Force and velocity measured for single kinesin molecules

We measured the force-velocity curves of single kinesin molecules attached to silica beads moving in an in vitro motility assay. Optical trapping interferometry was used to track movement with subnanometer precision and to apply calibrated, pN-sized forces to the beads. Velocity decreased linearly with increasing force, and kinesin molecules moved against applied loads of up to 5-6 pN. Comparison of force-velocity curves at limiting and saturating ATP concentrations suggests that the load-dependent diminution in kinesin velocity may be due to a decrease in the net displacement per molecule of ATP hydrolyzed, not simply to a slowing of the ATP turnover rate; kinesin would therefore appear to be a loosely coupled motor.