Aquaporins are membrane water channels that play critical roles in controlling the water contents of cells. These channels are widely distributed in all kingdoms of life, including bacteria, plants, and mammals. More than ten different aquaporins have been found in human body, and several diseases, such as congenital cataracts and nephrogenic diabetes insipidus, are connected to the impaired function of these channels. They form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes across the membrane. However, the water pores are completely impermeable to charged species, such as protons, a remarkable property that is critical for the conservation of membrane's electrochemical potential, but paradoxical at the same time, since protons can usually be transported readily through water molecules. The results of our simulations have now provided new insight into the mechanism underlying this fascinating property. Water molecules passing the channel are forced, by the protein's electrostatic forces, to flip at the center of the channel (see the animation), thereby breaking the alternative donor-acceptor arrangement that is necessary for proton translocation (read the complete story in our Science paper).

The group's recent studies have revealed several structural and functional features of aquaporins. Our first set of molecular dynamics (MD) simulations was performed on a fully hydrated model of aquaporin-1 (AQP1) in a membrane which was based on the electron microscopy structures. The calculations described the formation of a water single file inside the channel and revealed the critical role of the NPA motifs in the stability and function of the channel (Zhu, et al., FEBS Lett., 2001). The simulations clearly showed the sensitivity of the results to a proper description of hydrogen bonds between the asparagine residues in the NPA motifs and the necessity for a higher resolution structure.

In the year 2000, Stroud's lab in UCSF succeeded in solving the first high resolution structure of an aquaporin by x-ray crystallography. The studied structure was the one of the E. coli glycerol uptake facilitator (GlpF), which is an aquaglyceroporin, i.e., the channel is also permeable to small linear sugar molecules such as glycerol. Nanosecond MD simulations of tetrameric GlpF in a hydrated patch of POPE lipid bilayer characterized the complete pathway of substrate conduction in the channel. Analysis of hydrogen bond interactions of the substrate with the interior of the channel also explained for the first time why these channels incorporate in their architecture two characteristic loops, including energetically unfavorable secondary structure elements, which are conserved in the whole aquaporin family (Jensen et al., Structure, 2001).

Can we simulate the conduction the same way as in experiments? In order to directly compare the results of MD simulations with biochemical measurements of the conductivity of membrane water channels, in which osmotic pressure gradients are used to induce the flow of water across the membrane, we developed a new methodology for MD simulations. By applying small forces on water molecules in the bulk region (see figure), a hydrostatic pressure gradient is generated across the membrane. The pressure gradient induces conduction rates that can be studied with MD simulations, which are usually limited to a few nanoseconds. The method has been applied in the simulation of water permeation through the GlpF, and has resulted in a linear correlation between the applied pressure and the flux (Zhu et al., Biophys. J., 2002).

Aquaporin research in the news:

• Aquaporin simulation movie and the 2003 Nobel prize for Chemistry
  
• Precious Bodily Fluids (The Pittsburgh Supercomputer Center)
  
• Scientists document water molecule movement across cell walls (UIUC)
  
• Researchers Explain How Proteins Filter Water into Cells (PSC)
  
• This 'dance' is in us all (The News Gazette)
  
• Interview with Radio Free Europe in Persian (audio)
  

Aquaporin team:

• Klaus Schulten
  
• Emad Tajkhorshid
  
• Fangqiang Zhu
  
• Morten Jensen
  
• Sanghyun Park