Does RNA length change how protein droplets form and hold together?
Long RNAs sit in the core of protein-RNA droplets and make them more stable, while short RNAs gather at the surface and act like soap that lowers surface tension.
Source
Surfactants or scaffolds? RNAs of varying lengths control the thermodynamic stability of condensates differently
Study at a glance
- Design
- Computational / modelling — Multiscale molecular dynamics: residue-resolution Mpipi simulations of FUS or PR25 with polyU RNA of 20-400 nucleotides, plus a minimal patchy-colloid plus polymer model
- N
- No sample size; simulations used tens to hundreds of protein copies per box (48 FUS copies in the FUS runs) across RNA lengths and RNA/protein ratios.
- Population
- Simulated RNA-protein condensates (FUS, PR25 peptide, polyU RNA) and patchy colloids with self-avoiding polymers
- Outcome
- Critical temperature (condensate stability), density profiles across condensates, interfacial free energy
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors simulated droplets formed by liquid-liquid phase separation of two RNA-binding proteins, FUS (which can condense alone) and the peptide PR25 (which needs RNA), mixed with single-stranded polyU RNA of five lengths from 20 to 400 nucleotides. Keeping total RNA fixed, they varied chain length and RNA-to-protein ratio and estimated each droplet's critical temperature, its internal density profile and its interfacial free energy. They repeated the key tests with a much simpler model of sticky patchy colloids and flexible polymers.
What they found
Longer RNA raised droplet stability: for PR25 at the charge-neutral ratio the critical temperature rose by about 20% going from 20 to 400 nucleotides, while FUS gained only about 3%. Both systems showed reentrant behaviour, stabilising at moderate RNA and dissolving at high RNA, but longer RNA let droplets absorb more RNA before dissolving. In mixed-length droplets, long RNA concentrated in the core and short RNA at the surface, and an RNA-coated surface had roughly half the interfacial free energy of a protein-coated one. The patchy model reproduced the trends, with a 50% versus 8% boost for RNA-dependent versus self-condensing particles.
The limits
What it doesn't show
All results are from coarse-grained simulations with implicit water and a mean-field treatment of salt that ignores ion identity, ion condensation and divalent ions such as magnesium. Only uniform polyU RNA was modelled, not real sequences with secondary structure. Critical temperatures from the constant-pressure runs are bracketed intervals rather than exact values, and interfacial energy could not be computed for FUS droplets because they were too large to converge.
Key terms
- Liquid-liquid phase separation
- Spontaneous demixing of a solution into a dense liquid droplet phase and a dilute phase, like oil in water.
- Critical temperature
- The temperature above which two coexisting phases can no longer exist; used here as a measure of how stable a condensate is.
- Reentrant phase behaviour
- Phase separation that appears as a component is added, then disappears again as more is added.
- Surfactant
- A species that collects at an interface and lowers its free energy (surface tension).
- Electroneutral point
- The RNA-to-protein ratio at which negative RNA charge balances positive protein charge.
- Patchy particle
- A coarse model molecule represented as a sphere with a few sticky binding sites, capturing valency without chemical detail.
Flashcards
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Quiz yourself
Where did long RNA strands accumulate in mixed-length condensates?
Common questions
Why does longer RNA stabilise droplets if the total amount of RNA is the same?
Joining nucleotides into long chains lets one RNA bridge many proteins, increasing the connectivity of the liquid network, and covalent bonds hold together charges that would otherwise repel each other.
Why do short RNAs go to the surface of mixed droplets?
Putting long, well-connected RNA in the core maximises bonding and minimises excluded volume, while short, lower-valency RNA at the interface lowers the surface free energy, just as surfactants do.
Why use a crude patchy-particle model as well?
If a simple model with only valency, binding strength and polymer length reproduces the same trends, the behaviour is a general soft-matter effect rather than a quirk of specific protein sequences.
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