TRANSPORT OF CYTOSKELETAL ELEMENTS IN THE SQUID GIANT-AXON

In order to explore how cytoskeletal proteins are moved by axonal tran sport, we injected fluorescent microtubules and actin filaments as wel l as exogenous particulates into squid giant axons and observed their movements by confocal microscopy. The squid giant axon is large enough to allow even cytoskeletal assemblies to be injected without damaging the axon or its transport mechanisms. Negatively charged, 10- to 500- nm beads and large dextrans moved down the axon, whereas small (70 kDa ) dextrans diffused in all directions and 1000-nm beads did not move, Only particles with negative charge were transported. Microtubules and actin filaments, which have net negative charges, made saltatory move ments down the axon, resulting in a net rate approximating that previo usly shown for slow transport of cytoskeletal elements. The present ob servations suggest that particle size and charge determine which mater ials are transported down the axon.