Kinesin motion in the absence of external forces characterized by interference total internal reflection microscopy.

We study the motion of the kinesin molecular motor along microtubules using interference total internal reflection microscopy. This technique achieves nanometer scale resolution together with a fast time response. We describe the first in vitro observation of kinesin stepping at high ATP concentration in the absence of an external load, where the 8-nm step can be clearly distinguished. The short-time resolution allows us to measure the time constant related to the relative motion of the bead-motor connection; we deduce the associated bead-motor elastic modulus.