Two major models have been proposed to explain how the direction of organelle transport along microtubules is controlled, and beginning on page 155, Reese and Haimo provide significant new support favoring one model over the other. Kinesin and dynein, the microtubule motor families responsible for transporting organelles in opposite directions, have been studied extensively, but it remained unclear how the overall direction of transport is controlled. In one model, motors cyclically bind to, and dissociate from, organelles, and the types of motors present on an organelle determine its direction of movement. In the other model, both types of motors remain on organelles, but their activity is differentially regulated.
Using frog melanophores, the authors find that cytoplasmic dynein, dynactin, and kinesin II remain attached to pigment granules, whether the granules are being aggregated or dispersed. However, motors and dynactin from...
