Three-dimensional organization of a human water channel

Abstract

Aquaporins (AQP) are members of the major intrinsic protein (MIP) superfamily of integral membrane proteins and facilitate water transport in various eukaryotes and prokaryotes. The archetypal aquaporin AQP1 is a partly glycosylated water-selective channel that is widely expressed in the plasma membranes of several water-permeable epithelial and endothelial cells. Here we report the three-dimensional structure of deglycosylated, human erythrocyte AQP1, determined at 7 A resolution in the membrane plane by electron crystallography of frozen-hydrated two-dimensional crystals. The structure has an inplane, intramolecular 2-fold axis of symmetry located in the hydrophobic core of the bilayer. The AQP1 monomer is composed of six membrane-spanning, tilted alpha-helices. These helices form a barrel that encloses a vestibular region leading to the water-selective channel, which is outlined by densities attributed to the functionally important NPA boxes and their bridges to the surrounding helices. The intramolecular symmetry within the AQP1 molecule represents a new motif for the topology and design of membrane protein channels, and is a simple and elegant solution to the problem of bidirectional transport across the bilayer.

Point mutations in the aromatic/arginine region in aquaporin 1 allow passage of urea, glycerol, ammonia, and protons

“…Our data indicate that NH 3 permeability depends on the makeup of the ar͞R constriction. Comparing different classes of AQPs that pass or do not pass NH 3 , it appears that hydrophobic niches at the entry site are needed to permit NH 3 passage. Indeed, NH 3 permeability correlated well with the lipophilicity of the ar͞R constriction present in the different AQP1 mutants (Fig.…”

“…4B). However, because AQPs lack the ability to abstract a proton from an NH 4 ϩ ion, ammonia passage relies on the pH-dependent concentration of uncharged NH 3 .…”

“…In recent years, the protein structure of the prototypical mammalian AQP1 has been refined to 2.2-Å resolution (2)(3)(4)(5). Two highly conserved structural features within the channel were proposed as filters that exclude the passage of solutes larger than water and of charged molecules, including protons.…”

Regulation of the water channel aquaporin‐1: isolation and reconstitution of the regulatory complex

“…Subsequent experiments using radiolabeled inhibitors of water diffusion in red blood cells (RBC), followed by SDS-PAGE and autoradiography, reveal radiolabeled proteins migrating at the band 4.5 region, suggesting the radiolabeled bands to be possible water channels (Benga et al, 1986a,b). In the past decade, the water channel Aquaporin-1 (AQP-1), has been purified, cloned, and its crystal structure determined (Cheng et al, 1997;Denker et al, 1988;Preston and Agre, 1991;Preston et al 1992;Walz et al, 1997). A 2.2 angstrom resolution structure of the bovine AQP-1 has recently been obtained by X-ray crystallography (Sui et al, 2001), revealing the site of water molecules in a frozen state.…”

Structural basis of water-specific transport through the AQP1 water channel

“…Moderate resolution projection and low resolution 3-D maps of AQP1 derived from electron crystallographic studies provided the first structure based evidence for a general architecture consisting of six helices surrounding two putative helical structures within the membrane bilayer [8][9][10][11] . Models of AQP1 derived from electron crystallographic structural studies at about 4 Å resolution have recently been reported and independently confirmed the presence of two non-membrane spanning helices [12][13][14] .…”