Switch from an Aquaporin to a Glycerol Channel by Two Amino Acids Substitution

Title: Switch from an Aquaporin to a Glycerol Channel by Two Amino Acids Substitution
Authors: Lagree, Valerie; Froger, Alexandrine; Deschamps, Stephane; Hubert, Jean-Francois; Delamarche, Christian; Bonnec, Georgette; Thomas, Daniel; Gouranton, Jean; Pellerin, Isabelle
Publisher: Journal of Biological Chemistry
Date Published: March 12, 1999
Reference Number: 226
The MIP (major intrinsic protein) proteins constitute a channel family of currently 150 members that have been identified in cell membranes of organisms ranging from bacteria to man. Among these proteins, two functionally distinct subgroups are characterized: aquaporins that allow specific water transfer and glycerol channels that are involved in glycerol and small neutral solutes transport. Since the flow of small molecules across cell membranes is vital for every living organism, the study of such proteins is of particular interest. For instance, aquaporins located in kidney cell membranes are responsible for reabsorption of 150 liters of water/day in adult human. To understand the molecular mechanisms of solute transport specificity, we analyzed mutant aquaporins in which highly conserved residues have been substituted by amino acids located at the same positions in glycerol channels. Here, we show that substitution of a tyrosine and a tryptophan by a proline and a leucine, respectively, in the sixth transmembrane helix of an aquaporin leads to a switch in the selectivity of the channel, from water to glycerol.
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This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)

The major intrinsic proteins (MIP) are a large family of proteins that act as channels through which water or small solutes (depending on the MIP) can cross cell membranes. There are two MIP subgroups, distinguished by what elements they allow to channel through them:

  1. aquaporins (AQPs): MIPs that let water pass through them, and
  2. glycerol channels: MIPs that allow glycerol and small neutral solutes to pass through them.

To get an idea of the structure of an AQP, think of a string of beads (the beads are amino acid residues). A significant portion of the string lies in six coiled heaps within the cell membrane, that thin strip of tissue that encircles the cell, separating it from its environment. These coils are called transmembrane helices 1 - 6. Part of the string snakes outside the cell membrane into the extracellular environment, forming three curves called extracellular loops A, C and E. Part of the string snakes inside the cell, forming intracellular loops B and D. Both ends of the AQP, the amino-terminus and the carboxy-terminus, lie inside the cell with the intracellular loops. (Please refer to a diagram of the AQP structure.) Researchers have assumed that glycerol channels have the same structural organization as AQPs. Lagree, et al., wanted to understand the molecular difference between AQPs and the glycerol channels that accounted for one being a channel only for water and the other being a channel only for glycerol. To do so they examined the physio-chemical properties of amino acids located in similar positions on both MIP types: an insect AQP (AQPcic) and a glycerol channel from an E. coli (GlpF). They found five places on each MIP, the same position on each, where amino acids with highly different physio-chemical properties were located. These positions were labeled positions 1 - 5 (P1 - P5). P2, P3, and P4/P5 were located either in or very close to loop E (on both MIPs). This supports the idea that loop E plays an important role in the final structure the respective MIPs take to allow their respective substance (water or glycerol) to pass through. The authors next constructed mutants of AQPcic by removing the amino acids that occurred at P2, P3 and P4/P5 and replacing them with the amino acids from GlpF that occurred at its P2, P3 and P4/P5. They injected these mutants into laboratory cell cultures and measured their ability to transport water and/or glycerol. The authors found that the mutants of AQPcic could no longer function as channels through which water could cross cell membranes. Further, one of the mutants, AQPcic-W222P/W223L acted like a glycerol channel. That is, this mutant allowed glycerol, but not water to pass through it. Lagree, et al., found that by removing the tyrosine and tryptophan amino acids that normally occur at P4/P5 in the sixth helix of AQPcic and replacing them with the proline and leucine amino acids that occur at P4/P5 on the GlpF, they functionally changed the AQP to a glycerol channel. They found that this mutant loses its ability to shape itself into the form adopted by AQPs to transport water. This suggests that researchers undertaking structural and functional studies of MIPs should take into account amino acids that are necessary for AQPs to form the shape that allows them to transport water.