Actin (red) is involved in budding of clathrin-coated vesicles from the TGN (green).

A thick mess of cortical actin works intimately with clathrin to assist endocytosis at the plasma membrane, with the actin providing force for vesicle fission, propulsion, or both. Now, Carreno et al. (page 781) show that the less well-known actin around the trans-Golgi network (TGN) also aids in the trafficking of clathrin-coated vesicles from TGN to lysosomes.

The clue for such a link came from studies of Hip1R, a protein that links actin to clathrin-coated endocytic vesicles. When the team knocked down Hip1R expression using RNAi, they found abnormally swollen Golgi cisternae with many extra buds or vesicles stuck to the surface of the TGN. Subsequent fluorescent phalloidin staining of Hip1R-depleted cells also revealed abnormal actin formations at the Golgi surface. Similar actin-based curved structures have also been found when Hip1R is missing from budding sites for clathrin-coated vesicles at the plasma membrane. These parallels suggest that actin is involved in a general clathrin-coated vesicular budding mechanism.

The vesicles coming from the TGN contain the Arp2/3 complex as well as the mannose-6-phosphate receptor that targets proteins to the lysosome. Both depletion of Hip1R and treatment of cells with actin poisons halted proper trafficking of a lysosomal hydrolase, showing that these actin-dependent vesicles were traveling from the TGN to the lysosome. This new function for actin, along with the proximity of the Arp2/3 complex on these vesicles, raises the possibility that the intracellular pathogen Listeria monocytogenes could co-opt vesicle trafficking machinery rather than general cell cytoskeletal components for its movement. ▪