title: Crystal Structure of an Ammonia-Permeable Aquaporin.
abstract: Aquaporins of the TIP subfamily (Tonoplast Intrinsic Proteins) have been suggested to facilitate permeation of water and ammonia across the vacuolar membrane of plants, allowing the vacuole to efficiently sequester ammonium ions and counteract cytosolic fluctuations of ammonia. Here, we report the structure determined at 1.18 Å resolution from twinned crystals of Arabidopsis thaliana aquaporin AtTIP2;1 and confirm water and ammonia permeability of the purified protein reconstituted in proteoliposomes as further substantiated by molecular dynamics simulations. The structure of AtTIP2;1 reveals an extended selectivity filter with the conserved arginine of the filter adopting a unique unpredicted position. The relatively wide pore and the polar nature of the selectivity filter clarify the ammonia permeability. By mutational studies, we show that the identified determinants in the extended selectivity filter region are sufficient to convert a strictly water-specific human aquaporin into an AtTIP2;1-like ammonia channel. A flexible histidine and a novel water-filled side pore are speculated to deprotonate ammonium ions, thereby possibly increasing permeation of ammonia. The molecular understanding of how aquaporins facilitate ammonia flux across membranes could potentially be used to modulate ammonia losses over the plasma membrane to the atmosphere, e.g., during photorespiration, and thereby to modify the nitrogen use efficiency of plants.
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kirscht
authors.affiliation: The Max Planck Institute for Biophysical Chemistry, Computational Biomolecular Dynamics Group, Göttingen, Germany.
authors.forename: Shreyas S
authors.initials: SS
authors.lastname: Kaptan
authors.affiliation: Institut des Sciences de la Vie, Université catholique de Louvain, Louvain-la-Neuve, Belgium.|IPK-Leibniz Institute of Plant Genetics and Crop Plant Research Department of Physiology and Cell Biology, Gatersleben, Germany.
authors.forename: Gerd Patrick
authors.initials: GP
authors.lastname: Bienert
authors.affiliation: Institut des Sciences de la Vie, Université catholique de Louvain, Louvain-la-Neuve, Belgium.
authors.forename: François
authors.initials: F
authors.lastname: Chaumont
authors.affiliation: Danish Research Institute of Translational Neuroscience-DANDRITE, Nordic-EMBL Partnership for Molecular Medicine, Department of Molecular Biology and Genetics, Aarhus University, Aarhus C, Denmark.
authors.forename: Poul
authors.initials: P
authors.lastname: Nissen
authors.affiliation: The Max Planck Institute for Biophysical Chemistry, Computational Biomolecular Dynamics Group, Göttingen, Germany.
authors.forename: Bert L
authors.initials: BL
authors.lastname: de Groot
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Per
authors.initials: P
authors.lastname: Kjellbom
authors.affiliation: Danish Research Institute of Translational Neuroscience-DANDRITE, Nordic-EMBL Partnership for Molecular Medicine, Department of Molecular Biology and Genetics, Aarhus University, Aarhus C, Denmark.
authors.forename: Pontus
authors.initials: P
authors.lastname: Gourdon
authors.affiliation: Department of Biochemistry and Structural Biology, Center for Molecular Protein Science, Lund University, Lund, Sweden.
authors.forename: Urban
authors.initials: U
authors.lastname: Johanson
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D020346:Aquaporins / Q000737:chemistry* / Q000378:metabolism; D017360:Arabidopsis; D029681:Arabidopsis Proteins / Q000737:chemistry* / Q000378:metabolism; D003460:Crystallization; D015394:Molecular Structure
chemical_list: D020346:Aquaporins; D029681:Arabidopsis Proteins; C500688:TIP2;1 protein, Arabidopsis; D000641:Ammonia