Figure 7 Alternative models for the function of molecular motors in fast axonal transport. (From The Axon: Structure, Function and Pathophysiology by S. G. Waxman, J. D. Kocsis, and P. K. Stys. Copyright © 1995. Used by permission of Oxford University Press, Inc.).

in vitro motility assays, immunocytochemistry, and identification of particular cargoes by subcellular fractionation and immunoprecipitation.

2. Slow Transport

Less is known about the mechanisms of slow transport. The polymer sliding model proposes that cytos-keletal proteins, assembled in the cell body, travel down the axon in the form of individual microtubule and neurofilament polymers. Several recent experiments, however, suggest that cytoskeletal proteins are transported as small oligomers along microtubules. For example, in a modern variant of classical metabolic labeling studies, a recombinant adenoviral vector, encoding epitope-tagged neurofilament-M protein, was used to infect neurons of a transgenic mouse line in which axons normally lack neurofilaments. The viral-encoded subunits do not form polymers, but confocal and electron microscopy demonstrated that these nevertheless were transported

(at a slow rate of 5 mm/day) and that transport occurred in the close vicinity of microtubules.



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