Slow axonal transport: fast motors in the slow lane.

The bulk of neuronally synthesized proteins destined for the axon is transported in a phase of transport approximately 100 times slower (1mm/day) than the vesicular traffic of fast axonal transport (100mm/day). Of late, a number of studies have shed considerable light on the controversies and mechanisms surrounding this slow phase of axonal transport. Along-standing controversy has centered on the form of the transported proteins. One major transport cargo, neurofilament protein, has now been seen in a number of contexts to be transported primarily in a polymeric form, whereas a second cargo tubulin is transported as a small oligomer. The development of techniques to visualize the slow transport process in live cells has demonstrated that instantaneous motions of transported neurofilaments, and presumably other slow transport cargoes, are fast, bidirectional and interspersed with long pauses. This and additional biochemical efforts indicate that traditional fast motors, such as conventional kinesin and dynein, are responsible for these fast motions.