Slowly migrating axonal polypeptides. Inequalities in their rate and amount of transport between two branches of bifurcating axons

Polypeptides in the dorsal root ganglion (L5) of the adult rat were radioactively labeled, and components slowly migrating in the sciatic nerve (peripheral axons) and dorsal root (central axons) were analyzed, using SDS-polyacrylamide slab gel electrophoresis and fluorography. In particular, the transport rates and amounts of six major polypeptides, i.e., the triplet (reference 15; with mol wts of 200,000, 160,000, and 68,000 daltons), alpha- and beta-tubulins and actin were compared between the two axon branches. In peripheral axons, fronts of the triplet, tubulins, and actin migrate at 2-3 mm/d, 9-13 mm/d and approximately 19 mm/d, respectively. The corresponding values in central axons are 1-2 mm/d, 3-4 mm/d, and approximately 4 mm/d, indicating an obvious asymmetry in the transport rate between the two branches of bifurcating axons. A greater amount of labeled triplet, tubulins, and actin each is found to migrate in peripheral than in central axons. Another striking aspect of asymmetry between the two branches relates to the tubulins/triplet ratio which is significantly higher in the peripheral branch. Considerable proportions of radioactivities associated with tubulins and actin in the ganglion are nonmigratory, which are thought to derive mostly from periaxonal satellite cells. In contrast, most if not all of the labeled triplet is migratory, suggesting a virtual absence of triplet polypeptides in satellite cells. The possible significance of peripheral-central inequalities in slow axoplasmic transport is discussed from the viewpoints of axon volume and axonal outgrowth.