Early studies of skeletal muscle revealed the existence of two kinds of protein filaments: thick, ∼150 Å–diameter myosin filaments and thin, ∼60 Å–diameter actin filaments. These filaments combine to form myofibrils in developing and mature muscle cells. The year 1968 brought the discovery of another class of filaments ∼100 Å in diameter (Ishikawa et al., 1968). These intermediate filaments constitute the majority of free filaments in the cytoplasm of most eukaryotic cells.
“There were many reports of free, thin cytoplasmic filaments in many kinds of cells. At the time, these were generally thought to be actin,” says Howard Holtzer. “We thought that they might be a completely new kind of filament because of their failure to be decorated with heavy meromyosin. And, unlike actin, they were of indefinite lengths.”
Earlier experiments by Inoué (1952) and Tilney (1965) had shown that the mitotic inhibitor colchicine caused...
