A team of UC Berkeley physicists has developed a laser phase plate that dramatically sharpens images produced by electron microscopes, potentially enabling scientists to study the majority of human proteins that are currently too small to visualize clearly.
A physics team at the University of California – Berkeley has pulled off something biologists have wanted for decades: a way to see the tiniest proteins inside human cells in sharp, reliable detail. By mounting a powerful focused laser onto a state-of-the-art electron microscope, the researchers have boosted the image contrast of cryoelectron microscopy — known as cryo-EM — in a way that could reshape drug discovery and our fundamental understanding of disease. Their findings appear in the June 11 issue of the journal Science.
What Is Cryo-EM, and Why Does Contrast Matter?
Cryo-EM works by flash-freezing biological samples and bombarding them with electrons to produce highly detailed structural images of molecules. The technique earned its inventors the 2017 Nobel Prize in Chemistry, with the Nobel Committee crediting the method because it “both simplifies and improves the imaging of biomolecules. This method has moved biochemistry into a new era.”
Despite its power, cryo-EM has a persistent blind spot: proteins smaller than roughly 70 kilodaltons — a unit of molecular mass — produce images so faint they are nearly impossible to analyze. That size threshold excludes about 90% of the human proteome, the full collection of proteins our bodies produce.
The core problem is contrast. Just as early light microscopes struggled to distinguish transparent cell structures from their equally transparent surroundings, electron microscopes produce low-contrast images of small molecules because those molecules scatter very few electrons. Researchers have long known that a technique called phase contrast — which earned Dutch physicist Frits Zernike the 1953 Nobel Prize in Physics — could theoretically solve this, but applying it to electron beams proved extraordinarily difficult for nearly a century.
A Laser Bright Enough to Bend Electrons
Holger Müller, a UC Berkeley professor of physics and faculty scientist at Lawrence Berkeley National Laboratory, spent 15 years turning a 2010 theoretical proposal into a working instrument. The solution was an extraordinarily intense laser trapped inside a spherical mirrored cavity, bouncing back and forth more than 10,000 times to build up power before being focused onto the electron beam. Müller described the result in striking terms.
“It’s 75 kilowatts focused to a few microns,” Müller said in a news release. “That’s more powerful than what you use for welding. It’s more power than a military laser. It builds up the brightest continuous laser focus ever.”
When that focused laser intersects the electron beam, it shifts the phase of the electrons — essentially nudging the timing of their wave pattern — in a way that converts invisible phase information into visible contrast in the final image. The instrument, which Müller named Theia after the ancient Greek Titaness of light, is built around a customized Thermo Fisher Krios cryo-EM machine purchased with funding from the Chan Zuckerberg Biohub.
“Theia is the Formula 1 microscope,” Müller added. “It has extra electron optics that give it better resolution than the standard cryo-EM, even without the laser. With the addition of the laser phase plate, we hope that it really becomes the world’s best instrument overall.”
What the New Images Show
In their Science paper, Müller’s team tested Theia against six different biological samples of varying sizes and preparation quality. The laser phase plate improved resolution across the board, but the gains were most dramatic for smaller and lower-quality specimens.
“For the most challenging cases — small particles, bad specimens — the laser produces a very considerable advantage,” added Müller.
Among the molecules imaged was hemoglobin, the oxygen-carrying protein in red blood cells, which sits right at the lower limit of what conventional cryo-EM can handle. The laser phase plate pushed imaging capability down to 50 kilodaltons — smaller than hemoglobin — and Müller is targeting 17 kilodaltons, roughly the size of myoglobin, as a near-term goal.
He summarized the practical upshot plainly.
“The bottom line is, if you have a large protein and a really good sample — a fresh one or one frozen without bubbles, for example — you may not need the phase plate to get a single, high-quality image. But for a small protein and a bad sample, laser-on is best,” Müller said. “This could fill an enormous gap in our knowledge of protein structures that can’t be crystallized or are too small for today’s cryo-EM. And it will be revolutionary for cryo-ET.”
A Bigger Prize: Seeing Inside Living Cells
The technique Müller referenced — cryoelectron tomography, or cryo-ET — takes the concept further by stitching together images captured at many angles into a full 3D picture of a molecule inside an actual cell rather than in an isolated solution.
Bridget Carragher, founding technical director of imaging at Biohub in Redwood City, California, who is overseeing a parallel dual-laser instrument at the Biohub imaging lab, described the challenge cryo-ET currently faces.
“With cryo-ET, we’re looking at small, very complicated cellular material that’s incredibly crowded inside the cell,” Carragher said in the news release. “It’s like a forest of trees, and you’re trying to find one leaf on one tree in there. Cryo-ET needs a dramatic step forward in contrast, so we can start to see what’s going on inside the cell. That’s what the laser phase plate promises to give us.”
Müller framed the broader stakes in similarly direct terms.
“Cryo-EM has become the new, fastest-growing method for resolving the structure of biological macromolecules, and cryo-ET is expected to show how these molecules work together in their natural, cellular context,” he said. “But because of signal-to-noise limitations, the majority of human and animal proteins are too small to be analyzed by these methods. The increase in signal-to-noise ratio provided by this laser phase plate is expected to overcome these important limitations.”
Why It Matters for Medicine and Research
For students studying biochemistry, pharmacology or cell biology, the implications are significant. Many disease-related proteins — including those implicated in cancer, neurodegeneration and metabolic disorders — fall below the size threshold current cryo-EM machines can handle. A tool that can image those proteins in their native cellular environment could accelerate the development of targeted drugs and deepen understanding of how diseases begin at the molecular level.
Stephani Otte, Biohub’s vice president of imaging science, described the moment in sweeping terms.
“This technology is a step function change for biology,” Otte said. “We are going to be able to see how molecular machines operate inside the living cell, in context, for the first time. What was once invisible will become visible — and that changes everything about how we understand disease.”
Source: University of California, Berkeley
Source: EurekAlert