There are no water molecules between the ions in the selectivity filter of potassium

August 09, 2019

Do only potassium ions pass through the selectivity filter of a potassium channel, or are there water molecules between the ions? This question has been a source of controversy for years. Researchers led by Prof. Adam Lange from the Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) in Berlin have now been able to show that water molecules do not co-migrate through the potassium channel. Since the experiments were carried out on cell membranes under natural conditions for the first time, the researchers have strong proof in their hands. Their work has just been published in the journal Science Advances.

Our cells need potassium ions, for example to transmit nerve impulses or to control the heart rate. This is why virtually every human cell - or to be more precise, the membrane of a cell - is equipped with potassium channels. Because potassium channels are of fundamental importance for biological processes, and even the most minute changes can result in serious diseases, the tiny protein molecules are the focus of research efforts around the world. Indeed, in 2003, a US researcher was awarded the Nobel Prize in Chemistry for his elucidation of the structure of potassium channels.

Controversial debate on two different mechanisms

However, the question of how potassium actually passes through the channel in order to cross the cell membrane, still remained unclear. For a long time, it was assumed that each potassium ion was followed by a water molecule and that the elements then lined up, like links in a chain, and passed through the narrowest part of the potassium channel, the so-called selectivity filter, one after the other. This was based on the fact that potassium ions are positively charged and would repel each other without the intermediate water molecules. However, this mechanism was questioned in 2014 by Göttingen researchers led by Prof. Bert de Groot: Computer simulations showed that there are no water molecules in the selectivity filter of potassium channels. However, this did not end the debate. Subsequently, additional studies were published that seemed to support the older mechanism and apparently disproved the new one.

Now researchers from the FMP in Berlin have brought clarity into the controversial debate: Dr. Carl Öster and Kitty Hendriks of Prof. Adam Lange's research group and other colleagues at the FMP were able to show for the first time by means of solid-state nuclear magnetic resonance (NMR) spectroscopy that potassium ions actually do migrate through the potassium channels without water molecules in between. Their findings show that the potassium ions are positioned directly behind each other and push each other through the potassium channel, from bottom to top.

Under natural conditions the selectivity filter of potassium channels is free of water

"The technology we used enables us to look at membrane proteins in real cell membranes under natural conditions, for example at room temperature or physiological salt concentrations," explains Kitty Hendriks. "Thus we have been able to show that under these conditions there is definitely no water between the potassium ions in the selectivity filter."

The first indications of this came from computer simulations and there is also X-ray crystallographic data suggesting the absence of water molecules in the selectivity filter of potassium channels. "However, these investigations were conducted under artificial conditions," emphasizes Dr. Carl Öster. "With our supplementary data obtained through NMR spectroscopy, we now have a heavyweight argument in hand that the newer mechanism is the right one."

The FMP researchers and their colleagues from the Max Planck Institute for Biophysical Chemistry led by Prof. Bert de Groot, whose computer-aided molecular dynamics simulations were also incorporated into the study, have demonstrated that there are no water molecules between the potassium ions.

Progress for research

An important factor in elucidating the mechanism was the fact that the FMP is one of the world's leading centers for NMR spectroscopic investigations and is involved in ongoing further development of this complex technology. "Five years ago, we certainly would not have been able to demonstrate this in such a manner, but now we have reached a point where we are able to effectively answer this important question," said Prof. Adam Lange, head of the research group that focuses on the investigation of membrane proteins, such as ion channels. He adds: "Since the processes in potassium channels are fundamental to our health, our results have great significance that also extends beyond basic research."

The work was financially supported by the European Research Council as part of an ERC grant to Prof. Lange and by the German Research Foundation (DFG; Research Group 2518).

Forschungsverbund Berlin

Related Science Articles from Brightsurf:

75 science societies urge the education department to base Title IX sexual harassment regulations on evidence and science
The American Educational Research Association (AERA) and the American Association for the Advancement of Science (AAAS) today led 75 scientific societies in submitting comments on the US Department of Education's proposed changes to Title IX regulations.

Science/Science Careers' survey ranks top biotech, biopharma, and pharma employers
The Science and Science Careers' 2018 annual Top Employers Survey polled employees in the biotechnology, biopharmaceutical, pharmaceutical, and related industries to determine the 20 best employers in these industries as well as their driving characteristics.

Science in the palm of your hand: How citizen science transforms passive learners
Citizen science projects can engage even children who previously were not interested in science.

Applied science may yield more translational research publications than basic science
While translational research can happen at any stage of the research process, a recent investigation of behavioral and social science research awards granted by the NIH between 2008 and 2014 revealed that applied science yielded a higher volume of translational research publications than basic science, according to a study published May 9, 2018 in the open-access journal PLOS ONE by Xueying Han from the Science and Technology Policy Institute, USA, and colleagues.

Prominent academics, including Salk's Thomas Albright, call for more science in forensic science
Six scientists who recently served on the National Commission on Forensic Science are calling on the scientific community at large to advocate for increased research and financial support of forensic science as well as the introduction of empirical testing requirements to ensure the validity of outcomes.

World Science Forum 2017 Jordan issues Science for Peace Declaration
On behalf of the coordinating organizations responsible for delivering the World Science Forum Jordan, the concluding Science for Peace Declaration issued at the Dead Sea represents a global call for action to science and society to build a future that promises greater equality, security and opportunity for all, and in which science plays an increasingly prominent role as an enabler of fair and sustainable development.

PETA science group promotes animal-free science at society of toxicology conference
The PETA International Science Consortium Ltd. is presenting two posters on animal-free methods for testing inhalation toxicity at the 56th annual Society of Toxicology (SOT) meeting March 12 to 16, 2017, in Baltimore, Maryland.

Citizen Science in the Digital Age: Rhetoric, Science and Public Engagement
James Wynn's timely investigation highlights scientific studies grounded in publicly gathered data and probes the rhetoric these studies employ.

Science/Science Careers' survey ranks top biotech, pharma, and biopharma employers
The Science and Science Careers' 2016 annual Top Employers Survey polled employees in the biotechnology, biopharmaceutical, pharmaceutical, and related industries to determine the 20 best employers in these industries as well as their driving characteristics.

Three natural science professors win TJ Park Science Fellowship
Professor Jung-Min Kee (Department of Chemistry, UNIST), Professor Kyudong Choi (Department of Mathematical Sciences, UNIST), and Professor Kwanpyo Kim (Department of Physics, UNIST) are the recipients of the Cheong-Am (TJ Park) Science Fellowship of the year 2016.

Read More: Science News and Science Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to