Desmin and Intermediate Filaments in Muscle Cells

Abstract

With electron microscopy, a morphological class of filaments with an average diameter of 100 Å has been identified in many vertebrate cells. These filaments were initially referred to as intermediate filaments since their diameter is intermediate to those of the 60 Å actin filaments and the 250 Å microtubules in nonmuscle cells and intermediate to those of the 60 Å actin filaments and the 150 Å myosin filaments in muscle cells. Presently, intermediate filaments are commonly referred to as neurofilaments in neurons, glial filaments in glial cells, 10 nm filaments in fibroblastic cells, tonofilaments or keratin filaments in epiderimal and epithelial cells, and intermediate filaments in skeletal, cardiac, and smooth muscle cells. Due to their wide distribution and morphological similarity, it was originally believed that the major subunit of the intermediate filaments from these various sources would be highly conserved in its amino acid sequence as is the case with actin and tubulin. The recent isolation of intermediate filaments from a variety of avian or mammalian cell types has enabled investigators to characterize the subunit proteins of these different filament preparations and to determine the extent of their homology. By a number of criteria including peptide mapping, antibody cross-reactivity, isoelectric point and molecular weight, it was shown that the major protein subunit of the intermediate filaments from muscle cells (desmin), fibroblasts, glial cells, neurons, and epidermal and epithelial cells (keratins) was distinct (Benitz et al. 1976; Dahl and Bignami 1976; Davison et al. 1977; Schachner et al. 1977; Schlaepfer 1977; Franke et al. 1978; Lazarides 1978; Lazarides and Balzer 1978; Milstone and McGuire 1978; Schlaepfer and Freeman 1978; Starger et al. 1978; Sun and Green 1978).