Allocation of newly synthesized proteins in spinal motoneurons

When vinblastine sulfate is used in vitro to block slow and fast transport of proteins within frog lumbar spinal motoneurons, the amounts of both newly synthesized protein and total protein increase in motoneuronal perikarya. Analyses of motoneurons isolated from control and vinblastine-treated spinal cords show that 1) about 55 p. 100 of the newly synthesized protein is exported from motoneuron cell bodies during a 4 h incubation period; 2) only about 5 p. 100 of the total perikaryal protein is exported during the same period; and 3) less than 10 p. 100 of the labeled protein is exported by fast axonal transport. Thus, a substantial amount of the newly synthesized protein is quickly and preferentially exported from the cell body. It is not known how much of this exported protein reaches the axon by slow transport. However, when interpreted in conjunction with the studies of Schubert, Kreutzberg and Lux (Brain Res. 47, 331-343, 1972), the above data strengthen the possibility that substantial amounts of the new protein made by a motoneuron may be committed to its dendrites.