Cytoskeletal motors like kinesin-1 and dynein are necessary for intracellular transport and a variety of other functions in the cell. They have been well characterized in simplified, single-motor, in vitro systems, but less is known about their mechanical properties in vivo, in a more complex, multi-motor environment. In order to better study these properties and their impact on intracellular transport, we have built an optical trap to implement a recently developed theoretical technique which allows us to calibrate and measure forces in a living cell and other viscoelastic environments. We have found that lipid droplets in A549 cells and phagocytosed beads in Dictyostelium cells typically have 1 active plus-end directed motor and 1 active minus-end directed motor. Also, the plus-end motor’s stall force appears to be lower (2-3 pN) than kinesin-1’s in vitro stall force (5-7 pN), while the minus-end motor’s stall force (2 pN) is slightly higher but similar to many in vitro measurements of dynein’s stall force. Stall force measurements made in vitro by measuring the stall force of beads with both kinesin and dynein attached give results similar to those in vivo, supporting a synergistic transport model, in which dynein remains attached to the microtubule at all times, being dragged behind the kinesin and reducing its stall force when the cargo is moving in the plus direction, and being the sole active motor when the cargo is moving in the negative direction.
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