How much new protein travels by fast transport to the motor axon each day?
The experiments in the previous section did not distinguish among export of new protein to the axon, the dendrites or by exocytosis to the outside of the cell body. We attempted to estimate the fraction of new protein exported daily to the axon by fast transport (McIlwain and Hoke, 1988) by comparing the labeled protein in isolated frog motor neuron cell bodies with labeled protein within the ventral roots. Using a concentric incubation chamber in which axonal labeling via transport could be distinguished from labeling via the extracellular space, we found that only a small fraction (0.5-12%) of the newly synthesized protein appeared in the motor axon by fast transport during the 4h labeling period. We also found evidence that the exported protein did not mix with all the cell body protein before leaving it, indicating that some new proteins are preferentially exported from the cell body. An attempt to block fast axonal transport using monensin was unsuccessful, because this agent show
