Figure 4. Extended Kinesin Velocity Can Be Rescued by an Assisting Load(A) Schematic representation of a kinesin motor attached to a bead (green, not to scale) and trapped with a stationary laser beam (yellow), shown on the left. Displacement records of WT, 13P, and 14GS kinesin motors at 1 mM ATP show successive runs, stalling, and motor detachment events. The stall force distributions (mean ± SD, N = 100–300) are shown on the right.(B) Kinesin movement under a constant assisting load applied by a force-feedback controlled trap (diagram). Individual traces of WT, 13P, 14GS, and 26P under 3 pN (blue), 6 pN (green) and 9 pN (red) forward load at 1 mM ATP show stepwise movement (inserts).(C) Under forward load, the velocity of WT shows a very small increase, while 13P, 14GS, and 26P mutants speed up considerably with increasing load (mean ± SEM, N = 35–60).(D) Histograms of center-of-mass steps at a 9 pN assisting load show a larger average step size for 26P (11.51 ± 4.89 nm [mean ± SD], N = 305) than WT (7.98 ± 2.89 nm, N = 250).