Oligomeric tubulin in large transporting complex is transported via kinesin in squid giant axons.

Slow axonal transport depends on an active mechanism that conveys cytosolic proteins. To investigate its molecular mechanism, we now constructed an in vitro experimental system for observation of tubulin transport, using squid giant axons. After injecting fluorescence-labeled tubulin into the axons, we monitored the movement of fluorescence by confocal laser scanning microscopy and fluorescence correlation spectroscopy. Here, from the pharmacological experiments and the functional blocking of kinesin motor protein by anti-kinesin antibody, we show that the directional movement of fluorescent profile was dependent on kinesin motor function. The fluorescent correlation function and estimated translational diffusion time revealed that tubulin molecule was transported in a unique form of large transporting complex distinct from those of stable polymers or other cytosolic protein.