FIGURE 17-18 Arp2/3-dependent actin assembly during endocytosis. (a) Clathrin-mediated endocytosis is a rapid and ordered process. It has been best studied in yeast, in which the temporal order of specific steps has been delineated. In vivo imaging has shown that endocytosis assembly factors recruit NPFs that activate the Arp2/3 complex. A burst of Arp2/3-dependent actin assembly drives internalized endocytic vesicles away from the plasma membrane in a manner similar to the movement of Listeria. (b) Endosome movement can be reconstituted in vitro. Endosomes isolated from cells that had taken up fluorescently labeled transferrin (red) were added to a cell extract containing fluorescently labeled actin (green). The endosomes bind WASp, which then activates the Arp2/3 complex to assemble actin tails that propel them through the cytoplasm.