The Polypeptide Composition of Intra-axonally Transported Proteins: Evidence for Four Transport Velocities

Abstract

Using a method of gradient gel electron phoresis coupled with autoradiography, we have analyzed the polypeptide composition of the proteins being transported down the axons of the projecting neurons of the rabbit retina. This analysis reveals: (1) the molecular weight distribution of 43 transported polypeptides; (2) the existence of at least four components of intra-axonal protein transport, each characterized by an unique polypeptide composition as well as by an unique velocity of transport; (3) the disappearance of individual labeled intra-axonally transported proteins from the axons and synaptic terminals with half-timnes ranging from several hours to more than 8 days.The possibility that intra-axonal transport functions as a temporal mediator of alterations in axonal or synaptic events is discussed in relation to these findings.The axons and synaptic terminals of neurons are critically dependent upon proteins supplied to them by their cell bodies (1). To satisfy this requirement, proteins synthesized in the neuronal somata are transported intra-axonally to their axonal or synaptic sites of utilization (1-3). The phenomenon of intra-axonal transport of protein is generally considered to include a "slow" component (transported at a rate on the order of 1-5 mm/day) and a "fast" component (transported at a rate on the order of hundreds of millimeters per day) (1-3). Proteins transported "rapidly" differ from those transported "slowly" with regard to their subcellular distribution (4-6), their oligosaccharide content (4, 7-9), and their electrophoretic mobility (9-12). Thus, the "rapid" and "slow" components of intra-axonal transport are defined not only by their velocity of transport, but also by their composition.Several lines of evidence suggest that proteins are transported intra-axonally at additional velocities intermediate between the velocities of the "rapid" and "slow"' components (13-18). However, the existence of a compositionally distinct "intermediate" component of intra-axonal transport has not been demonstrated; on the contrary, it has been reported that proteins that appear to be transported at an intermediate velocity are electrophoretically indistinguishable from those of the "rapid" component (11). If in fact the "rapid" and "slow" components are the only compositionally distinct classes of intra-axonally transported proteins, the significance of intermediate velocities of transport would be questionable. Since a knowledge of the number of components involved is prerequisite for a comprehensive understanding of intra-axonal transport, it is critical to determine whether intra-axonal transport comprises exclusively a "slow" and a "fast" component, or whether a multiplicity of intermediate components exist.The centrally projecting fibers of the rabbit retina (the axons of the retinal ganglion cells) are well suited for studying this problem. Because of their proximity to the vitreous of the eye, the cell bodies of these fibers are able to take up intraocularly injected radioactive amino a...

Posttranslational modification of a neurofilament protein during axoplasmic transport: implications for regional specialization of CNS axons

“…At postinjection times >1 d, these waves were observed at progressively greater distances from the eye. When the positions of the wave crests for individual proteins at different postinjection intervals (1.5, 6, and 15 h, and 2, 4, 6, 15, 30, 45, 60, 90, and 120 d) were analyzed, at least five separate transport groups were identified moving at rates (in ram/d) of 0.1-0.4, 2-3, 8-12, 20-28, and > 150, corresponding to those previously described in central axons from other species (24,59,64). Prominently labeled proteins at 200,000, 140,000, and 70,000 corresponding in position on one-dimensional autoradiographs (not shown) and on two-dimensional autoradiographs (Fig.…”

Axonal transport of the cytoplasmic matrix.

“…SLOW AXONAL TRANSPORT : Slow axonal transport differs from fast axonal transport in its rate and the materials transported by it (3,35,64). No membranous proteins have been detected moving at the slow rates (61).…”

Axoplasmic flow of tritiated proline in the corticospinal tract of the rat

“…Three of four separate waves of slow polypeptide transport with velocities from 0.7 to 68 mm per day have been described in the optic nerve of the rabbit (Willard et al 1974;Willard and Hulebak 1977). Although our data was not analyzed in a way that would make it possible to identify each of these components, the fronts and peaks found do not support the concept of several slow component rates.…”

“…It is quite possible that the differences between the rates that we found in the corticospinal tract and the results of others relate to the slow component that is being measured. Perhaps in our animals tritiated proline labels mainly polypeptide 35 of Willard et al (1974). A more detailed analysis of the particular polypeptides labeled with tritiated proline would be required to clarify this point.…”

“…Willard et al (1974) identified a group of polypeptides which traversed the rabbit optic system from retina to lateral geniculate and superior colliculus at a rate of at least 240 mm per day. Their electrophoretic analysis ofpolypeptides labeled after intravitreous injection of [35S] methionine clearly demonstrates a rate of at least 240mm per day for these polypeptides, but gives no information as to the fraction of the injected protein that was transported at that rate.…”