Deciphering the role of mechanosensitive proteins

14 January 2019

A Croucher Foundation-funded doctoral student is involved in ground-breaking research related to a mysterious family of cellular proteins found to constitute a novel class of mechanosensitive ion channels.

Alex Tsui (Croucher Scholarship 2017), who is completing joint DPhil/PhD degrees at the University of Oxford and Scripps Research in California, has been collaborating with scientists at Scripps, who are uncovering the roles of proteins known as OSCAs and TMEM63s – the former present in plants, the latter in animals. Their findings have recently been published in the journal eLife.

Tsui conducted the computer simulation work that reconstituted the dynamic behaviour of an OSCA protein in its native environment, the cell membrane.

Mechanosensitive ion channels convert biologically relevant physical forces into biochemical signals. For example, a plant's response to environmental cues, such as wind, water currents or physical barriers, depends on mechanosensation. In mammals, sense of touch, pain sensation and blood pressure regulation are performed by mechanosensitive ion channels.

Despite their importance, very little is known about the molecules in plants and animals that perform these functions.

The team of scientists at Scripps Research has advanced understanding of the atomic structure of one member of the OSCA protein family, called OSCA1.2. This has led to insight into how these ion channels do their jobs, information that could be critical in identifying how dysfunctions in mechanosensing play a role in disease.

The work began with the identification and biochemical characterisation of OSCA proteins, largely completed at the Scripps lab of Prof Ardem Patapoutian. Members of Prof Andrew Ward's lab, also at Scripps, then focused on making the OSCA1.2 protein structure visible in high resolution for further study, adopting the Nobel prize-winning technique known as cryo-electron microscopy (cryo-EM) to do so.

The published research showed that OSCA channels are not only pressure-sensitive ion channels, but appear to have held on to their “mechanosensitive” properties as life evolved. Furthermore, similar proteins in animals, TMEM63s, were found to be mechanosensitive.

Tsui was involved in the computer simulation element of the research, performed in the Oxford lab of Prof Mark Sansom. “This computational technique, known as molecular dynamics, permits us to observe how molecules interact with one another within hundreds of nanoseconds,” Tsui explained. One billion nanoseconds are equivalent to one second.

“Our ‘starting material’ was a protein coordinate text file, usually just hundreds of kilobytes, that describes the positions of atoms of the protein in a three-dimensional space,” he said. “We then built a more holistic system, containing lipid molecules found in the cell membrane around the protein.

“A computer software called GROMACS was used to calculate how atoms in the entire system move relative to each other over time, according to principles of Newton's Laws of Motion. By iterating the underlying chemical forces in minuscule time steps, we ended up with a large trajectory file, usually gigabytes in size, that tracks the movement of each of the hundreds of thousands of atoms over time. Finally, we extracted key information about the system, specifically the distribution of lipids around the protein.”

Based on the simulation work, the team was able to observe how the lipid molecules interacted with specific sites on the OSCA1.2 protein, which may serve as the protein's opening switches when tension is applied.

“On the experimental side, there is currently no way of holding these proteins in a stretched state for us to see it at the molecular level,” he said. Current understanding is chiefly based on how they look in a “relaxed” form.

“The on-going computer simulation work may offer a way forward,” Tsui said. Many more parameters in the algorithms would need to be optimised before the stretching scenario could be emanated in a reliable way, he added.

Sansom explained the significance of the research: “This work provides fundamental insights into how a novel class of ion channels works as nanoscopic molecular machines, transforming mechanical stretch into electrical excitability of cell membranes.”

Understanding the basic mechanisms of ion channels in atomic detail was an essential first step in designing drugs to modify their behaviour, he said.

Alex Tsui is currently completing his joint DPhil/PhD degrees at the University of Oxford and Scripps Research. He is now based in Oxford and is due to join Scripps Research (California) in 2020 to work with Prof Andrew Ward. He receives his major funding from the prestigious Skaggs-Oxford Scholarship Programme, with additional support from the Croucher Foundation. He graduated with an MSc ETH in Biology (Structural Biology and Biophysics) from the Swiss Federal Institute of Technology Zurich (ETH Zürich). Prior to that, he completed a BSc in Biochemistry at University College London with First Class Honours (Dean's List) and top of his class.

To view Alex Tsui’s Croucher profile, please click here.