High-resolution mapping of intracellular fluctuations using carbon nanotubes

Motors stirring within the living cell Cytoskeletal dynamics is key to cellular function. At very short time scales, thermal motions probably dominate, whereas on time scales from minutes to hours, motor-protein-12–based directed transport is dominant. But what about the times in between? Fakhri et al. tracked kinesin molecules labeled with carbon nanotubes and monitored their motion in living cells for milliseconds to hours. The kinesins motored along microtubule tracks, but sometimes moved more randomly as the tracks themselves were moved by active, larger-scale cell movements. This active “stirring” of the cytoplasm may play a role in nonspecific transport. Science, this issue p. 1031 Random active stress fluctuations, detected by tracking labeled kinesin motors, stir the cytoplasm of eukaryotic cells. Cells are active systems with molecular force generation that drives complex dynamics at the supramolecular scale. We present a quantitative study of molecular motions in cells over times from milliseconds to hours. Noninvasive tracking was accomplished by imaging highly stable near-infrared luminescence of single-walled carbon nanotubes targeted to kinesin-1 motor proteins in COS-7 cells. We observed a regime of active random “stirring” that constitutes an intermediate mode of transport, different from both thermal diffusion and directed motor activity. High-frequency motion was found to be thermally driven. At times greater than 100 milliseconds, nonequilibrium dynamics dominated. In addition to directed transport along microtubules, we observed strong random dynamics driven by myosins that result in enhanced nonspecific transport. We present a quantitative model connecting molecular mechanisms to mesoscopic fluctuations.