First-passage-probability analysis of active transport in live cells.

The first-passage-probability can be used as an unbiased method for determining the phases of motion of individual organelles within live cells. Using high speed microscopy, we observe individual lipid droplet tracks and analyze the motor protein driven motion. At short passage lengths ( ) as a function of the passage length (L), averaged over a number of runs for a single lipid droplet, follow a power law distribution ~L(?), ?>2, at short times due to a passive subdiffusive process. This changes to another power law at long times where 1 ~t(?), where 0lipid droplets add additional independent Gaussian peaks to a cumulative first-passage-probability distribution indicating that the speeds of sequential phases of motion are independent and biochemically well regulated. As a result we propose a model for motor driven lipid droplets that exhibits a sequential run behavior with occasional pauses.