Why do moving protein filaments switch from swirls to aligned order?
As treadmilling FtsZ filaments get more crowded they straighten out, so their collective pattern changes from rotating chiral rings to a nematic, liquid-crystal-like order, and a flexible-filament model reproduces this.
Source
Chiral and nematic phases of flexible active filaments
Study at a glance
- Design
- Computational / modelling — In vitro reconstituted FtsZ filaments on supported lipid bilayers imaged by TIRF, STED and high-speed AFM, compared quantitatively with coarse-grained simulations of self-propelled semiflexible chiral filaments.
- N
- No single sample size; experiments span a range of FtsZ concentrations and simulations span density, flexibility and attraction; the defect count comes from four HS-AFM measurements.
- Population
- Membrane-bound treadmilling FtsZ filaments (wild type and the L169R mutant) and simulated active filaments
- Outcome
- Large-scale pattern (disordered, chiral rings, nematic), filament curvature, ring probability and lifetime, topological defect density
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The authors attached FtsZ, a bacterial filament-forming protein that moves by treadmilling, to a lipid membrane and raised its concentration from about 0.6 to 5.0 µM while filming the patterns with fluorescence microscopy and high-speed atomic force microscopy. They built a simulation of self-propelled, naturally curved, semiflexible filaments and varied density, stiffness and attraction to find which settings matched the experiments. They then tested a prediction with a mutant (L169R) whose filaments were expected to be straighter and stiffer.
What they found
At low density single filaments moved on curved paths about 1.16 µm across, at intermediate density they formed chiral rotating rings lasting about 5-6 minutes, and at high density the rings disappeared. Simulations matched these two transitions only for moderately flexible filaments (flexure number about 40), not very rigid or very floppy ones. High-speed AFM confirmed that filaments straighten as density rises (curvature fell twofold) and showed half-integer nematic defects; the stiffer mutant formed no rings or directional flows.
The limits
What it doesn't show
The model is deliberately minimal and ignores details such as finite filament lifetimes or stress-dependent growth, so agreement does not prove it is the only mechanism. Filament curvature could not be measured directly with fluorescence microscopy, and the defect density comes from only four AFM measurements. Results are from a flat reconstituted membrane, so links to the Z-ring inside dividing bacteria remain suggestive rather than tested.
Key terms
- Active matter
- Systems whose parts consume energy to move themselves, producing collective patterns not possible at equilibrium.
- Treadmilling
- Motion of a filament that grows at one end and shrinks at the other, so it appears to travel although its subunits stay put.
- Nematic order
- Alignment of rod-like objects along a common axis without a preferred head-tail direction.
- Flexure number
- Ratio of self-propulsion force to bending rigidity; larger values mean more flexible filaments.
- Topological defect
- A point where the local alignment pattern cannot be smoothly continued, such as a +1/2 or -1/2 defect in a nematic.
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Quiz yourself
What controlled the transition from chiral rings to nematic order?
Common questions
Why does crowding make the filaments straighter?
Neighbouring filaments push against each other sterically; activity makes local density fluctuate strongly, so many filaments feel a much higher density than average and straighten gradually.
Why do rings disappear at high density?
Rings rely on filaments being curved; once crowding straightens them, the chiral symmetry is lost and the system organises like a nematic liquid crystal instead.
How did the mutant test the model?
The L169R filaments were straighter, longer and stiffer; plugging these measured properties into the model predicted no polar streams or rings, which is what the experiments showed.
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