs are divided into three subfamilies (NIP I, II and III) which show low to absent water permeability but have acquired the ability to transport a variety of uncharged solutes ranging from glycerol to boric acid to protonated lactic acid. In the present work we characterize the transport and gating properties of an unusual NIP II protein from Arabidopsis pollen microspores, AtNIP7;1 which is related to the boric acid channels AtNIP6;1 and 5;1, necessary for the uptake of this critical micronutrient. Functional characterization of NIP7;1 shows that unlike NIP5;1 and NIP6;1 which form constitutive boric acid channels, the intrinsic boric acid transport activity of NIP7;1 is extremely low. Molecular modeling suggests that a conserved tyrosine residue (Tyr 81) located in the transport pore stabilizes a closed conformation of the pore. Molecular dynamics simulation suggests that the closed conformation is stabilized by hydrogen bonding between the Tyr81 hydroxyl group and Arg 220 of the canonical “aromatic-arginine” selectivity filter. Since boric acid is both essential nutrient as well as a toxic compound at high concentrations, it is proposed that Tyr 81 modulates transport and provides an additional level of regulation of uptake of boric acid in male gametophyte development.