�-Internexin, a neuronal intermediate filament protein implicated in neurodegenerative disease, coexists with the neurofilament (NF) triplet proteins (NF-L, NF-M, and NF-H) but has an unknown function. The earlier peak expression of �-internexin than the triplet during brain development and its ability to form homopolymers, unlike the triplet, which are obligate heteropolymers, have supported a widely held view that �-internexin and neurofilament triplet form separate filament systems. Here, we demonstrate, however, that despite a postnatal decline in expression, �-internexin is as abundant as the triplet in the adult CNS and exists in a relatively fixed stoichiometry with these subunits. �-Internexin exhibits transport and turnover rates identical to those of triplet proteins in optic axons and colocalizes with NF-M on single neurofilaments by immunogold electron microscopy. �-Internexin also coassembles with all three neurofilament proteins into a single network of filaments in quadruple-transfected SW13vim(�) cells. Genetically deleting NF-M alone or together with NF-H in mice dramatically reduces �-internexin transport and content in axons throughout the CNS. Moreover, deleting �-internexi
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