Abstract
This work is concerned with differential routing of different fast-transported proteins (FTP's) in dorsal root ganglion (DRG) cells from the bullfrog Rana catesbeiana. A single axon emerges from each cell body, this bifurcates into two processes--one entering the dorsal roots (DR) directed centrally and the other entering the sciatic nerve (SN) directed peripherally.The first goal was to determine to what degree, if any, the proteins transported down the two processes differ. Radioactively labeled, fast-transported proteins were collected at ligatures on each branch. The materials accumulated at the ligatures were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), fluorographed and used to expose X-ray film. In agreement with previous studies, the same species of proteins were found to be transported down each branch. For each protein species analyzed, the amount of radioactivity reaching each ligature was determined, and the ratio of radioactivity reaching the peripheral ligature to that reaching the central ligature was calculated. Not all proteins were transported in similar peripheral/central ratios, suggesting that there may be differential transport of fast-transported proteins in axonal branches.In the second study, the fast-transported proteins in the sciatic nerve were analyzed, specifically to identify those that may be deposited in the nerve (axolemma or released to surrounding cells) during transport, as opposed to being shipped to terminals. The newly synthesized proteins were pulse labeled, following which the ganglia were removed to prevent further initiation of transport. Following additional transport, two sections of nerve were compared--the area in which the moving wavefront was located and a more proximal section through which the wavefront had already passed. After separation of proteins by 2D-PAGE, computer analysis of the digitized video images of the fluorographs were used to calculate the amount of radioactivity in each protein. By this analysis, certain proteins were identified as being delivered primarily to axonal membrane, and others to more distal, probably terminal, areas.The first study reported here suggests that a neuron can control the abundance of a protein entering an axon branch, the second that it can further control the delivery of that protein to either systems of membranes in axon or to more distal areas.