Fast axonal transport depends on translocation of organelles in a direction specified by each organelle. How is this translocation powered and how is its direction specified? New insights into both questions have arisen by using videomicroscopy to visualize organelle movement. A component of the axonal transport mechanism has been extracted, purified, and reassembled from axoplasm extruded from the squid giant axon. Organelles translocation in both directions depends on direct interaction with microtubules. A ≈500 kDa protein, kinesin, purified from axoplasm (or brain) promotes translocation exclusively in the anterograde direction and a different translocator promotes retrograde movement. These new results suggest that the direction of movement depends on properties of organelles that enable binding or activation of the appropriate protein translocator.
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