“Despite how different Venus fly traps are from humans, studying the structure and function of these mechanosensitive channels gives us a broader framework for understanding the ways that cells and organisms respond to touch and pressure,” says co-senior author and Scripps Research professor Andrew Ward, PhD.

“Every new mechanosensitive channel that we study helps us make progress in understanding how these proteins can sense force and translate that to action and ultimately reveal more about human biology and health,” adds co-senior author Ardem Patapoutian, PhD, a Scripps Research professor who won the Nobel Prize in Physiology or Medicine for research on the mechanosensitive channels that allow the body to sense touch and temperature.

Mechanosensitive ion channels are like tunnels that span the membranes of cells. When jostled by movement, the channels open, letting charged molecules rush across. In response, cells then alter their behavior — a neuron might signal its neighbor, for instance. The ability for cells to sense pressure and movement is important for people’s senses of touch and hearing, but also for many internal body processes — from the ability of the bladder to sense that it’s full to the ability of lungs to sense how much air is being breathed.

Previously, scientists had homed in on three ion channels in Venus fly traps thought to be related to the ability of the carnivorous plant to snap its leaves shut when its sensitive trigger hairs get touched. One, Flycatcher1, caught researchers’ attention because its genetic sequence looked similar to a family of mechanosensitive channels, MscS, found in bacteria.

“The fact that variants of this channel are found throughout evolution tells us that it must have some fundamental, important functions that have been maintained in different types of organisms,” says co-first author Sebastian Jojoa-Cruz, a graduate student at Scripps Research.

In the new study, the researchers used cryo-electron microscopy — a cutting-edge technique that reveals the locations of atoms within a frozen protein sample — to analyze the precise arrangement of molecules that form the Flycatcher1 protein channel in Venus fly trap plants. They found that Flycatcher1 is, in many ways, similar to bacterial MscS proteins — seven groups of identical helices surrounding a central channel. But, unlike other MscS channels, Flycatcher1 has an unusual linker region extending outward from each group of helices. Like a switch, each linker can be flipped up or down. When the team determined the structure of Flycatcher1, they found six linkers in the down position, and just one flipped up.

“The architecture of Flycatcher1’s channel core was similar to other channels that have been studied for years, but these linker regions were surprising,” says Kei Saotome, PhD, a former postdoctoral research associate at Scripps Research and co-first author of the new paper.

To help elucidate the function of these switches, the researchers altered the linker to disrupt the up position. Flycatcher1, they found, no longer functioned as usual in response to pressure; the channel remained open for a longer duration when it would normally close upon removal of pressure.

“The profound effect of this mutation tells us that the conformations of these seven linkers is likely relevant for how the channel works,” says co-senior author Swetha Murthy, PhD, of Vollum Institute at Oregon Health and Science University, a former postdoctoral research associate at Scripps Research.

Now that they solved the molecular structure, the research team is planning future studies on the function of Flycatcher1 to understand how different conformations affect its function. More work is also needed to determine whether Flycatcher1 is solely responsible for the snapping shut of Venus fly trap leaves, or whether other suspected channels play complementary roles.

In addition to Jojoa-Cruz, Saotome, Murthy, Patapoutian and Ward, authors of the study, “Structural insights into the Venus flytrap mechanosensitive ion channel Flycatcher1,” are Che Chun Alex Tsui and Wen-Hsin Lee of Scripps Research, and Mark Sansom of University of Oxford.

This work and the researchers involved were supported by funding from the National Institutes of Health (R01 HL143297, R01 HL143297), a Ray Thomas Edwards Foundation grant, the Wellcome Trust (grant 208361/Z/17/Z), the Biotechnology and Biological Sciences Research Council (grants BB/N000145/1 and BB/R00126X/1), the Engineering and Physical Sciences Research Council (grant EP/R004722/1), a postdoctoral fellowship from the Jane Coffin Childs Memorial Fund for Medical Research, the Skaggs-Oxford Scholarship, the Croucher Foundation, and the Howard Hughes Medical Institute.

These detailed findings include 3D views of the structure and position of the virus’ envelope “spike” proteins (the Env protein, used when the virus binds with cells) in the context of the full virus. Normally, researchers view the protein particles separated from the virus or expressed as engineered or purified proteins. In another key development, the scientists shed new light on the glycan shields — the sugars on the proteins that can hide it from the body’s immune system.

“We’re looking at the whole virus particle and how this protein on the surface relates to the rest of the virus,” said lead author Kelly Lee, associate professor of medicinal chemistry in the UW School of Pharmacy. “And by looking at the intact virus structure, we can see how the different facets of this ‘face of the virus’ are being displayed and how they would be recognized by or hidden from the immune system.”

This intact view of the virus also allowed the scientists to gain new insights into positioning of the envelope spike protein on the surface relative to the internal protein structure, called the Gag lattice.

“This finding overturns previous models of how the parts of the viruses are assembled and helps to focus our attention on where the docking interaction of these two proteins is likely to be,” Lee said. “This interaction needs to be resolved in more detail, but at least the current work gives us the correct architectural model for the virus assembly.”

It was this particular finding that led to the title of the paper — “Cryo-ET of Env on intact HIV virions reveals structural variations and positioning on the Gag lattice” — published Feb. 4 in the journal Cell. Another finding that had not been previously observed, the scientists pointed out, is that the “stalk” supporting the envelope proteins is flexible and can tilt, presenting both opportunity and challenges to the immune system’s neutralizing antibodies that protect cells from infection.

“Structural biology has driven HIV vaccine design, so as we get a better and better picture of what it is we’re targeting, that inspires innovation and may lead to improved vaccines,” said co-corresponding author Michael Zwick, associate professor of immunology and microbiology at the Scripps Research Institute.

HIV’s envelope presents a particularly difficult target for vaccine development, Zwick added, because the virus displays so few spikes and camouflages them with sugar molecules so as to evade our immune system.

“All these features increase the dynamic variability that the HIV spike protein presents to the immune system,” said Lee, who also directs a UW lab exploring virus structure and dynamics. “This is something that people in HIV vaccine development have grappled with from the very beginning — this virus mutates and changes itself astronomically and rapidly. Each time it infects an individual, you end up with literally thousands of different variants within that one individual, and if you look across populations, it’s multiplied even more.”

In fact, in February, an even more deadly strain of HIV was found to have been circulating in the Netherlands. Luckily, while the strain is a “highly virulent variant,” it still responds to treatment.

“This is just another reminder that these viruses are always changing, so we need scientists to continue studying them,” Zwick said.

Co-authors are Vidya Mangala Prasad, former acting instructor at UW School of Pharmacy, currently at the Indian Institute of Science; Daniel Leaman, Department of Immunology and Microbiology, The Scripps Research Institute; Klaus Lovendahl, Jacob Croft and Edgar Hodge, Department of Medicinal Chemistry, UW School of Pharmacy; and Mark Benhaim, who worked on the project as a graduate student at UW.

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Materials provided by University of Washington. Original written by Jake Ellison. Note: Content may be edited for style and length.