Aquaporin-7: A Dynamic Aquaglyceroporin With Greater Water and Glycerol Permeability Than Its Bacterial Homolog GlpF

Abstract

Xenopus oocytes expressing human aquaporin-7 (AQP7) exhibit greater osmotic water permeability and 3 H-glycerol uptake vs. those expressing the bacterial glycerol facilitator GlpF. AQP7-expressing oocytes exposed to increasing extracellular [glycerol] under isosmolal conditions exhibit increasing swelling rates, whereas GlpF-expressing oocytes do not swell at all. To provide a structural basis for these observed physiological differences, we performed X-ray crystallographic structure determination of AQP7 and molecular-dynamics simulations on AQP7 and GlpF. The structure reveals AQP7 tetramers containing two monomers with 3 glycerols, and two monomers with 2 glycerols in the pore. In contrast to GlpF, no glycerol is bound at the AQP7 selectivity filter (SF), comprising residues F74, G222, Y223, and R229. The AQP7 SF is resolved in its closed state because F74 blocks the passage of small solutes. Molecular dynamics simulations demonstrate that F74 undergoes large and rapid conformational changes, allowing glycerol molecules to permeate without orientational restriction. The more rigid GlpF imposes orientational constraints on glycerol molecules passing through the SF. Moreover, GlpF-W48 (analogous to AQP7-F74) undergoes rare but long-lasting conformational changes that block the pore to H 2 O and glycerol.

Involvement of aquaglyceroporins in energy metabolism in health and disease

“…The AQP7 sequence has the particularity to contain conserved NPA motifs modified to NAA (in positions 94e96) and NPS (in positions 226e228) for the first and second motifs, respectively [54,55]. Xenopus laevis oocytes or cells expressing rat, mouse or human AQP7 permeate water, as well as glycerol, urea and H 2 O 2 [50,56,57]. Compared to orthodox AQPs, AQP7 displays lower water permeability in cell-based assays [58,59] but similar water permeability in proteopolymersome assay performed using a glycerol-free set up [60], or reduced water permeability in the presence of glycerol using molecular dynamics simulations [61].…”

“…Compared to orthodox AQPs, AQP7 displays lower water permeability in cell-based assays [58,59] but similar water permeability in proteopolymersome assay performed using a glycerol-free set up [60], or reduced water permeability in the presence of glycerol using molecular dynamics simulations [61]. The AQP7 structure has been elucidated in recent works using X-ray crystallography and molecular-dynamics stimulations [57,61]. Human AQP7 displays a pore of around 3 Å of diameter at the selectivity filter [61] comprising amino acid residues F74, G222, Y223 and R229 [57].…”

“…The AQP7 structure has been elucidated in recent works using X-ray crystallography and molecular-dynamics stimulations [57,61]. Human AQP7 displays a pore of around 3 Å of diameter at the selectivity filter [61] comprising amino acid residues F74, G222, Y223 and R229 [57]. AQP7 tetramers contain two monomers with three molecules of glycerol and two monomers with two molecules of glycerol within the pore [57].…”

“…Human AQP7 displays a pore of around 3 Å of diameter at the selectivity filter [61] comprising amino acid residues F74, G222, Y223 and R229 [57]. AQP7 tetramers contain two monomers with three molecules of glycerol and two monomers with two molecules of glycerol within the pore [57]. The AQP7 structure is quite flexible as its F74 residue undergoes rapid conformational changes from a closed to an opens state, allowing the passage of glycerol without strong orientation restriction [57], with a partly rotating movement that may serve to break hydrogen bonds and allow its movement along the channel pore [61].…”

“…AQP7 tetramers contain two monomers with three molecules of glycerol and two monomers with two molecules of glycerol within the pore [57]. The AQP7 structure is quite flexible as its F74 residue undergoes rapid conformational changes from a closed to an opens state, allowing the passage of glycerol without strong orientation restriction [57], with a partly rotating movement that may serve to break hydrogen bonds and allow its movement along the channel pore [61]. Water molecules pass along glycerol molecules to allow glycerol permeation as both molecules compete for hydrogen-bonding sites in the pore [62].…”