STABLE AND METASTABLE CYTOSKELETAL POLYMERS CARRIED BY SLOW AXONAL-TRANSPORT

The proteins carried by the slow axonal transport in the rat sciatic motor axons were radiolabeled by injecting 35S-methionine into the spinal cord, and the distribution of their solubility through the 2 main components of slow transport (SCa and SCb) was considered. For this purpose, a cytoskeleton-stabilizing buffer was designed in which a pellet enriched in macromolecular and polymeric structures was separated from the solubilized proteins. The monomer/polymer ratios for tubulin were quantified in the 2 rate components. Our results indicate that 90% of the total tubulin was carried with SCa. Of this, 75% was in a polymeric state, versus only 50% of the tubulin carried with SCb. The monomeric tubulin recovered in the soluble fraction was concomitantly transported with the polymerized microtubules, suggesting that it might represent metastable regions of these microtubules. The insoluble and soluble fractions of the transported actin were measured. Actin was mostly (70%) transported with SCb. Of this, more than 80% was recovered in the soluble fraction, but we cannot say whether it was in a monomeric or polymeric state, nor if it was transported free or bound to a structure solubilized during fractionation. The other 30% of the actin most of it transported with SCa, was recovered in the polymer-enriched fraction, probably bound to a stabilized polymer, such as the microtubules. The neurofilaments, all of which were recovered in the polymer fraction, exhibited a complex transport profile suggesting the existence of 2 populations of motor axons: one with a high neurofilament/microtubule ratio and an average SCa transport rate of 1.1 mm/d, and the other with a lower neurofilament/microtubule ratio and an average SCa transport rate of 2.3 mm/d. Our data strongly suggest that the cytoskeletal proteins are in a polymeric state when they move inside the axons with SCa and, possibly, with SCb.