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 whic h the type VI nestin and type III vimentin intermediate filament proteins a re 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 replace ment process may occur. First, we have demonstrated by cross linking experi ments that bacterially expressed forms of alpha-internexin and vimentin for m heterodimer molecules in vitro that assemble into copolymer intermediate filaments. We show using a urea disassembly assay that alpha-internexin mol ecules are likely to be more stable than those of vimentin. Second, by anal yses of the induced cross-links, we have determined the axial lengths of al pha-internexin homodimer and alpha-internexin-vimentin heterodimer molecule s and their modes of alignments in filaments. We report that these dimensio ns are the same as those reported earlier for vimentin homopolymer molecule s and, by implication are also the same for the other neuronal type IV prot eins. These data suggest that during neuronal development, alpha-internexin molecules are readily assimilated onto the pre-existing vimentin cytoskele tal intermediate filament network because the axial lengths and axial align ments of their molecules are the same. Furthermore, the dynamic replacement process may be driven by a positive equilibrium due to the increased stabi lity of the alpha-internexin network.