The Force–velocity Relationship of the Atp-Dependent Actin–Myosin Sliding Causing Cytoplasmic Streaming in Algal Cells, Studied Using a Centrifuge Microscope Available to Purchase

When uncoated polystyrene beads suspended in Mg-ATP solution were introduced into the internodal cell of an alga Chara corallina, the beads moved along the actin cables with directions and velocities (30–62 µm s⁻¹) similar to those of native cytoplasmic streaming. Bead movement was inhibited both in the absence of ATP and in the presence of Ca²⁺, as with native cytoplasmic streaming. These results indicate that bead movement is caused by cytoplasmic myosin molecules attached to the bead surface interacting with actin cables. The steady-state force–velocity relationship of the actin–myosin sliding that produces cytoplasmic 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 this shape are discussed in connection with the kinetic properties of the actin–myosin interaction in cytoplasmic streaming. It is suggested that the time for which a cytoplasmic myosin head is detached from actin in one cycle of actin–myosin interaction is very short. The Ca²⁺-induced actin–myosin linkages, responsible for the Ca²⁺-induced stoppage of cytoplasmic streaming, were shown to be much stronger than the rigor actin–myosin linkages.