Forces required of kinesin during processive transport through cytoplasm.

The purpose of this paper is to deduce whether the maximum force, steplike movement, and rate of ATP consumption of kinesin, as measured in buffer, are sufficient for the task of fast transport of vesicles in cells. Our results show that moving a 200-nm vesicle in viscoelastic COS7 cytoplasm, with the same steps as observed for kinesin-driven beads in buffer, required a maximum force of 16 pN and work per step of 1 +/- 0.7 ATP, if the drag force was assumed to decrease to zero between steps. In buffer, kinesin can develop a force of 6-7 pN while consuming 1 ATP/step, comparable to the required values. As an alternative to assuming that the force vanishes between steps, the measured COS7 viscoelasticity was extrapolated to zero frequency by a numerical fit. The force required to move the bead then exceeded 75 pN at all times and peaked briefly to 92 pN, well beyond the measured capabilities of a single kinesin in buffer. The work per step increased to 7 +/- 5 ATP, greatly exceeding the energy available to a single motor.