Kinesin passing permanent blockages along its protofilament track

Uncovering kinesin dynamics in neurites with MINFLUX

Neurons grow neurites of several tens of micrometers in length, necessitating active transport from the cell body by motor proteins. By tracking fluorophores as minimally invasive labels, MINFLUX is able to quantify the motion of those proteins with nanometer/millisecond resolution. Here we study the substeps of a truncated kinesin-1 mutant in primary rat hippocampal neurons, which have so far been mainly observed on polymerized microtubules deposited onto glass coverslips. A gentle fixation protocol largely maintains the structure and surface modifications of the microtubules in the cell. By analyzing the time between the substeps, we identify the ATP-binding state of kinesin-1 and observe the associated rotation of the kinesin-1 head in neurites. We also observed kinesin-1 switching microtubules mid-walk, highlighting the potential of MINFLUX to study the details of active cellular transport.

Toxic effects of zinc ions on kinesin - Potential molecular cause of impaired intracellular transport

In healthy organisms the metabolism of the trace element zinc is well balanced. If this balance becomes destroyed the free zinc level might increase and cause toxic effects. The present study demonstrates that under definite conditions zinc ions are able to inhibit the ATPase activity of neuron-specific KIF5A (kinesin-1). Correspondingly, the motility activity of KIF5A also decreased. The inhibition rates have been found to depend on the magnesium ion concentration. Lowering the magnesium concentration weakens the inhibition. In addition, also decreases of temperature or increasing the ATP concentration result in reduced inhibition. Zinc ion-mediated inhibition of KIF5A activity might be one molecular cause contributing to impaired transport processes within brain and other organs in cases of zinc dyshomeostasis.

Kinesin-1 motors can circumvent permanent roadblocks by side-shifting to neighboring protofilaments

Obstacles on the surface of microtubules can lead to defective cargo transport, proposed to play a role in neurological diseases such as Alzheimer's. However, little is known about how motor proteins, which follow individual microtubule protofilaments (such as kinesin-1), deal with obstacles on the molecular level. Here, we used rigor-binding mutants of kinesin-1 as roadblocks to permanently obstruct individual microtubule binding sites and studied the movement of individual kinesin-1 motors by single-molecule fluorescence and dark-field scattering microscopy in vitro. In the presence of roadblocks, kinesin-1 often stopped for ∼ 0.4 s before either detaching or continuing to move, whereby the latter circumvention events occurred in >30% after a stopping event. Consequently, and in agreement with numerical simulations, the mean velocity, mean run length, and mean dwell time of the kinesin-1 motors decreased upon increasing the roadblock density. Tracking individual kinesin-1 motors labeled by 40 nm gold particles with 6 nm spatial and 1 ms temporal precision revealed that ∼ 70% of the circumvention events were associated with significant transverse shifts perpendicular to the axis of the microtubule. These side-shifts, which occurred with equal likelihood to the left and right, were accompanied by a range of longitudinal shifts suggesting that roadblock circumvention involves the unbinding and rebinding of the motors. Thus, processive motors, which commonly follow individual protofilaments in the absence of obstacles, appear to possess intrinsic circumvention mechanisms. These mechanisms were potentially optimized by evolution for the motor's specific intracellular tasks and environments.

Effects of Obstacles on the Dynamics of Kinesins, Including Velocity and Run Length, Predicted by a Model of Two Dimensional Motion

Kinesins are molecular motors which walk along microtubules by moving their heads to different binding sites. The motion of kinesin is realized by a conformational change in the structure of the kinesin molecule and by a diffusion of one of its two heads. In this study, a novel model is developed to account for the 2D diffusion of kinesin heads to several neighboring binding sites (near the surface of microtubules). To determine the direction of the next step of a kinesin molecule, this model considers the extension in the neck linkers of kinesin and the dynamic behavior of the coiled-coil structure of the kinesin neck. Also, the mechanical interference between kinesins and obstacles anchored on the microtubules is characterized. The model predicts that both the kinesin velocity and run length (i.e., the walking distance before detaching from the microtubule) are reduced by static obstacles. The run length is decreased more significantly by static obstacles than the velocity. Moreover, our model is able to predict the motion of kinesin when other (several) motors also move along the same microtubule. Furthermore, it suggests that the effect of mechanical interaction/interference between motors is much weaker than the effect of static obstacles. Our newly developed model can be used to address unanswered questions regarding degraded transport caused by the presence of excessive tau proteins on microtubules.

The Kinesin-8 Kip3 switches protofilaments in a sideward random walk asymmetrically biased by force

Molecular motors translocate along cytoskeletal filaments, as in the case of kinesin motors on microtubules. Although conventional kinesin-1 tracks a single microtubule protofilament, other kinesins, akin to dyneins, switch protofilaments. However, the molecular trajectory-whether protofilament switching occurs in a directed or stochastic manner-is unclear. Here, we used high-resolution optical tweezers to track the path of single budding yeast kinesin-8, Kip3, motor proteins. Under applied sideward loads, we found that individual motors stepped sideward in both directions, with and against loads, with a broad distribution in measured step sizes. Interestingly, the force response depended on the direction. Based on a statistical analysis and simulations accounting for the geometry, we propose a diffusive sideward stepping motion of Kip3 on the microtubule lattice, asymmetrically biased by force. This finding is consistent with previous multimotor gliding assays and sheds light on the molecular switching mechanism. For kinesin-8, the diffusive switching mechanism may enable the motor to bypass obstacles and reach the microtubule end for length regulation. For other motors, such a mechanism may have implications for torque generation around the filament axis.

Behavior of Kinesin Driven Quantum Dots Trapped in a Microtubule Loop

We report the observation of kinesin driven quantum dots (QDs) trapped in a microtubule loop, allowing the investigation of moving QDs for a long time and an unprecedented long distance. The QD conjugates did not depart from our observational field of view, enabling the tracking of specific conjugates for more than 5 min. The unusually long run length and the periodicity caused by the loop track allow comparing and studying the trajectory of the kinesin driven QDs for more than 2 full laps, i.e., about 70 μm, enabling a statistical analysis of interactions of the same kinesin driven object with the same obstacle. The trajectories were extracted and analyzed from kymographs with a newly developed algorithm. Despite dispersion, several repetitive trajectory patterns can be identified. A method evaluating the similarity is introduced allowing a quantitative comparison between the trajectories. The velocity variations appear strongly correlated to the presence of obstacles. We discuss the reasons making this long continuous travel distances on the loop track possible.

Measuring transport of motor cargos

In this chapter, we describe experimental techniques used in vitro to illuminate how small teams of motors can work to translocate cargos. We will focus on experiments utilizing in vitro reconstitution, artificial or ex vivo purified cargos, and fluorescence imaging. A number of studies have been able to recapitulate the activities of cargo transport driven by small teams of motors elucidating how multiple motors can work together to transport cargos within the cell. Here, we describe some of the methods employed and highlight important experimental details needed to perform these experiments.

Biosensing MAPs as "roadblocks": kinesin-based functional analysis of tau protein isoforms and mutants using suspended microtubules (sMTs)

The concept of a reconstructed microtubule kinesin-based transport system was originally introduced for studies of underlying biophysical mechanisms of intracellular transport and its potential applications in bioengineering at micro- and nanoscale levels. However, several technically challenging shortcomings prohibit its use in practical applications. One of them is the propensity of microtubules to bind various protein molecules creating "roadblocks" for kinesin molecule movement and subsequently preventing efficient delivery of the molecular cargo. The interruption in kinesin movement strictly depends on the specific type of "roadblock", i.e. the microtubule associated protein (MAP). Therefore, we propose to use the "roadblock" effect as a molecular sensor that may be used for functional characterization of particular MAPs with respect to their role in MT-based transport and associated pathologies, such as neurodegeneration. Here, we applied a kinesin-based assay using a suspended MT design (sMT assay) to functionally characterize known MAP tau protein isoforms and common mutations found in familial frontotemporal dementia (FTD). The proposed sMT assay is compatible with an on-chip format and may be used for the routine characterization of MT associated molecules applicable to diagnostics and translational research.

Kinesin assembly and movement in cells

Long-distance transport in eukaryotic cells is driven by molecular motors that move along microtubule tracks. Molecular motors of the kinesin superfamily contain a kinesin motor domain attached to family-specific sequences for cargo binding, regulation, and oligomerization. The biochemical and biophysical properties of the kinesin motor domain have been widely studied, yet little is known about how kinesin motors work in the complex cellular environment. We discuss recent studies on the three major families involved in intracellular transport (kinesin-1, kinesin-2, and kinesin-3) that have begun to bridge the gap in knowledge between the in vitro and in vivo behaviors of kinesin motors. These studies have increased our understanding of how kinesin subunits assemble to produce a functional motor, how kinesin motors are affected by biochemical cues and obstacles present on cellular microtubules, and how multiple motors on a cargo surface can work collectively for increased force production and travel distance.