Structure of Aquaporin Reveals Mechanism for Transport Selectivity

Structure of Aquaporin Reveals Mechanism for Transport Selectivity

  • Published: December 22, 2003
  • DOI: 10.1371/journal.pbio.0000075

Biochemists aren't much accustomed to seeing their work in the popular press, save for annual coverage of the Nobel prize in chemistry. This year, Roderick MacKinnon was recognized for working out the atomic structure of an ion channel and Peter Agre for discovering that a major protein found in red blood cells functions primarily as a water channel. Agre went on to establish the family of related channels, which he named “aquaporins.” Channel proteins have aqueous pores that cross the cell membrane and regulate the flow of molecules in and out of cells. Water passively pours though aquaporins by osmosis, moving from low to high concentrations of solutes. Solving the structure of these channels provided a platform for exploring the underlying molecular mechanisms that allow the proteins to function as filters and maintain osmotic equilibrium. Robert Stroud and colleagues recently solved the atomic structure of an aquaporin (GlpF) and have now solved the structure of another water channel from Escherichia coli, called aquaporin Z, that selectively conducts only water at high rates.

Aquaporins form a large, diverse family of proteins and have been found in bacteria, plants, and animals. There are 11 family members in the human proteome. Less than a decade ago, scientists discovered the aquaporin Z gene (aqpZ) in E. coli, pointing to the protein's role in regulating water transport in this prokaryote. The high-resolution X-ray structures of recombinant aquaglyceroporin glycerol facilitator (GlpF)—a channel protein that transports both glycerol and water in E. coli—determined by the Stroud group in 2000 and of bovine aquaporin 1 (AQP1) from red blood cells, determined a year later, revealed how these aquaporins regulate their transport and selectivity. The aquaporin Z channel protein in E. coli can accommodate a flow of water at rates six times higher than GlpF, making it the prime subject for studying the selectivity of a high-conducting water channel. And because the two main classes of aquaporins occur in E. coli—which means they're exposed to the same cellular environment—and were both expressed recombinantly, the opportunities for comparative structural and functional analyses, combined with site-directed mutagenesis, promise to provide valuable insights into the molecular underpinnings of the selectivity of functionally different aquaporins.

After fabricating and growing a recombinant form of AqpZ in E. coli, David Savage in the Stroud group recovered the proteins from the bacterial colonies, then purified and concentrated them. The proteins were crystallized—capturing five water molecules inside—and then analyzed by state-of-the-art high-resolution X-ray diffraction techniques. The architecture of aquaporin Z, the researchers report, is typical of aquaporins, with a spiral of eight oxygens providing water-binding sites inside the channel and amino acid side chains determining the size and chemistry of the channel. The outer membrane and cytoplasmic ends of the channel are wider than the interior, which is long and narrow. This structure confirms that aquaporin selectivity arises in part from erecting a physical barrier: small molecules, like water, can easily pass, but larger ones simply can't fit. And the strategic positioning of amino acid residues with hydrophilic or hydrophobic properties along the channel helps police the influx of molecules based on their affinity for water. While it seems two amino acid chains located in the middle of the channel also provide a water-friendly surface, Stroud et al. say they play a more intriguing role. Noting that the water molecules occupy the channel in single file, the scientists explain that such an orientation would normally facilitate the random flow of protons (or hydrogen ions), which would be lethal to the cell. This central amino acid pair, they say, restricts the behavior of water molecules in the center of the channel in such a way that prevents “proton jumping” yet keeps the water flowing.

With two structural models of aquaporins down to the atomic level in the same species, scientists can now begin to investigate the molecular mechanisms that facilitate their selectivity. The importance of understanding these widely distributed channel proteins was underscored by the Nobel awards this year. Water transport is fundamental to life, and aquaporins are found throughout the body. Knowledge of their structure will help reveal the molecular mechanics of their specialized feats and promise to offer insights into a wide range of human disorders.

Structure of aquaporin Z