Evidence that myosin does not contribute to force production in chromosome movement.

Abstract

Antibody against cytoplasmic myosin, when microinjected into actively dividing cells, provides a physiological test for the role of actin and myosin in chromosome movement. Anti-Asterias egg myosin, characterized by Mabuchi and Okuno, completely and specifically inhibits the actin activated Mg2+-ATPase of myosin in vitro and, when microinjected, inhibits cytokinesis in vivo. Microinjected antibody has no observable effect on the rate or extent of anaphase chromosome movements. Neither central spindle elongation nor chromosomal fiber shortening is affected by doses up to 8-fold higher than those required to uniformly inhibit cytokinesis in all injected cells. Such doses are sufficient to completely inhibit myosin ATPase activity in these cells. Cells injected with buffer alone, with myosin-absorbed antibody, or with nonimmune .gamma.-globulin, proceed normally through both mitosis and cytokinesis. Control .gamma.-globulin, labeled with fluorescein, diffuses to homogeneity throughout the cytoplasm in 2-4 min and remains uniformly distributed. Antibody is not excluded from the spindle region. Prometaphase chromosome movements, fertilization, pronuclear migration and pronuclear fusion are also unaffected by microinjected antimyosin. Antimyosin blocks the actomyosin interaction thought to be responsible for force production in cytokinesis but has no effect on mitotic or meiotic chromosome motion. Myosin is not involved in force production for chromosome movement.