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

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Abstract
Molecular motors: By combining optical tweezers and high‐speed particle tracking, individual steps of microtubule motor proteins transporting organelles can be detected under known force loads in living mammalian cells (see figure). 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‐millisecond time resolution. Using an EM‐CCD 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 different force regimes.