Abstract
Intracellular transport is propelled by kinesin and cytoplasmic dynein motors that carry membrane-bound vesicles and organelles bidirectionally along microtubule tracks. Much is known about these motors at the molecular scale, but many questions remain regarding how kinesin and dynein cooperate and compete during bidirectional cargo transport at the cellular level. The goal of the present study was to use a stochastic stepping model to identify specific motor properties that determine the speed, directionality, and transport dynamics of a cargo carried by one kinesin and one dynein motor. The computational model incorporated load-dependent properties of kinesin-1 and dynein-dynactin-BicD2 (DDB) taken from published optical tweezer experiments. Model performance was evaluated by comparing simulations to recently published experiments of kinesin-DDB pairs connected by complementary oligonucleotide linkers. Using motor parameters from single molecule studies, the simulations recapitulated mean experimental cargo velocities, but displayed considerable directional switching and positional fluctuations not seen in experiments. Instantaneous velocity distributions from kinesin-DDB experiments showed a single peak centered around zero, whereas simulated velocity distributions showed a slow plus-end directional velocity peak and two additional peaks corresponding to fast unloaded kinesin and DDB velocities. We hypothesized that directional switching in the simulations resulted from frequent motor detachment events and non-negligible durations during which only one motor is attached. To investigate this hypothesis, we explored how specific parameters in the model contributed to the overall cargo dynamics and found that making DDB detachment insensitive to load and increasing the kinesin-1 reattachment rate to 50 s-1, together with small adjustments in the kinesin stall force and cargo stiffness brought the simulations into alignment with the experiments. These results provide new insights into motor dynamics during bidirectional transport and put forth hypotheses that can be tested by future experiments.
Statement of significance Bidirectional transport of vesicles along microtubules is vital for cellular function, particularly in the highly elongated axons and dendrites of neurons, and transport defects are linked to neurodegenerative diseases. For developing future therapeutic strategies, a better understanding is needed for how motors, cargo adapters, and accessory proteins coordinate their activities to transport cargo to their proper cellular locations. We approached this problem by simulating how antagonistic kinesin and dynein motors compete in pairs. We constrain our simulations by recent experimental results and conclude that the motors spend nearly all their time attached to the microtubule and competing against one another. This behavior is not predicted by existing single-molecule experiments and thus provides new insights into bidirectional transport.
Competing Interest Statement
The authors have declared no competing interest.