Water channel structures analysed by electron crystallography

• Published in: Biochimica et biophysica acta Vol. 1840; no. 5; pp. 1605 - 1613
• Main Authors: Tani, Kazutoshi, Fujiyoshi, Yoshinori
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.05.2014
• Subjects: Aquaporin; aquaporins; Aquaporins - chemistry; brain; Cell adhesion; Cryo-electron microscopy; crystallography; Crystallography - methods; crystals; Electron crystallography; Electrons; hydrogen bonding; lipid bilayers; lipids; Models, Molecular; permeability; Protein Conformation; Water - chemistry; Water channel; AQP; Electron crystallography; Water channel; H-bond; Cryo-electron microscopy; 2D; AQP4M23; MD; Cell adhesion; ar/R; AQP4M1; NPA; Aquaporin
• ISSN: 0304-4165; 0006-3002; 1872-8006
• DOI: 10.1016/j.bbagen.2013.10.007

The mechanisms underlying water transport through aquaporin (AQP) have been debated for two decades. The water permeation phenomenon of AQP seems inexplicable because the Grotthuss mechanism does not allow for simultaneous fast water permeability and inhibition of proton transfer through the hydrogen bonds of water molecules. The AQP1 structure determined by electron crystallography provided the first insights into the proton exclusion mechanism despite fast water permeation. Although several studies have provided clues about the mechanism based on the AQP structure, each proposed mechanism remains incomplete. The present review is focused on AQP function and structure solved by electron crystallography in an attempt to fill the gaps between the findings in the absence and presence of lipids. Many AQP structures can be superimposed regardless of the determination method. The AQP fold is preserved even under conditions lacking lipids, but the water arrangement in the channel pore differs. The differences might be explained by dipole moments formed by the two short helices in the lipid bilayer. In addition, structure analyses of double-layered two-dimensional crystals of AQP suggest an array formation and cell adhesive function. Electron crystallography findings not only have contributed to resolve some of the water permeation mechanisms, but have also elucidated the multiple functions of AQPs in the membrane. The roles of AQPs in the brain remain obscure, but their multiple activities might be important in the regulation of brain and other biological functions. This article is part of a Special Issue entitled Aquaporins. [Display omitted] •Electron crystallography solved the first atomic structure of AQP.•Electron crystallography determined the structures of AQP1, AQP0, and AQP4.•Electron crystallography can be used to observe membrane proteins in lipids.•H-bond isolation mechanism is proposed to explain water channel functions.•Double-layered crystals of AQP0 and AQP4 revealed the cell adhesion function.