A coat of filamentous actin prevents clustering of late-endosomal vacuoles in vivo

The endocytic pathway depends on the actin cytoskeleton. Actin contributes to internalization at the plasma membrane [1, 2] and to subsequent trafficking steps like propulsion through the cytoplasm [3, 4], fusion of phagosomes with early endosomes [5], and transport from early to late endosomes [6,7]. In vitro studies with mammalian endosomes and yeast vacuoles implicate actin in membrane fusion [8, 9]. Here, we investigate the function of the actin coat that surrounds late endosomes in Dictyostelium [10]. Latrunculin treatment leads to aggregation of these endosomes into grapelike clusters and completely blocks progression of endocytic marker. In addition, the cells round up and stop moving. Because this drug treatment perturbs all actin assemblies in the cell simultaneously, we used a novel targeting approach to specifically study the function of the cytoskeleton in one subcellular location. To this end, we constructed a hybrid protein targeting cofilin, an actin depolymerizing protein, to late endosomes. As a consequence, the endosomal compartments lost their actin coats and aggregated, but these cells remained morphologically normal, and the kinetics of endocytic marker trafficking were unaltered. Therefore, the actin coat prevents the clustering of endosomes, which could be one safeguard mechanism precluding their docking and fusion.