Key points
A theoretical paper by Sorkin & Wingreen (Princeton University) proposes that active phase separation — not molecular motors — can drive directed, self-sustained propulsion of micrometer-scale particles. Below is a structured English summary of the original Chinese forum post.
The core mechanism
The model requires only three components:
1. A phase-separating protein that is sticky (drives droplet formation) in an activated state (e.g., phosphorylated) and soluble in an inactive state. 2. A deactivating enzyme, uniformly distributed in solution, breaking up droplets. 3. A reactivating enzyme, anchored on a micrometer-scale colloidal surface, rebuilding droplets near the colloid.
The resulting inactivation–activation feedback loop works as follows:
- The surface enzyme creates sticky proteins near the colloid, nucleating a droplet.
- The droplet repels the colloid (mutual exclusion assumed), pushing it aside.
- Exposed to the deactivating enzyme, the old droplet dissolves; meanwhile a new droplet nucleates ahead of the colloid.
- Droplets therefore continuously form on one side and vanish on the other — a self-sustaining "molecular propeller."
- Directed intracellular transport is usually attributed to motor proteins (kinesin, dynein, myosin). The authors carefully frame active phase separation as complementary, not a replacement — known motor-linked condensates (e.g., P-granules in *C. elegans* embryos) act as position markers via dissolution/condensation cycles rather than self-propulsion.
- The deeper principle: any system sustaining chemical non-equilibrium can convert free energy into mechanical work. Related phenomena include enzyme-enhanced diffusion and diffusiophoresis of catalytic droplets.
- The finding resonates with Oparin's coacervate hypothesis (1924): primordial droplets concentrating chemistry may also have been able to move without motors, ATP, or genetic information, potentially giving protocells a primitive motility.
- Experimental realization requires protein engineering where phosphorylation state controls phase separation, with one enzyme soluble and the other surface-anchored.
- Droplet–colloid repulsion is a key model assumption that may be more complex in real systems.
- Condensate aging: biomolecular condensates tend to harden into gels or pathological aggregates (as in neurodegenerative disease); a self-propelling droplet must remain liquid.
- Relevance to real cells remains unproven; observed condensate movements typically still involve microtubules and motors.
- Sorkin, B. & Wingreen, N. S. (2026). *Propelling catalytic structures using active phase separation*. arXiv:2604.27965.
- Brangwynne, C. P. et al. (2009). Germline P granules are liquid droplets that localize by controlled dissolution/condensation. *Science*, 324, 1729–1732.
- Alberti, S. & Hyman, A. A. (2021). *Nature Reviews Molecular Cell Biology*, 22, 196–213.
- Ramm, B. et al. (2021). A diffusiophoretic mechanism for ATP-driven transport without motor proteins. *Nature Physics*, 17, 850.
- Demarchi, L. et al. (2023). *Physical Review Letters*, 130, 128401.
- Oparin, A. I. (1924). *The Origin of Life*. Moscow.
- Douliez, J.-P. et al. (2014). *Nature Chemistry*, 6, 527.
The system is isotropic and homogeneous: the direction of motion emerges via spontaneous symmetry breaking, with random fluctuations amplified into a stable heading by positive feedback.
Performance against thermal noise
For a 1 μm particle in water, the Stokes–Einstein diffusivity is roughly \(D = k_B T / (6\pi \eta r) \approx 0.2\,\mu\text{m}^2/\text{s}\), so pure Brownian motion displaces it only ~0.6 μm per second. Simulations show active phase separation can instead propel such colloids at tens to over a hundred micrometers per second — one to two orders of magnitude faster than diffusion. The propulsion is robust: it persists against external forces up to the nanonewton scale, and a trapped colloid experiences repulsive forces of a few nanonewtons (note: single motor proteins like kinesin generate piconewton-scale forces; the two are not directly comparable, since droplet propulsion is a collective emergent effect).
Biological and origins-of-life implications
Open challenges
Conclusion
The study demonstrates in a minimal, autonomous framework that a homogeneous solution containing only phase-separating protein, two enzymes, and a catalytic surface can spontaneously generate persistent directed motion. Thermal noise, far from being an insurmountable barrier, is tamed by non-equilibrium chemistry: fluctuations are amplified by positive feedback into direction, and passive phase separation becomes active propulsion.