Axonal Transport of Neurotubule Protein

Abstract

IT has been known for some time that there is a constant somatofugal transport of materials down axons¹, but it has only recently been clearly demonstrated that more than one rate of transport may exist, and that transport cannot therefore be considered to be a single process^2–4. As far as transport of protein is concerned, at least two mechanisms seem to be operating, at significantly different rates. First, there is a “fast” protein component, which, as our previous studies have shown, advances at a rate of about 50 mm per day in goldfish optic nerve⁵; its counterpart in mammalian nerve has been found to have a rate of up to about 500 mm per day^6,7. Second, there is a “slow” component whose rate of advance is about two orders of magnitude slower; the value in goldfish optic nerve was found to be about 0.4 mm/day (ref. 8), while in mammalian nerve, values of 1–10 mm per day have been observed². It is clear, moreover, that the fast and slow types of transport involve different cellular constituents, for the fast component consists almost entirely of insoluble—that is, particulate—protein, while the slow component contains about 40–50 per cent soluble protein^5,9.