Two Populations of Axonally Transported Tubulin Differentiated by Their Interactions with Neurofilaments

Abstract

In the sensory fibers of the rat sciatic nerve (fibers of the dorsal root ganglion cells), 2 components of tubulin transport were observed that differed in the rate of transport, solubility in Triton and subunit composition. The faster component, migrating ahead of the neurofilament proteins, was soluble in 1% Triton. The slower component, migrating with the neurofilament proteins, was insoluble in 1% Triton and contained a unique polypeptide, NAP, in the tubulin region that was not present in the faster component. NAP was not a subspecies of tubulin as evidenced by peptide mapping. It seems to be a neurofilament-associated protein. When a complete separation of the main tubulin wave from the neurofilament wave was achieved in the motor axons of the same nerve (axons of the ventral motoneurons) under the effect of .beta.,.beta.''-iminodipropionitrile, a portion of tubulin was still associated with the retarded neurofilament wave. The subunit composition of this portion was similar to the slower, neurfilament-associated component in the sensory fibers under normal conditions, i.e., enriched in NAP and the most acidic subtype of .beta.-tubulin. Two populations of transported tubulin may exist that are differentiated by the extent of their interaction with neurofilaments.