Somal site of synthesis of fast transported materials in mammalian nerve fibers

Abstract

Abstract The crest of labelled materials carried down the fibers of cat sciatic nerve by the fast transport system was blocked by puromycin or cycloheximide injected into the lumbar 7th ganglion 20–30 min before the precursor 3 H‐ leucine. The protein synthesis blocking agents were less effective when given 5 min after 3 H‐leucine injection and ineffective when injected 20 min or more afterwards. Differential centrifugation of the labelled components of nerve showed a relatively high specific activity (counts/min/mg protein) present in the small particulate and supernatant fractions of nerves removed 5–6 hrs after ganglion injection. Sephadex G‐200 column gel filtration of the supernatant fraction showed the presence of labelled soluble proteins with molecular weights of 450,000 (peak Ia) and 68,000 (peak Ib), polypeptides with a molecular weight of 5,000, and a small amount of free leucine (peak II). In nerves taken 16–22 hrs after ganglion injection, a relatively greater specific activity was found in the peak Ia soluble protein. Labelled materials carried down by fast transport were allowed to enter the axons for a period of 5–6 hrs, and then further outflow from the somas prevented by ligations made just distal to the ganglia. After an additional 16 hrs in the animal it was found that the polypeptides and free leucine, which had entered these nerve fibers earlier, were not then later synthesized into the higher molecular weight proteins characteristic of the later downflows. These studies show a rapid synthesis of labelled materials carried down the fiber by the fast transport system and a translocation in the somas of another portion, mainly protein which is then carried out into the axon at later times.