Germanium nanospheres for ultraresolution picotensiometry of kinesin motors

Force spectroscopy on single molecular machines generating piconewton forces is often performed using optical tweezers. Since optical forces scale with the trapped particle volume, piconewton force measurements require micron-sized probes practically limiting the spatiotemporal resolution. Here, we have overcome this limit by developing high-refractive index germanium nanospheres as ultraresolution trapping probes. Using these probes, we have dissected the molecular motion of the cytoskeletal motor kinesin-1 that transports vesicles along microtubule filaments. With a superior spatiotemporal resolution, we have resolved a controversy unifying its stepping and detachment behavior. We found that single motors took 4-nm-center-of-mass steps with alternating force dependence of their dwell times. At maximum force, motors did not detach but switched to a weakly bound state. In this state, motors slid on the microtubule with 8-nm steps on a microsecond time scale. Kinesins remained in this intermediate slip state before either truly detaching or reengaging in directed motion. Surprisingly, reengagement and, thus, rescue of directed motion occurred in about 80 percent of events. Such rescue events suggest that macroscopically observed run lengths of individual motors are concatenations and rescues need to be accounted for to understand long-range transport. Furthermore, teams of motors involved in transport may be synchronized through the weakly bound slip state. Apart from ultraresolution optical trapping, germanium nanospheres are promising candidates for applications ranging from nanophotonics to energy storage.