title: Distinct transport selectivity of two structural subclasses of the nodulin-like intrinsic protein family of plant aquaglyceroporin channels.
abstract: Major intrinsic proteins (MIPs) are a diverse class of integral membrane proteins that facilitate the transport of water and some small solutes across cellular membranes. X-ray structures of MIPs indicate that a tetrad of residues (the ar/R region) form a narrow pore constriction that constitutes the selectivity filter. In comparison with mammalian and microbial species, plants have a greater number and diversity of MIPs with greater than 30 genes encoding four phylogenetic subfamilies with eight different classes of ar/R sequences. The nodulin 26-like intrinsic protein (NIP) subfamily in Arabidopsis can be subdivided into two ar/R subgroups: the NIP subgroup I, which resembles the archetype of the family, soybean nodulin 26, and the NIP subgroup II, which is represented by the Arabidopsis protein AtNIP6;1. These two NIPs 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. A comparison of the water and solute tranport properties of the two proteins was performed by expression in Xenopus laevis oocytes. Nodulin 26 is an aquaglyceroporin with a modest osmotic water permeability (P(f)) and the ability to transport uncharged solutes such as glycerol and formamide. In constrast, AtNIP6;1 showed no measurable water permeability but transported glycerol, formamide, as well as larger solutes that were impermeable to nodulin 26. By site-directed mutagenesis, we show that the H2 position is the crucial determinant that confers these transport behaviors. A comparison of the NIPs and tonoplast-intrinsic proteins (TIP) shows that the H2 residue can predict the transport profile for water and glycerol with histidine found in TIP-like aquaporins, tryptophan found in aquaglyceroporins (NIP I), and alanine found in water-impermeable glyceroporins (AtNIP6;1).
authors.affiliation: Department of Biochemistry and Cellular and Molecular Biology, The University of Tennessee, Knoxville, Tennessee 37996-0840, USA.
authors.forename: Ian S
authors.initials: IS
authors.lastname: Wallace
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Roberts
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D051397:Aquaglyceroporins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D020346:Aquaporins / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D001692:Biological Transport; D005260:Female; D005559:Formamides / Q000378:metabolism; D005990:Glycerol / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D009154:Mutation / Q000235:genetics; D009865:Oocytes / Q000378:metabolism; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D018075:RNA, Complementary / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D014508:Urea / Q000378:metabolism; D014867:Water / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D051397:Aquaglyceroporins; D020346:Aquaporins; D029681:Arabidopsis Proteins; D005559:Formamides; D008565:Membrane Proteins; D010940:Plant Proteins; D018075:RNA, Complementary; C110065:major intrinsic protein, plant; C045625:nodulin; D014867:Water; C031066:formamide; D014508:Urea; D005990:Glycerol