Eukaryotic cells use molecular motors to transport nutrients, chemical signals, and cellular components along cytoskeleton filaments within the cell. Cytoplasmic dynein is the molecular motor responsible for transporting cargo in the retrograde direction along microtubules from the cell periphery toward the cell center. Dynein moves cargos through successive stepping motions where the step size is an integer multiple of the microtubule’s pitch (8 nm). However, it is not yet understood how adenosine triphosphate (ATP) hydrolysis generates the required force to move the cargo, or how many ATPs are hydrolyzed per step. We have developed a purpose-built high-resolution optical microscope which can measure dynein’s molecular motion with molecular resolution of ∼0.1 nm on molecular timescales of ∼1 μs. Whereas previous studies have only been able to measure inter-step statistics (step lengths and dwell times) due to limited resolution, our microscope has sufficient spatiotemporal resolution to resolve the dynamics of a dynein cargo during the short time (∼100 μs) that a dynein motor makes its minimal step of 8 nm. We demonstrate these capabilities by tracking cargos undergoing retrograde axonal transport in neurons, where we observe a combination of stepping and stochastic subdiffusive motion.
Axelrod et al. (Sun,) studied this question.