Adenosine 5'-triphosphate (ATP) forms protein-free and responsive condensates in crowded environments

This study demonstrates that macromolecular crowding enables ATP to overcome electrostatic and entropic barriers to form protein-free, responsive liquid-like condensates that create distinct microenvironments for selective molecular enrichment and RNA protection, thereby expanding ATP's role beyond an energy carrier to a structural and regulatory architect in cellular and prebiotic contexts.

The Gist

The Big Idea: ATP is More Than Just a Battery

For a long time, scientists thought of ATP (Adenosine 5'-triphosphate) as the cell's "battery." It's the tiny fuel packet that powers almost everything our cells do. We also knew it could stick to proteins to help form "condensates"—which are like tiny, membrane-less bubbles inside cells where chemical reactions happen.

But there was a big mystery: Could ATP form these bubbles all by itself?

The answer used to be "No." Imagine trying to build a sandcastle with sand that is constantly repelling itself and flying apart. ATP is a tiny, highly charged molecule. Because it's so small and electrically "sticky" (in a repulsive way), it usually refuses to clump together. It acts like a "hydrotrope," which is a fancy word for a molecule that keeps things dissolved and prevents them from sticking.

The Discovery:
This paper reveals a surprising twist: If you crowd the room enough, ATP molecules will actually hold hands and form their own liquid bubbles, even without any proteins around.

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The Analogy: The "Crowded Dance Floor"

Imagine a dance floor (the cell) where everyone is trying to dance.

1. The Problem (The Empty Room): If you put a few people (ATP molecules) in a huge, empty room, they will just run around wildly. They are too small, too fast, and they are all wearing "static electricity" suits that push them apart. They can't form a group.
2. The Solution (The Crowded Room): Now, imagine you fill that room with thousands of other people (called crowders, like PEG in the experiment) who are just standing there, taking up space.
3. The Result: Suddenly, the ATP molecules have nowhere to run! The crowd pushes them together. Because they are squeezed so tightly, they can't escape their own repulsion anymore. They start to stick together, forming a dense, liquid-like puddle in the middle of the room.

The Magic Mechanism:
It's not just about being squeezed. The paper found two secret ingredients that make this happen:

• The "Static Shield": The crowd helps screen out the electrical repulsion between ATP molecules, like putting a blanket over static electricity so they stop pushing apart.
• The "Molecular Glue": Once they are close, they start forming hydrogen bonds (like tiny Velcro strips) that hold them together.

What Makes These Bubbles Special?

These ATP bubbles are very different from the usual protein bubbles scientists study.

• They are "Jelly-like," not "Jello-like": They are very fluid. If you poke them, they wiggle and merge instantly. They are much more liquid than the thick, sticky gels usually found in cells.
• They are "Chameleons": They are incredibly sensitive to their environment.
  • Temperature: Heat them up, and they form. Cool them down, and they vanish.
  • Acidity (pH): Change the acidity, and they appear or disappear.
  • Dilution: Add a little water, and they dissolve. Evaporate the water, and they reform.
  • Analogy: Think of them like a smart cloud. If the weather (temperature/pH) changes, the cloud forms or dissolves instantly. It's a dynamic, breathing compartment.

The Superpower: Protecting the "Blueprints"

The most exciting part of the discovery is what happens inside these bubbles.

The researchers tested what happens when they put RNA (the cell's instruction manual) and a "scissor enzyme" (DNAzyme) inside these ATP bubbles.

• In normal water: The scissors cut the RNA immediately.
• Inside the ATP bubble: The scissors stop working. The RNA is safe!

Why?

1. The Acid Trap: The inside of the ATP bubble is surprisingly acidic (like lemon juice). The scissors hate acid and refuse to cut in that environment.
2. The Confusion: The ATP molecules themselves seem to confuse the scissors, making it hard for them to grab the RNA.

The Takeaway:
It's like putting a fragile, important document inside a fortress made of acid. Even if you bring a pair of scissors into the fortress, they can't cut the paper because the environment neutralizes them.

Why Does This Matter?

This changes how we see ATP. It's not just a battery; it's also a structural architect.

• For Biology: It suggests that cells might use ATP to create temporary, protective pockets to store important molecules (like RNA) and shield them from damage when things get chaotic.
• For the Origin of Life: Before life existed, there were no proteins. This discovery suggests that simple molecules like ATP could have formed the very first "protocells" (tiny bubbles) on early Earth. These bubbles could have protected the first genetic codes from being destroyed, allowing life to get its start.

In a nutshell: ATP is like a shy molecule that needs a crowded party to feel comfortable enough to form a club. Once inside that club, it creates a safe, acidic sanctuary that protects precious genetic blueprints from being destroyed.

Technical Summary

1. Problem Statement

Adenosine 5'-triphosphate (ATP) is universally recognized as the primary energy currency of cells and a known modulator of biomolecular condensates formed by proteins and nucleic acids. However, a fundamental thermodynamic paradox exists regarding ATP's ability to form condensates by itself (via homotypic interactions):

• Electrostatic Barriers: ATP is a small molecule with high charge density (triphosphate groups), leading to strong electrostatic repulsion that prevents self-association.
• Entropic Barriers: Its small size confers high diffusivity and translational entropy, making phase separation thermodynamically unfavorable.
• Hydrotropic Nature: Previous studies suggest ATP acts as a hydrotrope, solubilizing protein aggregates rather than condensing.

Consequently, it was widely assumed that ATP requires binding partners (like positively charged proteins) to form condensates. The authors investigate whether ATP can overcome these intrinsic barriers to form protein-free, liquid-like condensates under specific environmental conditions.

2. Methodology

The study employs a multidisciplinary approach combining biophysics, spectroscopy, and chemical biology to characterize ATP phase behavior in crowded environments.

• System Setup: The researchers utilized macromolecular crowding (specifically Polyethylene Glycol, PEG) to mimic the crowded intracellular environment. They tested various crowders (PEG, PEtOx, Dextran, Ficoll) and molecular weights.
• Phase Diagram Construction: By varying ATP and PEG concentrations, they mapped binodal curves to define the boundaries between homogeneous solutions and phase-separated condensates.
• Characterization Techniques:
  • Fluorescence Microscopy: Used FITC-Dextran and Rhodamine-PEG to visualize phase segregation and droplet morphology.
  • ¹H Nuclear Magnetic Resonance (NMR): Analyzed proton chemical shifts of ATP functional groups (adenine base, ribose sugar) to detect changes in hydrogen bonding and electronic environments.
  • Rheology & Dynamics: Measured viscosity and diffusion coefficients using Fluorescence Recovery After Photobleaching (FRAP) and single-particle tracking (Mean Squared Displacement analysis).
  • Perturbation Studies: Systematically introduced salts (NaCl, KCl, MgCl₂, CaCl₂), alcohols (ethanol, 1,6-hexanediol), and hydrogen-bond disruptors (urea) to elucidate interaction mechanisms.
  • Environmental Stimuli: Tested responsiveness to temperature (LCST behavior), pH, and concentration changes.
  • Functional Assay: Utilized a DNAzyme-mediated RNA cleavage reaction as a model to test enzymatic activity and RNA protection within the condensates.

3. Key Contributions

• Discovery of Protein-Free ATP Condensates: Demonstrated that ATP can undergo Liquid-Liquid Phase Separation (LLPS) via homotypic interactions in the presence of macromolecular crowders, without requiring protein binding partners.
• Mechanistic Elucidation: Identified a synergistic mechanism where screened electrostatic repulsion (facilitated by crowding and counterions) and enhanced hydrogen bonding overcome the entropic and enthalpic barriers of ATP self-association.
• Dynamic Responsiveness: Characterized ATP condensates as highly dynamic, reversible systems that respond to temperature, pH, and concentration, contrasting with the stability of typical ATP-protein condensates.
• Functional Regulation: Revealed that ATP condensates create a unique acidic microenvironment that selectively enriches hydrophilic molecules and protects RNA from enzymatic cleavage, suggesting a regulatory role beyond energy transfer.

4. Key Results

A. Formation and Properties of ATP Condensates

• Segregative LLPS: In the presence of PEG, ATP and PEG demix. ATP concentrates in the droplet phase, while PEG remains in the continuous phase.
• Liquid-like Behavior: The condensates exhibit rapid coalescence, spherical morphology, and fast fluorescence recovery (FRAP), indicating liquid-like fluidity.
• Viscosity: The viscosity of ATP condensates is approximately 17.9 mPa·s (about 10x water but significantly lower than protein condensates which are often $10^3$–$10^5$ mPa·s), confirming their highly fluid nature.
• Crowder Dependence: Condensation is driven by excluded volume effects and hydrophobicity. High molecular weight PEG (2K, 8K) and PEtOx induce condensation, while highly hydrophilic crowders (Dextran, Ficoll) do not.

B. Interaction Mechanisms

• Electrostatic Screening: Adding salts (especially divalent cations like Mg²⁺ and Ca²⁺) lowers the critical concentration for phase separation, confirming that screening ATP's negative charges is crucial.
• Hydrogen Bonding: The addition of urea (a hydrogen bond disruptor) dissolves condensates, while alcohols (which lower dielectric constants) promote them.
• NMR Evidence: Increasing PEG concentration causes downfield shifts in ATP proton signals (adenine and ribose), indicating enhanced hydrogen bonding and deshielding, distinct from the upfield shifts seen with simple concentration increases (stacking).

C. Environmental Responsiveness

• Thermal Response: ATP condensates exhibit Lower Critical Solution Temperature (LCST) behavior; they form at higher temperatures (e.g., 60°C) and dissolve upon cooling (20°C).
• pH Response: They show reentrant phase separation. Condensates form at low pH (protonated ATP, reduced repulsion), dissolve at neutral pH, and re-form at high pH (due to high Na⁺ concentration screening repulsion).
• Concentration Response: Dilution triggers rapid dissolution via a "dialytaxis" mechanism (droplets swim to dissolve), while evaporation triggers re-nucleation and growth.

D. Selective Partitioning and RNA Protection

• Partitioning: Hydrophilic molecules (RNA, peptides, calcein) are enriched inside condensates, while hydrophobic molecules are excluded.
• RNA Protection: In a DNAzyme cleavage assay, RNA substrates were enriched >10-fold inside ATP condensates. Paradoxically, cleavage was suppressed by ~84-fold compared to bulk buffer.
  • Causes: (1) The condensate interior is highly acidic (pH ~2.49 vs. 3.42 in bulk), inhibiting enzyme activity; (2) ATP molecules may directly interfere with enzyme-substrate binding.

5. Significance

• Redefining ATP's Role: This work expands the canonical view of ATP from a mere energy carrier to a structural and regulatory architect capable of forming dynamic, protein-free compartments.
• Prebiotic Chemistry: The ability of small molecules like ATP to form dynamic, responsive condensates in crowded prebiotic environments offers a plausible mechanism for the origin of cellular compartments (protocells) before the evolution of complex proteins.
• Biological Implications: The findings suggest that ATP-rich microdomains in cells could serve as protective reservoirs for RNA, regulating gene expression and preventing degradation under stress.
• Biomedical Applications: The stimuli-responsive nature of these condensates (reversible assembly/disassembly) makes them promising candidates for drug delivery systems or synthetic biology applications where controlled release of cargo is required.

In summary, the paper establishes that macromolecular crowding can overcome the intrinsic thermodynamic barriers of ATP, enabling it to self-assemble into functional, responsive, and protective liquid condensates, fundamentally altering our understanding of ATP's role in cellular organization and prebiotic evolution.