Heterogeneity of intermediate filaments assembled in vitro

Intermediate filaments (IFs) constitute one of the three types of cytoskeletal structural proteins of eu-karyote cells. They have been isolated from a wide variety of tissues and cell types either as intact fila-ments or as soluble subunits. Whereas the principal subunits of microtubules (tubulin) and microfilaments (actin) have been highly conserved throughout evo-lution, the subunits of IFs of different cells vary widely in their size, complexity, solubility, and antigenicity. Much of the present understanding of the IFs in cells has arisen from immunofluorescence studies, and five distinct immunologic subclasses of IFs have been de-lineated: epithelial cells (keratin), muscle (desmin), neuronal (neurofilaments), astroglial cells (glial fila-ments), and mesenchymal cells or tissues (decamin) (for recent reviews, see Gilbert 1978; Goldman et al. 1979; Lazarides 1980; Zackroff et al. 1981). Similar immunofluorescence techniques have also revealed the coexistence of two of these immunologic subclasses in some types of cells (Lazarides 1980; Zackroff et al. 1981). Examples are (1) BHK-21 cells and developing myotubes inculture contain the muscle and fibroblasti