How do motors make microtubule bundles stretch?
Adding a crowding agent packs microtubules more tightly and changes how the filaments move inside a bundle, even though the bundle as a whole keeps stretching.
Source
Structure and dynamics of motor-driven microtubule bundles
Study at a glance
- Design
- Other — In vitro active-matter experiments on reconstituted microtubule-kinesin-14 bundles across PEG depletant concentrations, with photobleaching, single-filament tracking and small-angle X-ray scattering plus scattering-model fits.
- N
- No single sample count reported; many bundles and tracer filaments were analysed per PEG concentration.
- Population
- Reconstituted bundles of stabilized microtubules driven by kinesin-14 motors with 0-1% PEG
- Outcome
- Bleach-line splitting and extension speeds, tracer velocity statistics, and bundle packing lattice from SAXS
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What they did
The authors mixed stabilized microtubules with kinesin-14 motors, which both crosslink and slide filaments, and varied the concentration of the crowding polymer PEG between none and 1%. They photobleached pairs of stripes on bundles to watch how filaments moved apart, tracked rare fluorescently labelled tracer microtubules, and used small-angle X-ray scattering compared with computed scattering curves to infer how filaments were packed.
What they found
Bundles extended along their length and thinned across it at all PEG levels, but the overall extension speed fell from about 8.8 to about 4.5 nm per second as PEG rose. Without PEG each bleached stripe split into two lines moving apart, showing antiparallel sliding, whereas at 1% PEG stripes only broadened; intermediate concentrations showed a mix. Scattering fits indicated open hexagonal packing with about 45.8 nm spacing without PEG, and a tight rectangular lattice at 1% PEG.
The limits
What it doesn't show
The experiments do not show where the motors actually sit inside the bundles, so the idea that tight packing excludes motors from the interior is speculation. The scattering fits use lattices of fewer than ten microtubules and peaks are broad, meaning the packing has only short-range order. The expected exponential (telescoping) extension could not be distinguished from linear growth over the short distances measured, and a faster net extension despite mostly sliding motion at low PEG remains unexplained.
Key terms
- Active matter
- Materials whose components consume energy locally to generate forces and motion, driving the system away from equilibrium.
- Kinesin-14
- A motor protein with one moving motor domain and one passive microtubule-binding domain, so it both crosslinks filaments and slides them past each other.
- Depletion agent (PEG)
- A crowding polymer that creates an effective attraction between large objects like microtubules, pushing them together.
- Extensile bundle
- A bundle that lengthens along its axis while narrowing across it because internal forces push filaments apart lengthwise.
- Small-angle X-ray scattering (SAXS)
- A technique that measures X-rays scattered at small angles to infer nanometre-scale spacings and arrangements in a sample.
Flashcards
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Quiz yourself
What did splitting of a bleached stripe indicate?
Common questions
What does a bleached stripe splitting in two mean?
It means filaments within the stripe move in two opposite directions at a steady speed, the signature of antiparallel filaments sliding past each other.
If sliding dominates at low PEG, why is net extension faster there?
The authors do not fully explain this; they suggest other factors, such as an effective friction inside the bundle, set how much sliding turns into net extension.
Why does PEG change the packing?
PEG creates an osmotic pressure that pushes filaments together; at high enough levels microtubules pack into a dense rectangular lattice, as previously seen in passive bundles.
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