Researchers at the Max Delbrück Center have captured the first high-resolution 3D structures of type-1 ryanodine receptors (RyR1) in their native membrane, resolving six distinct stages of channel opening and identifying the physical interface that lets neighboring channels open in synchrony. The work, published in Nature Communications, resolves a mechanistic question that has stood open for nearly 30 years.

The study, led by Vasilii Mikirtumov, PhD, a former doctoral student in the in situ structural biology lab of Misha Kudryashev, PhD, suggests the channel-to-channel interface itself is a candidate therapeutic target for RyR1-linked skeletal muscle diseases, including malignant hyperthermia and congenital myopathies.

Why image RyR1 in its native membrane?

Skeletal muscle contraction depends on calcium release from the sarcoplasmic reticulum (SR), an internal store whose membrane carries thousands of RyR1 channels — the largest known ion channels. Proper contraction requires the channels to open together through "coupled gating," a mechanism described almost 30 years ago but never physically explained. As the authors note, "the physical mechanism orchestrating this cooperativity has remained unknown."

Previous structural work relied on channels extracted from membranes with detergents. "Because RyR1 is a membrane protein, you have to pull it out with detergents to purify it. But that environment can be disruptive to such a sensitive protein," said Mikirtumov, now a postdoctoral researcher in Christian Spahn's lab at Charité – Universitätsmedizin Berlin. "We wanted to capture the structure of the channel in its native membrane and find out whether its opening mechanism looks different there."

The team isolated SR from rabbit muscle and imaged it at the Core Facility for Cryo-Electron Microscopy, run jointly by Charité, the Max Delbrück Center, and the Leibniz-Forschungsinstitut für Molekulare Pharmakologie. "We shoot electrons through the sample and take thousands of pictures, each with many copies of the same protein," Mikirtumov explained. "Then we average them all together, and that gives us a high-resolution 3D reconstruction."

What did the structures show?

By adding small molecules to trigger opening, the researchers trapped RyR1 at six stages between fully closed and fully open. Comparing them revealed the full motion:

• The bulky outer part of the channel rotates within the plane of the membrane, "like turning the ring of a camera lens."
• The pore widens to roughly twice its original width.
• Neighboring channels stay in physical contact throughout the transition.

That contact mediates coupled gating: as one channel rotates open, it strains the interface with its neighbor, lowering the energetic barrier for that channel to open too. "It's like the cogs in a clock," said Kudryashev. "Once one cog turns, it primes its neighbors to turn, too."

Cryo-electron tomography of channel pairs at five opening stages added functional weight to the structural result. Neighboring channels were more likely to be synchronized, and two interacting closed channels were more stable than two closed channels in isolation — evidence that channels hold each other shut. The structures reveal "direct, corner-to-corner physical contacts between receptors, providing a high-resolution view of the interface that mediates coupled gating," the authors wrote.

How does this connect to disease?

Mutations in the RYR1 gene cause malignant hyperthermia, a life-threatening reaction to certain anesthetics, and congenital myopathies that weaken muscle. Many of these mutations map to exactly the region where channels touch their neighbors.

"A lot of these mutations don't seem to affect how a single channel opens, but rather how channels cooperate with their neighbors," Mikirtumov said. "We mapped several of them onto the interface, and we think that in these cases, it's the cooperation between channels that breaks down."

The researchers propose that disruption at the interface makes channels leaky, releasing calcium when they should retain it — reframing RyR1 disease mechanisms from single-channel defects to failures of a mechanically coupled receptor lattice. "We propose a mechanism for RyR1 activation, shifting the focus from an individual channel event to a cooperative process orchestrated by a mechanically coupled receptor lattice," they stated.

The team is already testing the therapeutic implication. "We need to prevent the channels from opening spontaneously," said Kudryashev. "Now that we know how the inactive state is organized, we can design biologics or small molecules to stabilize this closed state." The authors conclude that the results "provide new perspectives on EC-coupling and the development of targeted therapeutics for RyR1-linked channelopathies."