Slow axonal transport driven by directional actin turnover

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Although actin and associated proteins are essential for axonal extension, how they are transported along axons remains unclear. Here we show that actin filaments (F-actins) and associated proteins migrate toward the axonal growth cone by means of directional actin polymerization/depolymerization, called treadmilling. F-actins migrating along axonal shafts underwent treadmilling, with their polymerizing ends oriented toward the growth cone. F-actins were anchored to the plasma membrane through the cell adhesion molecule L1-CAM and the linker protein shootin1. Migration of F-actins depended on their polymerization and substrate anchorage, and directional counter-forces associated with F-actin migration were detected on the substrate. Actin-associated proteins co-migrated with F-actins by interacting with the filaments. Our findings reveal a new mode of intracellular transport, which supplies actin and associated proteins to the axonal leading edge, thereby promoting its protrusion.