Molecules in cells move following complex patterns, which operate on a range of different timescales: Brownian fluctuations typically dominate at short timescales whereas directed motion takes over at longer scales. Frederick MacKintosh, Christoph Schmidt and colleagues at Georg-August University, Rice University and Vrije University now report a study of intracellular dynamics that ranges over five orders of magnitude in time and identifies an intermediate regime of molecular transport.
The researchers follow the motion of kinesin proteins — molecular motors that move along microtubule tracks — by mapping the fluorescent signal from carbon nanotubes attached to the proteins. By acquiring the fluorescent signal at an intermediate rate of four frames per second, they show that, in addition to thermal diffusion and directed motor activity, kinesins exhibit a vigorous random motion, while still attached to the microtubules.
Schmidt and colleagues attribute this finding to the action of myosin proteins that indirectly agitate kinesin's tubulin tracks. Tubulin forms strong filaments that are embedded in a network of more flexible actin filaments. The myosins exert a mechanical stress on the actin network. This stress is then released in the form of random stirring of the whole filament network, including the microtubules. This random stirring, independently of the directed kinesin motion, enhances molecular transport at intermediate timescales.