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." 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, as reported in this issue of PLoS Biology, have now solved the structure of the water channel from Escherichia coli called aquaporin Z. This channel is especially interesting in that it selectively conducts only water at high rates.
Aquaporins form a large, diverse family of proteins and have been found in bacteria, plants, and animals. 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 aquaporin Z channel protein in E. coli can accommodate a flow of water at rates six times higher than GlpF (aquaglyceroporin glycerol facilitator, a channel protein that transports both glycerol and water in E. coli) 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--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 producing a recombinant form of AqpZ in E. coli, 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 is typical of aquaporins, with a spiral of eight oxygens providing water-binding sites inside the channel. The outer membrane and cytoplasmic ends of the channel are wider than the interior, which is long and narrow. This structure demonstrates 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.
Philip Bernstein | PLoS
A Map of the Cell’s Power Station
18.08.2017 | Albert-Ludwigs-Universität Freiburg im Breisgau
On the way to developing a new active ingredient against chronic infections
21.08.2017 | Deutsches Zentrum für Infektionsforschung
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
21.08.2017 | Materials Sciences
21.08.2017 | Health and Medicine
21.08.2017 | Materials Sciences