Characterization of fast and slow transported proteins in dorsal root and sciatic nerve of cat

Labeled components, carried by axoplasmic transport into the cat dorsal roots and sciatic nerves following injection of 3 H‐leucine into the lumbar seventh (L7) dorsal root ganglia, were studied. The pattern of activity present in the subcellular fractions was similar for the two branches of the ganglion neurons entering the nerve and the dorsal root. Fractionation of the proteins present in the high speed supernatant fraction shows that slow transport carries a major proportion of high molecular weight soluble proteins. Further fractination of the soluble proteins with Sephadex G‐200 showed labeled activity present in three peaks: Ia, proteins of 480,000 M. W., lb, proteins of 60,000–120,000 M. W. containing colchicine binding (microtubular) protein, and peak II polypeptides, 5,000–10,000 M. W. Isoelectrofocusing of la proteins gave three peaks at pl 4.0, 4.05, and 4.15, and of lb proteins gave peaks at pl 4.0, 4.45, and 4.8 with several minor peaks also present. A gradual shift in pl values of the labeled proteins from acidic to neutral pH was seen from 3 hr to 7 days after the injections. A small amount of soluble proteins and some free leucine is carried down by fast transport, which is characterized by a high amount of activity in the particulate component. A comparison of the distribution of labeled incorporation in the crest and plateau region of fast transported materials showed a somewhat smaller percentage of activity in the small particulate fraction in the plateau, with more labeling of the soluble fraction and with no free leucine present. The results are in conformity with the concept of a compartmentalization of incorporated proteins in the cell body from which materials of somewhat different compositions later enter the axon for transport.