<i>Arabidopsis thaliana</i> NIP7;1: An Anther-Specific Boric Acid Transporter of the Aquaporin Superfamily Regulated by an Unusual Tyrosine in Helix 2 of the Transport Pore

Plant nodulin-26 intrinsic proteins (NIPs) are members of the aquaporin superfamily that serve as multifunctional transporters of uncharged metabolites. In <i>Arabidopsis thaliana</i>, a specific NIP pore subclass, known as the NIP II proteins, is represented by <i>AtNIP5;1</i> and <i>AtNIP6;1</i>, which encode channel proteins expressed in roots and leaf nodes, respectively, that participate in the transport of the critical cell wall nutrient boric acid. Modeling of the protein encoded by the <i>AtNIP7;1</i> gene shows that it is a third member of the NIP II pore subclass in <i>Arabidopsis</i>. However, unlike AtNIP5;1 and AtNIP6;1 proteins, which form constitutive boric acid channels, AtNIP7;1 forms a channel with an extremely low intrinsic boric acid transport activity. Molecular modeling and molecular dynamics simulations of AtNIP7;1 suggest that a conserved tyrosine residue (Tyr81) located in transmembrane helix 2 adjacent to the aromatic arginine (ar/R) pore selectivity region stabilizes a closed pore conformation through interaction with the canonical Arg220 in ar/R region. Substitution of Tyr81 with a Cys residue, characteristic of established NIP boric acid channels, results in opening of the AtNIP7;1 pore that acquires a robust, transport activity for boric acid as well as other NIP II test solutes (glycerol and urea). Substitution of a Phe for Tyr81 also opens the channel, supporting the prediction from MD simulations that hydrogen bond interaction between the Tyr81 phenol group and the ar/R Arg may contribute to the stabilization of a closed pore state. Expression analyses show that <i>AtNIP7;1</i> is selectively expressed in developing anther tissues of young floral buds of <i>A. thaliana</i>, principally in developing pollen grains of stage 9–11 anthers. Because boric acid is both an essential nutrient as well as a toxic compound at high concentrations, it is proposed that Tyr81 modulates transport and may provide an additional level of regulation for this transporter in male gametophyte development.