The 68,000-dalton neurofilament-associated polypeptide is a component of nonneuronal cells and of skeletal myofibrils

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Abstract
Purified preparations of 10 nm neurofilaments from rat spinal cord and bovine or porcine brain contain a predominant 68,000 dalton polypeptide. This polypeptide is also a major component of the neurofilaments that copurify with brain tubulin isolated by cycles of polymerization and depolymerization. A protein that has the same isoelectric point and MW as the neurofilament-associated polypeptide was identified as a cytoskeletal protein in a variety of avian and mammalian cell types, including baby hamster kidney (BHK-21) mouse fibroblasts 3T3, Novikoff rat hepatoma, chicken fibroblast and chicken muscle cells. This protein is also a component of isolated chicken skeletal myofibrils. One-dimensional peptide maps of the 68,000 dalton proteins purified by 2-dimensional isoelectric focusing/NaDodSO4/polyacrylamide gel electrophoresis from myofibrils, cycled tubulin, purified neurofilaments and various cultured cell types were identical. In immunofluorescence this protein was associated with cytoplasmic intermediate filaments and myofibril Z discs. The neurofilament-associated polypeptide apparently is a conserved protein that is present in many different cell types in addition to neuronal cells. Because some of these cells contain the major components of 2 other intermediate filament classes, desmin and vimentin, a given cell type may contain the subunits of at least 3 distinct intermediate filament types.