Experimental Force-Velocity Relationship for Multiple Motors

Abstract

The relationship between the velocity of one kinesin motor as it works against an increasing opposing load has been well studied. The relationship between the velocity of a cargo being moved against an opposing load by multiple kinesin motors is not well established experimentally. A major difficulty in determining the multiple motors relationship is measuring the number of motors acting against the load.We have developed a method for measuring the number of kinesins moving a microtubule during a gliding assay. When a 4 pN transverse force is applied to the trailing end of a gliding microtubule, the microtubule detaches one-by-one from its anchor points. Using video microscopy, we can determine the distance, d, between the anchor points. We measured d for several values of the kinesin surface density σ. We find that d scales as σ⊥(-2/5), as predicted by the theory of Duke, Holy, and Liebler (1995). From d, we can estimate the number of motors N on a microtubule of length L: N=L/d.In separate experiments, we measured the average velocity of a microtubule gliding anti-parallel to a magnetic force applied to its trailing end. We find that the average velocity, when scaled by the average number of motors, decreases linearly with the force. This suggests that the motors may correlate their stepping during gliding assays.