Fig. 8. Force distribution on single dynein motors constrained to move at a fixed velocity.

We show the frequency distributions of forces acting on a single motor in an assay where a cargo with one dynein motor attached is moved towards the minus end of the microtubule at fixed velocities of 500nm/s (a), 200nm/s (b) and 50nm/s (c), all at an ATP concentration of 2mM. Panels (d), (e) and (f) show the corresponding traces of the forces plotted versus time to show the predicted force fluctuations. To demonstrate the switching between the high and low-load states as shown in Fig. 8, we used a reduced linker stiffness of k_l = 0.02pN/nm and suppressed detachment. For higher linker stiffness k_l, the force exhibits larger and more frequent fluctuations since a single step of the motor adds a load of k_lΔx, broadening the two peaks of the bimodal force distribution in (b) such that they cannot be resolved, and eliminating the sustained intervals of high and low forces in the force traces (not shown).