Fast and slow components in axonal transport of protein.

(a) After injection of labeled leucine into the eye of goldfish, radioactive protein rapidly accumulates in the contralateral optic tectum in the layer containing the synaptic endings of the optic fibers. This material reaches the tectum 6-12 hr after the isotope injection, a fact which indicates that the rate of transport is at least 40 mm per day. (b) This rapidly transported material has been shown to consist exclusively of protein, in which the label remains attached to leucine. (c) Inhibition of protein synthesis in the retina prevents the appearance of the transported protein in the tectum, but inhibition of protein synthesis in the tectum does not. Substances having some of the same properties as leucine, such as cycloleucine and norepinephrine, are not transported to the tectum. These experiments all indicate that the transported protein is synthesized in the retina. However, inhibition of retinal protein synthesis after this protein has been formed does not interfere with the transport mechanism itself. (d) The fast component consists of about 85% particulate material. It may be distinguished from a slowly moving component, transported at 0.4 mm per day, which contains about 5 times as much radioactivity as the fast component, and which consists of 60% particulate matter and 40% soluble protein.