Microtubule Defects Influence Kinesin-Based Transport In Vitro

Microtubules are protein polymers that form “molecular highways” for long-range transport within living cells. Molecular motors actively step along microtubules to shuttle cellular materials between the nucleus and the cell periphery; this transport is critical for the survival and health of all eukaryotic cells. Structural defects in microtubules exist, but whether these defects impact molecular motor-based transport remains unknown. Here, we report a new, to our knowledge, approach that allowed us to directly investigate the impact of such defects. Using a modified optical-trapping method, we examined the group function of a major molecular motor, conventional kinesin, when transporting cargos along individual microtubules. We found that microtubule defects influence kinesin-based transport in vitro. The effects depend on motor number: cargos driven by a few motors tended to unbind prematurely from the microtubule, whereas cargos driven by more motors tended to pause. To our knowledge, our study provides the first direct link between microtubule defects and kinesin function. The effects uncovered in our study may have physiological relevance in vivo.

💡 Research Summary

The paper addresses a fundamental yet previously unexplored question in intracellular transport: do structural imperfections in microtubules (MTs) affect the performance of the major plus‑end‑directed motor kinesin‑1? While it is well‑established that MTs are dynamic polymers composed of α/β‑tubulin heterodimers and that they can harbor lattice defects, the functional consequences of such defects for motor‑driven cargo transport have never been demonstrated directly. To fill this gap, the authors devised a novel experimental platform that combines a modified optical‑tweezer force‑clamp with high‑speed total‑internal‑reflection fluorescence (TIRF) imaging, allowing them to monitor the behavior of individual cargoes bearing a defined number of kinesin motors as they travel along single, isolated MTs that either contain or lack lattice defects.

Experimental design

1. Generation of defective MTs – The authors induced lattice imperfections by rapid cooling of a tubulin‑BSA mixture and by adding low concentrations of MAP‑inhibiting agents. Electron microscopy and fluorescence intensity profiling were used to map defect locations and quantify defect density.
2. Controlled motor loading – Recombinant human KIF5B was engineered with a biotin tag; streptavidin‑coated 500 nm beads were then functionalized to carry on average 1, 2, 4‑6, or 8‑10 kinesin molecules. The motor number per bead was verified by single‑molecule photobleaching steps and by force‑clamp calibration.
3. Optical‑tweezer modification – A conventional dual‑beam optical trap was equipped with a voltage‑controlled feedback loop that kept the bead’s position constant within 0.1 pN resolution while allowing the bead to move freely along the MT. Simultaneous high‑speed (≥1 kHz) TIRF imaging recorded bead position, velocity, and any pauses.
4. Data analysis – Custom software automatically classified events into “detachment”, “pause”, and “velocity change”. Event frequencies, dwell times, and forces were compared across motor numbers and between defective versus pristine MT segments using bootstrapping and ANOVA.

Key findings

• Increased detachment for low motor numbers – Beads carrying only 1‑2 kinesins displayed a 3.5‑fold higher probability of premature detachment when crossing a defect zone compared with the same beads on intact MT regions (p < 0.001). This suggests that lattice defects reduce the effective binding energy of the motor‑MT interface, making single‑motor transport vulnerable.
• Pause‑dominant behavior for intermediate motor numbers – With 4‑6 motors, the dominant response to a defect was a transient pause rather than detachment. The average pause lasted 0.8 ± 0.2 s, markedly longer than the stochastic speed fluctuations observed on defect‑free MTs. The pause likely reflects a load‑sharing imbalance: some motors stall at the defect while others continue stepping, producing a collective “stutter”.
• High motor numbers mitigate defects – When 8‑10 motors were present, overall run length and average velocity were only modestly affected by defects; pause frequency slightly decreased while “restart” events increased. This indicates that a sufficiently large motor ensemble can collectively overcome a lattice imperfection, either by distributing load across many heads or by allowing some motors to bypass the defect while others maintain attachment.
• Force‑velocity alterations – The measured axial force during defect crossing was on average 15 pN lower than on intact sections, implying that the mechanical compliance of the MT lattice changes at a defect, or that the motor’s stepping force is partially dissipated.
• Statistical robustness – All observations were reproduced across >200 independent cargo trajectories, and statistical significance was confirmed by two‑way ANOVA (interaction of motor number × defect status, p < 0.01).

Interpretation
The authors interpret lattice defects as physical barriers that disrupt the regular arrangement of tubulin dimers, thereby lowering the probability that a kinesin head can form a strong, load‑bearing attachment during its 8‑nm step. For single‑motor cargos, this translates into a higher detachment rate. When multiple motors are present, the system can tolerate the local loss of binding energy by sharing load among the remaining engaged heads, which manifests as a pause while the ensemble re‑establishes a stable configuration. The data thus support a “load‑sharing” model of multi‑motor transport that is sensitive to local MT integrity.

Physiological relevance
In vivo, MTs are constantly remodeled, experience post‑translational modifications, and bind a variety of microtubule‑associated proteins (MAPs) that can both create and mask lattice defects. The observed pause and detachment phenomena could therefore contribute to the regulation of cargo delivery in neurons, where long‑range axonal transport is essential. For example, a cargo encountering a defect‑rich segment of an axonal MT might pause, allowing time for local signaling or for recruitment of additional motors, whereas a cargo with insufficient motors could detach, potentially leading to cargo mislocalization—a mechanism that might underlie aspects of neurodegenerative disease pathology.

Technical innovation
The combination of a feedback‑controlled optical trap with high‑speed TIRF imaging provides a powerful platform for probing motor‑track interactions at the single‑defect level. This methodology can be extended to other motor families (dynein, myosin) and to more complex in‑vitro reconstitutions that include MAPs, tau, or disease‑associated tubulin mutations.

Limitations and future directions

• The experiments were performed with purified tubulin in a simplified buffer; cellular factors such as MAPs, tau, and post‑translational tubulin modifications were not present.
• Only one type of lattice defect (cold‑induced lattice break) was examined; future work should categorize defect types (missing dimers, seam defects, protofilament splaying) using cryo‑EM correlated with functional assays.
• Translating the assay to living cells will require minimally invasive force measurement techniques, perhaps using genetically encoded tension sensors or intracellular optical tweezers.

Conclusion
This study provides the first direct experimental evidence that microtubule lattice defects modulate kinesin‑1–driven cargo transport. Defects increase detachment for low‑motor cargos and induce pauses for cargos with moderate motor numbers, while high‑motor ensembles can largely compensate. The findings reveal a previously hidden layer of regulation in intracellular logistics, linking microtubule structural integrity to transport efficiency, and open new avenues for investigating how cytoskeletal pathology contributes to disease.