The structural basis of water permeation and proton exclusion in aquaporins (Review)

Abstract

Aquaporins (AQPs) represent a ubiquitous class of integral membrane proteins that play critical roles in cellular osmoregulations in microbes, plants and mammals. AQPs primarily function as water-conducting channels, whereas members of a sub-class of AQPs, termed aquaglyceroporins, are permeable to small neutral solutes such as glycerol. While AQPs facilitate transmembrane permeation of water and/or small neutral solutes, they preclude the conduction of protons. Consequently, openings of AQP channels allow rapid water diffusion down an osmotic gradient without dissipating electrochemical potentials. Molecular structures of AQPs portray unique features that define the two central functions of AQP channels: effective water permeation and strict proton exclusion. This review describes AQP structures known to date and discusses the mechanisms underlying water permeation, proton exclusion and water permeability regulation.

The role of aquaporins in excretion in insects

“…Proton transport along water chains requires uniform orientation of the H-bonded water molecules which permits reorientation during proton transfer. This forced orientation of the water molecule filling the narrow region of the pore by the paired asparagines is sufficient to completely disrupt proton movement along the water chain, rendering the channel impervious to protons (Chakrabarti et al, 2004;Fu and Lu, 2007;Tajkhorshid et al, 2002). Most AQPs are also reversibly inhibited by Hg 2+ , the inhibition being relieved by the reducing agent β-mercaptoethanol.…”

Gene expression and localization of two types of AQP5 inXenopus tropicalisunder hydration and dehydration

“…A pair of Asn-Pro-Ala motifs (squares) and the mercurial-inhibition site (diamond) are conserved in all the aquaporins (AQPs). The phenylalanine, histidine, cysteine, and arginine residues (stars) are presumed to be important for aromatic/arginine constriction, as in mammalian AQP1 (16,21). Potential phosphorylation sites for protein kinase C (open triangles) and PKA (solid triangles) in AQP-xt5a were conserved in AQP-xt5b, AQP-x5(b), mammalian AQP5, or mammalian AQP2, except Ser-227.…”

Expression of Nitric Oxide Synthase and Aquaporin-3 in Cyclophosphamide Treated rat Bladder

“…From a functional perspective, the AQPs can be divided into two subfamilies: AQP 0, 1, 2, 4, 5, 6, and 8 move water directly; AQP 3, 7, 9, and 10 as aquaglyceroporins move water with additional small neutrally charged solute such as glycerol, urea, and pyrimidines [32]. Human urothelium is known to channel complex with the two functions and rat bladder can have similar physiological function.…”