THE FORCE-VELOCITY RELATIONSHIP OF THE ATP-DEPENDENT ACTIN-MYOSIN SLIDING CAUSING CYTOPLASMIC STREAMING IN ALGAL CELLS, STUDIED USING A CENTRIFUGE MICROSCOPE

When uncoated polystyrene beads suspended in Mg-ATP solution were intr oduced into the internodal cell of an alga Chara corallina, the beads moved along the actin cables with directions and velocities (30-62 mu m s(-1)) similar to those of native cytoplasmic streaming. Bead moveme nt was inhibited both in the absence of ATP and in the presence of Ca2 + as with native cytoplasmic streaming. These results indicate that be ad movement is caused by cytoplasmic myosin molecules attached to the bead surface interacting with actin cables. The steady-state force-vel ocity relationship of the actin-myosin sliding that produces cytoplasm ic streaming was determined by applying constant centrifugal forces to the beads moving on the actin cables. The force-velocity curve in the positive load region was nearly straight, and the implications of thi s shape are discussed in connection with the kinetic properties of the actin-myosin interaction in cytoplasmic streaming. It is suggested th at the time for which a cytoplasmic myosin head is detached from actin in one cycle of actin-myosin interaction is very short. The Ca2+-indu ced actin-myosin linkages, responsible for the Ca2+-induced stoppage o f cytoplasmic streaming, were shown to be much stronger than the rigor actin-myosin linkages.