Demonstration of a rapidly transported neuronal protein associated with the myelin fraction of nerve
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An in vitro preparation was utilized to determine if a rapidly transported protein which appears in the myelin fraction of nerve is similar to rapidly transported proteins which have previously been observed released from nerve axons. Three sciatic nerves, and their accompanying eighth and ninth dorsal root ganglia were isolated from frogs (Rana catesbieana), and placed in a modified Warburg flask. A center well of the flask contained the ganglia, bathed in 0.2 ml Ringer solution containing 100 (mu)Ci ('3)H-leucine. The nerves passed from the center well through a hole to an outer well containing 3.0 ml Ringer solution with cycloheximide, but without radiolabeled leucine. After 24 hours, the nerves were removed, and the ganglia excised and discarded. Myelin proteins were isolated from the nerves and separated by sodium dodecyl sulfate gel electrophoresis, and the radiolabeled profile of the gel determined. Radiolabeled transported proteins released by the nerve into the outer well were also electrophoresed, and their radiolabeled profile determined. To compare the myelin proteins synthesized by the Schwann cells with the myelin proteins labeled by the neuron and with the proteins released from the axons, the nerves, without the ganglia, were incubated in 3.0 ml Ringer containing 300 (mu)Ci ('3)H-leucine for 24 hours. The myelin proteins were examined as above. The results show that the neuronally labeled and transported protein isolated with myelin is of high molecular weight, about 100k daltons, whereas the proteins synthesized by Schwann cells correspond to the major myelin proteins of the peripheral nervous system, the P(,0), P(,1), and P(,2) proteins, with molecular weights of 40k, 28k, and 19k daltons, respectively. The protein released by the nerve axons is similar to the neuronally labeled and transported protein isolated with myelin, in that it also has a molecular weight of about 100k daltons. This conclusion was strengthened by the observation that the two proteins co-electrophorese when placed on the same gel. The results show that the neuronally labeled and axonally transported protein from isolated myelin can be released from isolated myelin by treatment with 10 mM EDTA, thus indicating that the bonding to myelin is dissociable.