SINGLE-PARTICLE TRACKING - MODELS OF DIRECTED TRANSPORT

Single-particle tracking techniques make it possible to measure motion of individual particles on the cell surface. In these experiments, in dividual trajectories are observed, so the data analysis must take int o account the randomness of individual random walks. Methods of data a nalysis are discussed for models combining diffusion and directed moti on. In the uniform flow model, a tracer simultaneously diffuses and un dergoes directed motion. In the conveyor belt model, a tracer binds an d unbinds to a uniform conveyor belt moving with constant velocity. If a tracer is bound, it moves at the velocity of the conveyor belt; if it is unbound, it diffuses freely. Trajectories are analyzed using par ameters that measure the extent and asymmetry of the trajectory. A met hod of assessing the usefulness of such parameters is presented, and p itfalls in data analysis are discussed. Joint probability distribution s of pairs of extent and asymmetry parameters are obtained for a pure random walk. These distributions can be used to show that a trajectory is not likely to have resulted from a pure random walk.