In large cells, cytoplasmic streaming evenly disperses some cellular components and may help to concentrate others at specific locations. In Drosophila oocytes, slow streaming occurs during the localisation of pattern formation determinants; fast streaming cuts in later to evenly mix the yolk. On ,Serbus and colleagues report that while all cytoplasmic streaming in Drosophila oocytes requires the plus-end-directed microtubule motor kinesin-1, dynein (a minus-end-directed motor) and the actin cytoskeleton impede kinesin's fast-streaming activity. The researchers use time-lapse microscopy, suppression of dynein activity, and analysis of kinesin heavy chain mutations to propose a model in which a novel competition between dynein and kinesin initially prevents the parallel ordering of microtubules, allowing only short-range currents. The release of a dynein inhibitor subsequently allows the establishment of a self-amplifying loop of plus-end-directed organelle motion and parallel microtubule orientation, which drives fast streaming.
