Axonal transport of a subclass of tau proteins: evidence for the regional differentiation of microtubules in neurons.

Tubulin, the major constituent of microtubules, is anterogradely transported within the axon as part of slow component a (SCa; 0.2-1.0 mm/day). This raises the possibility that the microtubule-associated proteins (MAPs) may be transported at the same rate. To examine this question, the high molecular weight and tau MAPs obtained from whole brain preparations of microtubules were compared with the proteins of SCa in guinea pig retinal ganglion cell axons by using phosphocellulose chromatography and one- and two-dimensional polyacrylamide gel electrophoresis. Only two of the tau proteins were found to be cotransported with axonal tubulin, although four tau and two high molecular weight MAPs were synthesized in the retina. This result suggests either that the retinal ganglion cell synthesizes only those two tau proteins or that it synthesizes several of the MAPs, but commits to axonal transport just two of the tau proteins. In either case, these observations are consistent with the transport of intact microtubules and demonstrate that axonal microtubules represent a distinct subset of brain microtubules. Such a distinction may be related to unique properties of the axonal cytoskeleton.