Myosin powers cytokinesis

The stage was set in 1977 for Issei Mabuchi and Makoto Okuno to show that myosin interacted with actin to provide the force behind cell cleavage (Mabuchi and Okuno, 1977). Several groups had spotted nonmuscle myosin in a variety of cell types using electron microscopy and antibodies (e.g., see Fujiwara and Pollard, 1976). And numerous ultrastructural studies showed that the contractile ring contained microfilaments, which had been identified as actin in newt eggs, crane fly spermatocytes and human HeLa cells by the early 1970s. Furthermore, the force exerted at the cleavage furrow had been measured in echinoderm eggs as comparable to the tension in skeletal muscle (Rappaport, 1967).

Finally, Mabuchi himself had isolated myosin from the cortex of dividing sea urchin and starfish eggs (Mabuchi, 1973; Mabuchi, 1974). From all of this, the University of Tokyo duo concluded, “it would be reasonable to suspect that actin and myosin would interact to produce the force of constriction.” To test this suspicion, Mabuchi says he decided the microinjection of myosin antibodies would be “the only promising method.” He injected an antibody raised against starfish egg myosin into dividing eggs in the first experiment to use antibodies as protein inhibitors in live cells.

The antibody inhibited the actin-activated ATPase activity of myosin in a dose-dependent manner. And when it was injected into eggs during the second interphase, it stopped all subsequent cleavage events. Mitosis still occurred normally in most of the injected cells, even if cleavage was inhibited. The study solidified a role for actin–myosin contraction in cytokinesis but not nuclear division—and, equally important, a role for antibodies in disrupting cellular phenomena.