High-resolution x-ray structure of human aquaporin 5

Abstract

Human aquaporin 5 (HsAQP5) facilitates the transport of water across plasma membranes and has been identified within cells of the stomach, duodenum, pancreas, airways, lungs, salivary glands, sweat glands, eyes, lacrimal glands, and the inner ear. AQP5, like AQP2, is subject to posttranslational regulation by phosphorylation, at which point it is trafficked between intracellular storage compartments and the plasma membrane. Details concerning the molecular mechanism of membrane trafficking are unknown. Here we report the x-ray structure of HsAQP5 to 2.0-Å resolution and highlight structural similarities and differences relative to other eukaryotic aquaporins. A lipid occludes the putative central pore, preventing the passage of gas or ions through the center of the tetramer. Multiple consensus phosphorylation sites are observed in the structure and their potential regulatory role is discussed. We postulate that a change in the conformation of the C terminus may arise from the phosphorylation of AQP5 and thereby signal trafficking.membrane protein ͉ trafficking ͉ crystallography ͉ water channel protein ͉ heterologous overexpression A quaporins (1) facilitate the flow of water across cellular membranes while preserving ion concentration gradients. By maintaining water homeostasis within cells, aquaporin family members play essential physiological roles within all kingdoms of life. They form a large superfamily containing both pure water channels, and channels also permeable to other small polar molecules such as glycerol (2). Thirteen human aquaporin (AQP) isoforms have been identified, and they govern a broad spectrum of physiological functions (2, 3). Examples include concentration of urine in the kidneys, release of tears and maintenance of lens transparency in the eye, maintenance of water homeostasis within the brain, the extrusion of sweat from the skin, control of glycerol concentration in fat metabolism, and facilitation of cell migration during angiogenesis.X-ray and electron diffraction studies have yielded crystal structures of mammalian AQP0 (4-6), AQP1 (7, 8), AQP2 (9), and AQP4 (10), plant SoPIP2;1 (11, 12), bacterial AQPZ (13,14), and GlpF (15), and the archaeal AQPM (16). These structures establish that phylogenetically and functionally diverse AQPs arrange as homotetramers, each protomer containing six highly conserved transmembrane (TM) ␣-helices. Two half-helices form a pseudoseventh TM helix because of the insertion of loops B and E into the membrane from opposite sides, placing both copies of the highly conserved Asn-Pro-Ala (NPA) AQP signature motif near the center of the water channel. The conserved aromatic/arginine (ar/R) constriction region imposes substrate selectivity on the channel (17). A consensus has emerged from molecular dynamics simulations regarding the mechanism of water transport and ion exclusion (18) establishing that an electrostatic potential barrier peaking at the NPA region prevents the cotranslocation of protons through the channel.Although the tissue-specific expressio...

An emerging consensus on aquaporin translocation as a regulatory mechanism

“…(Harries et al, 2004, Ho et al, 2009, Horsefield et al, 2008, Murata et al, 2000 revealing that the family shares a conserved architecture ( Figure 1). AQPs exist as homotetramers with four AQP subunits per functional AQP tetramer; selective transport of water molecules, glycerol and/or ions occurs through each of the four pores.…”

“…The high resolution structures of several AQPs have been resolved (see Tornroth-Horsefield et al, 2010 for review) and these include the structures of human AQP1, 4 and 5 (Murata et al, 2000, Ho et al, 2009, Horsefield et al, 2008. (Harries et al, 2004, Ho et al, 2009, Horsefield et al, 2008, Murata et al, 2000 revealing that the family shares a conserved architecture ( Figure 1).…”

Defective channels lead to an impaired skin barrier

“…Two short half-helices abut in the centre of the membrane to form a virtual seventh TMH that is crucial to the selectivity of the channel (Jung et al, 1994). Human AQP5 has been crystallised as a tetramer, which is considered to be the physiologically relevant form of aquaporin (Horsefield et al, 2008). However, unlike gap junctions, where the molecules pass through the hemichannel formed by the connexin subunits, water molecules move through the channel formed by each of the four aquaporin monomers, rather than through the central pore of the aquaporin tetramer (Jung et al, 1994).…”

Crystal structure of human aquaporin 4 at 1.8 Å and its mechanism of conductance

“…To date, there are only 3 crystal structures reported for mammalian AQPs, 2 purified from naturally rich sources, AQP1 in red blood cells, AQP0 from the eye lens (23,24), and human AQP5, from protein heterologously expressed in Pichia pastoris (25).…”