Probing Dynein and Kinesin Stepping with Mechanical Manipulation in a Living Cell

We report a label free assay for simultaneous optical manipulation and tracking of endogenous lipid droplets as actively transported cargoes in a living mammalian cell with sub milli-second time resoultion. Using an EM-Cd camera as a highly sensitive quadrant detector. we can detect steps of dynein- and kinesin-driven cargoes under known force loads. We can distinguish single and multiple motor-driven cargoes and show that the stall forces for inward and outward transported cargoes are similar By combining the stall force observable with the ability to detect individual steps, we can characterize kinesin and dynein-driven active transport in difference force regimes.