Axonal transport of synapsin I-like proteins in rabbit retinal ganglion cells

Synapsin I is a neuronal phosphoprotein that is associated with the cytoplasmic surface of small, clear synaptic vesicles in neuronal synaptic terminals; it may play an important role in synaptic transmission. In vitro, it can interact with fodrin, a relative of the erythrocyte protein spectrin. We have investigated the delivery of synapsin I from its site of synthesis in neuronal cell bodies to synaptic terminals by means of the process of axonal transport. We labeled the newly synthesized proteins of rabbit retinal ganglion cells by injecting 35S-methionine into the vitreous humour, and subsequently observed the appearance of radioactive synapsin I (identified by its 2-dimensional electrophoretic mobility) in tissues containing the axons and synaptic terminals of these neurons. A portion of the newly synthesized synapsin I was axonally transported at the velocity of the most rapidly transported (group I) proteins, which comprise membrane-associated proteins and may include elements of synaptic vesicles. However, the subsequent time course of labeling of synapsin I in the axons suggests that greater than 90% of the axonally transported synapsin I may comprise 2 additional populations--one transported rapidly, the other slowly--that are released from the cell bodies only after a delay of more than 1 d. The delayed, slowly transported population moves at the velocity (approximately 6 mm/d) of groups III and IV (which include fodrin and other proteins of the membrane cytoskeleton). We consider whether such distinct populations may correspond to functionally specialized variants of synapsin I-like proteins that may be transported in association with different organelles. The electrophoretic mobility of labeled synapsin I-like proteins in the axons changed subtly with time. Additional subtle differences between labeled synapsin I-like proteins in the axons and the terminal-containing tissues suggest that certain posttranslational modifications occur specifically in the terminals.