Rab3a, a small GTP-binding protein, undergoes fast anterograde transport but not retrograde transport in neurons.

Rab3a is a small GTP-binding protein that is associated with synaptic vesicles in neurons and that undergoes membrane dissociation-association paralleling the synaptic membrane cycle. Using the sciatic nerve as a model, we have studied the axonal transport of Rab3a by quantitative immunocytochemistry and immunoblotting and electron microscopy. Ligation of the sciatic nerve resulted in a progressive accumulation of Rab3a proximal of the ligation site in virtually all axons with an onset of less than 1 h. Accumulation of Rab3a was accompanied by accumulation of synaptophysin and synapsin I, two synaptic vesicle membrane proteins, and accumulation of clathrin light chain. Immunogold electron microscopy revealed that organelles labeled for Rab3a were mainly small round vesicles with an average diameter of 50 to 60 nm, indistinguishable from those labeled for the synaptic vesicle protein synaptophsin. In contrast, no retrograde accumulation of Rab3a was observed in most axons, with sparse labeling being confined to few thin axons. For other synaptic vesicle membrane proteins as well as for clathrin light chain, substantial accumulations were observed on the distal side of the ligation. We conclude that Rab3a associates with synaptic vesicle precursors in the cell soma before entering the axon and being transported to the synapse. At the end of its useful life-span, however, Rab3a does not associate with retrogradely transported membrane material, suggesting that it is degraded within the nerve terminal.