Distinct Transport Selectivity of Two Structural Subclasses of the Nodulin-like Intrinsic Protein Family of Plant Aquaglyceroporin Channels

摘要

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).