Figure 1: Stochastic model of bidirectional cargo transport.

Kinesin and dynein motors are attached to a shared cargo and motor stepping and detachment/reattachment modeled using published parameters. Single-molecule forward and backward stepping rates, k_for and k_back, depend on load, and motor velocity is equal to the net stepping rate multiplied by the 8 nm step size. The stall force, F_stall, is defined as the force at which k_for = k_back and thus velocity is zero. Motor detachment is modeled as a slip-bond with an exponential dependence on load, $k_{det}\left(F\right)=k_{det}^0{e}^{\frac{F}{F_{det}}}$, where k⁰_det is the unloaded off-rate and F_det is the detachment force parameter. Motors that detach from the microtubule reattach with first-order on-rate, k_reattach. At every time step, each motor can step forward or backward, or detach or reattach from the microtubule. See Methods for model details and parameters.