Molecular parameters of type IV alpha-internexin and type IV-type III alpha-internexin-vimentin copolymer intermediate filaments.

During neuronal development, a dynamic replacement mechanism occurs in which the type VI nestin and type III vimentin intermediate filament proteins are replaced by a series of type IV proteins beginning with alpha-internexin. We have explored molecular details of how the type III to type IV replacement process may occur. First, we have demonstrated by cross-linking experiments that bacterially expressed forms of alpha-internexin and vimentin form heterodimer molecules in vitro that assemble into copolymer intermediate filaments. We show using a urea disassembly assay that alpha-internexin molecules are likely to be more stable than those of vimentin. Second, by analyses of the induced cross-links, we have determined the axial lengths of alpha-internexin homodimer and alpha-internexin-vimentin heterodimer molecules and their modes of alignments in filaments. We report that these dimensions are the same as those reported earlier for vimentin homopolymer molecules and, by implication, are also the same for the other neuronal type IV proteins. These data suggest that during neuronal development, alpha-internexin molecules are readily assimilated onto the pre-existing vimentin cytoskeletal intermediate filament network because the axial lengths and axial alignments of their molecules are the same. Furthermore, the dynamic replacement process may be driven by a positive equilibrium due to the increased stability of the alpha-internexin network.