Association of proteins undergoing slow axonal transport with goldfish visual system myelin

[3H]Proline derived radioactivity which is slowly transported in the goldfish optic nerve has been isolated in association with purified preparations of optic tectal myelin. Acrylamide gel analysis reveals that most of the slowly transported radioactivity is distributed among the high molecular weight myelin proteins with a major peak comigrating with a band (approximately 42,000 daltons) which is prominent in goldfish brain myelin. A similar peak is not seen in gels of whole synaptosome protein. Relatively small but measureable amounts of radioactivity comigrate with the basic and proteolipid proteins. A low density myelin subfraction is 3-5 fold lower in specific radioactivity (disint./min/mug protein) than a corresponding high density fraction and shows particularly low specific radioactivities in the basic and proteolipid bands. The results are discussed with reference to the possible adherence of specific axonal membrane fragments as well as other potential sources of myelin label.