Aquaporins: Channels for the Molecule of Life

Abstract

Liquid water has unique properties that make it a universal solvent. Water is an essential component of almost all physiological and biochemical reactions; therefore, its presence is required everywhere within an organism. The circulatory systems of animals and the vascular system of plants move fluids over long distances. In the tallest trees, water moves across a height gradient of 100 m or more, thereby connecting roots and distant leaves. On a cellular and tissue level, water movement is facilitated by intrinsic membrane proteins called aquaporins. These water channels are found in all life forms. Aquaporins have been mostly studied in mammals and plants, where water channels play important physiological roles. This article gives an overview of the discovery, structure and regulation of aquaporins. Their roles in different life forms are discussed.

Key Concepts

  • Water can diffuse through the lipid bilayer, but without aquaporins flow across membranes is slow.
  • Aquaporins are integral membrane transport proteins, and they facilitate water movement in cells, tissues and entire organisms.
  • A single water channel allows the passive passage of more than one billion water molecules per second.
  • The aquaporin protein family is ancient, and aquaporins can be found in a wide range of organisms.
  • Aquaporins appear to play important physiological roles in mammals, plants and other organisms.

Keywords: cell membranes; hydraulic conductivity; kidney function; membrane permeability; MIP family; water flow; water‐relations

Figure 1. Expression of CHIP28 water channel activity in Xenopus oocytes. Oocytes obtained from female Xenopus are injected with in vitro transcribed RNA of CHIP28. After a period of incubation, the water permeability of test oocytes expressing the protein and of control oocytes is tested in distilled water. Almost immediately, ‘the test oocytes were highly permeable to water and exploded like popcorn’ (Agre, ).
Figure 2. All aquaporin proteins share a common hourglass structure. They are composed of six transmembrane helices (H1–H6) connected by five loops (LA–LE); both N‐ and C‐termini are located on the cytoplasmic side of the membrane. Two short helical domains (HB and HE) of LB and LE form a seventh ‘broken’ helix; they both contain a signature motif NPA (asparagine, proline, alanine) located in the middle of the pore. The aromatic/arginine constriction site (ar/R) is located closer to the extracellular side. Near the constriction sites, water molecules move in single‐file configuration.
Figure 3. Confocal laser scanning micrograph showing the localisation of PIP2 proteins in a balsam poplar (Populus balsamifera) leaf midvein. The image was colour coded with an intensity look‐up table (displayed in the upper right corner) in which black was used to encode background, and blue, green, yellow and red to represent increasing signal intensities. Strong PIP2 signals were observed in the plasma membrane of phloem cells (P), developing xylem cells (X) and in phloem fibres (F). Arrows point to labelling in phloem and living/developing xylem cells. Controls with no primary antibody indicated minimal labelling (not shown); methods as described previously (Laur and Hacke, ). Immunolabelling and imaging was conducted by Ryan Stanfield (University of Alberta).
Figure 4. In situ mRNA hybridisation of the aquaporin gene PgPIP1;2 in a young stem of white spruce (Picea glauca). Regions of aquaporin expression are indicated by dark purple staining. Strong labelling was found in xylem (X) rays (arrows), cambium cells (C), phloem (P) and resin ducts (R). Pi = pith. Rays allow water to move radially between xylem and phloem. Negative controls hybridised with sense probes showed no/minimal labelling (not shown). Methods as described previously (Laur and Hacke, ).
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Further Reading

Beitz E (ed). ISBN: 978‐3‐540‐79884‐2 (Print) 978‐3‐540‐79885‐9 (Online) (2009) Aquaporins. Handbook of Experimental Pharmacology Volume 190. Springer.

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Hacke, Uwe G, and Laur, Joan(Feb 2016) Aquaporins: Channels for the Molecule of Life. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001289.pub2]