Low access resistance perforated patch recordings using amphotericin B

Membrane channel proteins of the aquaporin family are highly selective for permeation of specific small molecules, with absolute exclusion of ions and charged solutes and without dissipation of the electrochemical potential across the cell membrane. We report the crystal structure of the Escherichia coli glycerol facilitator (GlpF) with its primary permeant substrate glycerol at 2.2 angstrom resolution. Glycerol molecules line up in an amphipathic channel in single file. In the narrow selectivity filter of the channel the glycerol alkyl backbone is wedged against a hydrophobic corner, and successive hydroxyl groups form hydrogen bonds with a pair of acceptor, and donor atoms. Two conserved aspartic acid-proline-alanine motifs form a key interface between two gene-duplicated segments that each encode three-and-one-half membrane-spanning helices around the channel. This structure elucidates the mechanism of selective permeability for linear carbohydrates and suggests how ions and water are excluded.

https://science.sciencemag.org/content/sci/290/5491/481.full.pdf

Structure of a glycerol-conducting channel and the basis for its selectivity.

The high water permeability characteristic of mammalian red cell membranes is now known to be caused by the protein AQP1 This channel freely permits movement of water across the cell membrane, but it is not permeated by other small, uncharged molecules or charged solutes AQP1 is a tetramer with each subunit containing an aqueous pore likened to an hourglass formed by obversely arranged tandem repeats Cryoelectron microscopy of reconstituted AQP1 membrane crystals has revealed the three-dimensional structure at 3-6 A AQP1 is distributed in apical and basolateral membranes of renal proximal tubules and descending thin limbs as well as capillary endothelia Ten mammalian aquaporins have been identified in water-permeable tissues and fall into two groupings Orthodox aquaporins are water-selective and include AQP2, a vasopressin-regulated water channel in renal collecting duct, in addition to AQP0, AQP4, and AQP5 Multifunctional aquaglyceroporins AQP3, AQP7, and AQP9 are permeated by water, glycerol, and some other solutes Aquaporins are being defined in numerous other species including amphibia, insects, plants, and microbials Members of the aquaporin family are implicated in numerous physiological processes as well as the pathophysiology of a wide range of clinical disorders

Cellular and molecular biology of the aquaporin water channels.

Water molecules confined to nonpolar pores and cavities of nanoscopic dimensions exhibit highly unusual properties. Water filling is strongly cooperative, with the possible coexistence of filled and empty states and sensitivity to small perturbations of the pore polarity and solvent conditions. Confined water molecules form tightly hydrogen-bonded wires or clusters. The weak attractions to the confining wall, combined with strong interactions between water molecules, permit exceptionally rapid water flow, exceeding expectations from macroscopic hydrodynamics by several orders of magnitude. The proton mobility along 1D water wires also substantially exceeds that in the bulk. Proteins appear to exploit these unusual properties of confined water in their biological function (e.g., to ensure rapid water flow in aquaporins or to gate proton flow in proton pumps and enzymes). The unusual properties of water in nonpolar confinement are also relevant to the design of novel nanofluidic and molecular separation devices or fuel cells.

/pdf/water-in-nonpolar-confinement-from-nanotubes-to-proteins-and-412uecrowv.pdf

Water in Nonpolar Confinement: From Nanotubes to Proteins and Beyond *

Despite longstanding interest by nephrologists and physiologists, the molecular identities of membrane water channels remained elusive until recognition of CHIP, a 28-kDa channel-forming integral membrane protein from human red blood cells originally referred to as "CHIP28." CHIP functions as an osmotically driven, water-selective pore; 1) expression of CHIP conferred Xenopus oocytes with markedly increased osmotic water permeability but did not allow transmembrane passage of ions or other small molecules; 2) reconstitution of highly purified CHIP into proteoliposomes permitted determination of the unit water permeability, i.e., 3.9 x 10(9) water molecules.channel subunit-1 x s-1. Although CHIP exists as a homotetramer in the native red blood cell membrane, site-directed mutagenesis studies suggested that each subunit contains an individually functional pore that may be reversibly occluded by mercurial inhibitors reacting with cysteine-189. CHIP is a major component of both apical and basolateral membranes of water-permeable segments of the nephron, where it facilitates transcellular water flow during reabsorption of glomerular filtrate. CHIP is also abundant in certain other absorptive or secretory epithelia, including choroid plexus, ciliary body of the eye, hepatobiliary ductules, gall bladder, and capillary endothelia. Distinct patterns of CHIP expression occur at these sites during fetal development and maturity. Similar proteins from other mammalian tissues and plants were later shown to transport water, and the group is now referred to as the "aquaporins." Recognition of CHIP has provided molecular insight into the biological phenomenon of osmotic water movement, and it is hoped that pharmacological modulation of CHIP function may provide novel treatments of renal failure and other clinical problems.

Aquaporin CHIP: the archetypal molecular water channel

THEORY: Osmotic Equilibrium Osmotic Transport (Osmosis) Induced by an Impermeant Solute Tracer Diffusion of Water Single-file Transport Osmotic Transport (Osmosis) LIPID BILAYER MEMBRANES: The Unmodified Membrane Nystatin an Amphotericin B. Gramicidin A PLASMA MEMBRANES: General Considerations The Red Cell Membrane Epithelia: Antidiuretic Hormone (ADH)-Induced Water Permeability Index.

Alan. Finkelstein

Papers

Water movement through lipid bilayers, pores, and plasma membranes