Actin in nonmuscle cells

In the late 1960s, Howard Holtzer's group at the University of Pennsylvania made the unexpected observation that virtually all eukaryotic cells assemble a variety of actin-based structures (Ishikawa et al., 1969). At that time, scientists thought that actin and myosin were restricted to muscle cells and ascribed contractile activity in other cells to a variety of molecules and structures. Holtzer's fluorescent antibodies to sarcomeric myosin decorated muscle but not any other cell type. “That's why,” he says, “we were so surprised to find actin filaments in nonmuscle cells.”

Holtzer's work followed a classic study by Huxley (1963). Huxley had reported that in a cell-free system heavy meromyosin (HMM), a proteolytic fragment of myosin, could be incubated with polymerized, filamentous actin and form polarized arrowhead complexes that could be readily visualized in...