Physicists at the University of California, Berkeley, have developed a laser phase plate (LPP) that significantly improves the image resolution of small proteins in cryo-electron microscopy (cryo-EM). By using an intense, continuous-wave laser to shift the electron beam phase, the system provides the contrast necessary to visualize proteins smaller than 70 kilodaltons, a range that previously remained largely invisible to standard imaging techniques.
Why small proteins have challenged cryo-EM
Cryo-EM has fundamentally changed structural biology, but it faces a persistent hurdle: signal-to-noise limitations. According to research published in Science, the technique struggles to capture high-resolution structures for proteins below 70 kilodaltons. This size threshold excludes approximately 90% of the human proteome. While cryo-EM earned a Nobel Prize in 2017 for its ability to bypass the need for protein crystallization, its reliance on electron scattering means that smaller biological molecules often fail to produce a signal strong enough to be distinguished from background noise, effectively leaving most of the human body’s molecular machinery unmapped.
The laser focus used in the new LPP system reaches 75 kilowatts, which UC Berkeley physicist Holger Mueller notes is more powerful than industrial welding equipment.
How the laser phase plate changes imaging
The new LPP functions by shifting the phase of the electron beam itself rather than relying on physical apertures that can dim or destabilize the beam. By applying a continuous-wave laser focus, the system increases the signal-to-noise ratio, allowing researchers to perceive structural details that were previously lost. Holger Mueller, the lead physicist on the project, explains that while large proteins or high-quality, bubble-free samples may not require this intervention, the LPP is essential for small proteins and suboptimal samples. The current research aims to push this capability to visualize proteins as small as 17 kilodaltons.
What happens next for protein structure research
The integration of this technology into broader biological research is already underway. Biohub is currently developing a dual-laser version of the system to reduce component wear and minimize optical aberrations, according to Stephani Otte, Biohub’s vice president of imaging science. This advancement is expected to impact cryo-electron tomography (cryo-ET) as much as traditional cryo-EM. By making the invisible visible, researchers anticipate a clearer understanding of how proteins function in disease states, which could lead to more targeted drug discovery efforts.
If you are working with small, hard-to-crystallize proteins, prioritize samples with minimal impurities. Even with advanced phase-plate technology, sample quality remains the primary determinant of high-resolution success.
Frequently Asked Questions
Why can’t standard cryo-EM see small proteins?
Standard cryo-EM relies on electron scattering to create an image. Small proteins (under 70 kDa) do not scatter enough electrons to create a distinct signal against the background noise of the sample environment.
What is the primary advantage of the laser phase plate?
The LPP creates “true phase contrast” by manipulating the electron beam with a laser, which increases the signal-to-noise ratio without the beam instability or dimming associated with traditional physical phase plates.
How small can this technology currently image?
The research team is currently targeting a threshold of 17 kilodaltons, a size range that would grant access to the vast majority of the human proteome.
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