In extracto cryo-EM reveals eEF2 as a major hibernation factor on 60S and 80S particles

This study introduces "in extracto" cryo-EM to achieve near-atomic resolution (~2.2 Å) of cellular lysates, revealing that elongation factor 2 (eEF2) is the predominant hibernation factor bound to both 80S and 60S ribosomes, alongside other regulatory proteins, thereby elucidating diverse native ribosome inactivation mechanisms.

The Gist

Imagine your body is a bustling city, and inside every cell, there are millions of tiny, high-speed factories called ribosomes. These factories are responsible for building the proteins that keep you alive. Usually, when we study these factories, scientists have to take them apart, clean them up, and put them in a test tube to see how they work. It's like studying a car engine by taking it out of the car and putting it on a workbench. You can see the parts clearly, but you miss how it actually runs in the real world, surrounded by traffic, noise, and other cars.

This new paper introduces a revolutionary way to look at these factories: "In Extracto Cryo-EM."

Think of it as taking a high-speed snapshot of the city street while the traffic is still moving, but without the chaos of the whole city getting in the way. Instead of taking the engine out, the scientists gently opened the cell, let the "traffic" (the cellular soup) flow out, and froze it instantly. This allowed them to see the ribosomes exactly as they exist in nature, surrounded by all their natural neighbors.

Here are the key discoveries from this study, explained simply:

1. The "Hibernating" Factories

The scientists found something surprising. In a healthy, busy cell, most ribosomes are working hard, building proteins. But in certain conditions (like when a cell is stressed or hungry), many ribosomes stop working. They don't just shut down; they go into "hibernation."

Imagine a factory worker who stops building and sits down, putting a heavy blanket over the machine to protect it from dust and damage. In the cell, these "hibernating" ribosomes are covered by special proteins that act like a shield. This protects the delicate machinery so it can be woken up quickly when the cell needs to start building again.

2. The Surprise Bodyguard: eEF2

For years, scientists thought a specific protein called eEF2 was only a "construction worker." Its job was to help move the assembly line forward when building proteins.

But this study found that eEF2 is actually a bodyguard too!

• The Discovery: The researchers found eEF2 attached to the ribosomes that were not working (the hibernating ones). Even more surprisingly, they found eEF2 hanging out on the ribosome's "engine block" (the 60S subunit) even when the ribosome was completely empty and not attached to the rest of the machine.
• The Analogy: It's like finding the construction foreman standing guard over a parked, empty truck, even when there's no cargo to move. He's not moving anything; he's just making sure the truck doesn't get damaged while it waits.

3. The "Shield" Team

The study revealed that these hibernating ribosomes are covered by a team of different proteins, each plugging a specific hole to keep the ribosome safe:

• eEF2: Guards the main control center.
• SERBP1 and LARP1: These proteins act like a plug in the "mRNA tunnel" (the pipe where instructions enter). They stop anything from getting in or out.
• eIF5A, CCDC124, and IFRD2: These act as extra locks on the other side of the machine.

Together, they create a "fortress" around the ribosome, ensuring that when the cell is under stress (like a famine or a virus attack), the ribosomes don't get chewed up by the cell's cleanup crew. They are kept in reserve, ready to spring into action the moment the stress passes.

4. Why This Method is a Game-Changer

The paper highlights a new technique called "In Extracto Cryo-EM."

• Old Way (In Vitro): Like studying a car engine on a workbench. You get a very clear picture, but you might miss parts that only appear when the engine is running in a car.
• The Hard Way (In Situ): Like trying to take a photo of a car engine while it's still inside a moving car, surrounded by other cars and people. It's very hard to get a clear picture because everything is crowded.
• The New Way (In Extracto): Like taking the engine out of the car but leaving all the oil, grease, and other fluids around it. You get the clarity of the workbench plus the realistic environment of the car.

This method allowed the scientists to see the ribosomes at a resolution so high (~2.2 Ångströms) that they could see individual atoms, all while the ribosomes were still wearing their "hibernation coats" and surrounded by their natural cellular environment.

The Bottom Line

This study changes how we understand how cells survive stress. We used to think ribosomes just stopped working when things got tough. Now we know they have a sophisticated, multi-layered security system (involving eEF2 and other factors) that actively protects them, keeping them safe and ready to restart production the moment the danger is gone. It's a beautiful example of nature's efficiency: don't throw the machine away; just put a blanket over it and stand guard.

Technical Summary

1. Problem Statement

Structural biology faces a dichotomy in studying macromolecular complexes:

• Single-particle cryo-EM (in vitro): Offers near-atomic resolution (~2–3 Å) but relies on purified components. This approach often misses transient interactions, native cellular contexts, and undiscovered regulatory factors because the complex is artificially reconstituted from a limited set of purified proteins.
• In situ cryo-EM/Cryo-ET: Visualizes complexes within intact cells or tissues, preserving the native environment. However, it suffers from low resolution (typically >10 Å), requires laborious sample preparation (e.g., FIB milling), and involves complex data processing that often fails to reach atomic detail.

There is a critical gap in methods that can achieve high-resolution structural insights while maintaining the complexity of the cellular environment without the need for extensive purification or cellular milling.

2. Methodology: "In Extracto" Cryo-EM

The authors developed a workflow termed "in extracto cryo-EM," which combines the high-resolution capabilities of single-particle cryo-EM with the native context of cellular lysates.

• Sample Preparation:
  • Cell Lines: Used MCF-7 (human breast cancer), BSC-1 (monkey kidney), and Rabbit Reticulocyte Lysate (RRL).
  • Lysis: Employed a mild, rapid lysis protocol using digitonin (a non-ionic detergent) to permeabilize cell membranes while preserving translation-competent ribosome states. The process takes <10 minutes.
  • Conditions: Samples were prepared under normal growth, nutrient-deprived (starvation), and translation-active (with reporter mRNA) conditions.
• Data Collection & Processing:
  • Challenge: Lysates are dense and viscous, containing filaments and organelles that obscure ribosomes, causing standard particle picking algorithms to fail.
  • Solution: Utilized High-Resolution 2D Template Matching (2DTM).
    • A high-resolution 3D template of the vacant 60S ribosomal subunit was used to search micrographs.
    • This method successfully identified ribosomes in dense environments where standard picking failed, yielding particle counts in the hundreds of thousands (e.g., ~525k particles for BSC-1).
  • Reconstruction: Used focused 3D maximum-likelihood classification (in cisTEM/Frealign) with masks on functional sites (A-site, GTPase center) to separate distinct conformational and compositional states.
  • Resolution: Achieved resolutions ranging from ~2.2 Å to ~3.0 Å, comparable to purified single-particle studies.

3. Key Contributions

• Methodological Innovation: Validated "in extracto" cryo-EM as a robust, high-throughput alternative to both purified single-particle cryo-EM and in situ cryo-ET. It allows for the study of native complexes with near-atomic resolution.
• Discovery of eEF2 as a Hibernation Factor: Identified Elongation Factor 2 (eEF2) as the most abundant factor bound to non-translating (hibernating) ribosomes, a role previously underappreciated.
• Unexpected 60S Binding: Revealed that eEF2 binds not only to 80S ribosomes but also to isolated 60S subunits, suggesting a mechanism for preserving subunit integrity during stress.
• Structural Basis of Hibernation: Provided atomic-level details of how hibernation factors (eEF2, SERBP1, LARP1, eIF5A, CCDC124, IFRD2) shield ribosomal functional centers (SRL, Decoding Center, Peptidyl Transferase Center) to prevent degradation.

4. Key Results

A. Ribosome States in Mammalian Lysates (MCF-7 & BSC-1)

• Elongation States: Under normal conditions, ~60–70% of ribosomes are in elongation states (bound to mRNA and tRNAs).
• Stress Response: In nutrient-deprived MCF-7 cells, elongating ribosomes decreased (~58%), while hibernating (non-translating) ribosomes increased from 18% to 27%.
• eEF2 on 60S: A significant fraction of 60S subunits were found bound to eEF2, a finding unexpected as eEF2 is traditionally viewed as an 80S translocase.

B. Rabbit Reticulocyte Lysate (RRL) Analysis

• Hibernation Dominance: In RRL (a standard translation system), only 12% of ribosomes were actively translating a reporter mRNA. The vast majority (60%) were in hibernating states lacking mRNA.
• Composition of Hibernating Complexes:
  • eEF2: Bound to >95% of hibernating 80S ribosomes and 60S subunits.
  • Protective Shielding: The hibernating complexes feature a "shield" of proteins covering all functional centers:
    • eEF2: Binds the Sarcin-Ricin Loop (SRL) and Decoding Center (DC).
    • SERBP1: Occupies the mRNA tunnel (P-site).
    • eIF5A: Extends from the E-site to the Peptidyl Transferase Center (PTC).
    • LARP1: Identified in a new hibernation state, occupying the mRNA tunnel and interacting with eEF2 and other factors (CCDC124 or IFRD2).
• 60S-eEF2 Complexes: eEF2 binds to free 60S subunits in both "open" and "compact" conformations, stabilized by interactions with the SRL and the N-terminal tail of ribosomal protein uL14.

C. Structural Mechanisms

• eEF2 Stabilization: Unlike in translocation (where eEF2 requires GTP and a rotated ribosome), hibernating eEF2 is found in the GDP-bound state.
• uL14 Interaction: The N-terminal tail of ribosomal protein uL14 adopts a specific conformation in hibernating complexes, contacting the switch-I helix of eEF2 and the GDP molecule. This interaction stabilizes eEF2 on the ribosome/subunit without the need for mRNA or tRNA.
• LARP1 Discovery: The study identified LARP1 in hibernating 80S ribosomes, suggesting a role in repressing TOP-mRNA translation and potentially preparing these mRNAs for rapid re-initiation upon stress relief.

5. Significance

• Biological Insight: The study fundamentally changes the understanding of ribosome hibernation. It reveals that eEF2 is a primary hibernation factor, stabilizing ribosomes in a GDP-bound state via interactions with uL14 and the SRL, independent of mRNA.
• Mechanism of Protection: The structural data demonstrates that hibernation factors act as a comprehensive shield, blocking the SRL, Decoding Center, and PTC to protect ribosomes from nucleolytic cleavage during stress.
• Methodological Impact: "In extracto" cryo-EM proves to be a superior method for discovering native cellular interactions that are lost during purification. It offers a flexible, high-resolution platform to study translation regulation, time-resolved dynamics, and stress responses without the artifacts of chemical inhibitors or the resolution limits of in situ tomography.
• Future Applications: This approach can be applied to study other cellular processes, allowing researchers to tune substrate concentrations (e.g., specific mRNAs) in lysates to capture transient intermediates that are difficult to isolate via traditional purification.

In summary, this paper establishes a new paradigm for structural biology, demonstrating that cellular lysates can be used to resolve macromolecular complexes at near-atomic resolution, leading to the discovery of eEF2 as a central regulator of ribosome hibernation and the identification of novel protective complexes involving LARP1 and other factors.