Microtubules, motor proteins, and anomalous mean squared displacements

We report an experimental study of the dynamics of particles driven along biopolymer tracks by active protein motors. We use extracts of Xenopus laevis eggs, which support the formation of cytoskeletal networks (microtubules and actin filaments) in vitro and provide all the biochemical factors needed for particle motion. 3 μm polystyrene beads adsorb motor proteins non-specifically and serve as the transport substrate. We observe an enhanced diffusive motion with time scaling of the mean squared displacement as t^3/2. At the shortest times, the data indicate a crossover to a subdiffusive or saturated regime, due to intermittent adhesion of the motor proteins to the microtubules. The time interval over which this behavior was observed varied widely. On inhibiting one or the other family of the microtubule-associated motor proteins, the interval on which such saturation was observed increased, yet the t^3/2 behavior at longer times was unaffected.