ASSEMBLY OF TYPE-IV NEURONAL INTERMEDIATE FILAMENTS IN NONNEURONAL CELLS IN THE ABSENCE OF PREEXISTING CYTOPLASMIC INTERMEDIATE FILAMENTS

We report here on the in vivo assembly of alpha-internexin, a type IV neuronal intermediate filament protein, in transfected cultured cells, comparing its assembly properties with those of the neurofilament tri plet proteins (NF-L, NF-M, and NF-H). Like the neurofilament triplet p roteins, alpha-internexin coassembles with vimentin into filaments. To study the assembly characteristics of these proteins in the absence o f a preexisting filament network, transient transfection experiments w ere performed with a non-neuronal cell line lacking cytoplasmic interm ediate filaments. The results showed that only alpha-internexin was ab le to self-assemble into extensive filamentous networks. In contrast, the neurofilament triplet proteins were incapable of homopolymeric ass embly into filamentous arrays in vivo. NF-L coassembled with either NF -M or NF-H into filamentous structures in the transfected cells, but N F-M could not form filaments with NF-H. Alpha-internexin could coassem ble with each of the neurofilament triplet proteins in the transfected cells to form filaments. When all but 2 and 10 an-amino acid residues were removed from the tail domains of NF-L and NF-M, respectively, th e resulting NF-L and NF-M deletion mutants retained the ability to coa ssemble with alpha-internexin into filamentous networks. These mutants were also capable of forming filaments with other wild-type neurofila ment triplet protein subunits. These results suggest that the tail dom ains of NF-L and NF-M are dispensable for normal coassembly of each of these proteins with other type IV intermediate filament proteins to f orm filaments.