title: Water channel structures analysed by electron crystallography.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Cellular and Structural Physiology Institute, Nagoya University, Furo-cho, Chikusa, Nagoya, Japan.
authors.forename: Kazutoshi
authors.initials: K
authors.lastname: Tani
authors.affiliation: Cellular and Structural Physiology Institute, Nagoya University, Furo-cho, Chikusa, Nagoya, Japan. Electronic address: yoshi@cespi.nagoya-u.ac.jp.
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Fujiyoshi
mesh_terms: D020346:Aquaporins / Q000737:chemistry*; D003461:Crystallography / Q000379:methods*; D004583:Electrons; D008958:Models, Molecular; D011487:Protein Conformation; D014867:Water / Q000737:chemistry
chemical_list: D020346:Aquaporins; D014867:Water
keywords: Aquaporin; Cell adhesion; Cryo-electron microscopy; Electron crystallography; Water channel