Processive behaviour of kinesin observed using micro-fabricated cantilevers.

The mechanical characterization of biomolecular motors requires force sensors with sub-piconewton resolution. The coupling of a nanoscale motor to this type of microscale sensors introduces structural deformations in the motor according to the thermally activated degrees of freedom of the sensor. At present, no simple solution is available to reduce these effects. Here, we exploit the advantages of micro-fabricated cantilevers to produce a force sensor with essentially one degree of freedom and a spring constant of 0.03 pN nm ! 1 for the study of the molecular motor protein kinesin-1. During processive runs, the cantilever constrains the movement of the cargo binding domain of kinesin in a straight line, parallel to the microtubule track, and excludes specific reaction coordinates such as cargo rotation. In these conditions, we measured a step size of 8.0 ± 0. 4n m and a maximal unloaded velocity of 820 ± 80 nm s ! 1 at saturated adenosine triphosphate (ATP) concentration. We concluded that the motor does not need to rotate its tail as it moves through consecutive stepping cycles.