Localization and Distribution of Actin Fibers in Normal, Transformed and Revertant Cells

Actin is not restricted to muscle tissue but is a major protein component of most, if not all, eukaryotic cells (see review of Pollard and Weihing 1973). Electron microscopy of various cells grown in culture has shown that there are at least three major fibrous structures found in a wide variety of cells: microfilaments, 100-Å filaments and microtubules (Goldman and Knipe 1972). Of these three structures, only microfilaments can be decorated with heavy meromyosin, the proteolytic fragment of skeletal muscle myosin still capable of binding to actin (Ishikawa et al. 1969). Although this result establishes that microfilaments contain actin, it cannot be taken to suggest that microfilaments are the only form of organization of actin in eukaryotic cells. Indeed, electron microscopy studies of different cell types containing approximately the same amount of actin show a great range of microfilament expression. A striking example of nonequivalence of microfilaments and actin occurs...