Although

dependent a for muscle muscle cel (Shimmen, kinetic prop in non-musc cells, in which cytoplasmic streaming is caused by sliding between actin filament arrays (actin cables) and myosin molecules in the cytoplasm (Kamiya and Kuroda, 1956; Nagai and Rebhum, 1966; Kersey and Wessels, 1976; Kato and Tonomura, 1977). The velocity of cytoplasmic streaming (50mms21 or more; Shimmen, 1992) is very rapid, being many times larger than the maximum velocity of myofilament sliding in muscle (5–10mms21). Algal internodal cells have been widely used for in vitro assays of myosin-based motility, since latex beads coated with skeletal, cardiac and smooth muscle myosins slide on the actin cables in an ATP-dependent manner in these cells. Unfortunately, however, it has not been possible to produce sliding between algal cell cytoplasmic myosin and the actin cables responsible for native cytoplasmic streaming. und TP ter esu o th bea stre the steady-state force–velocity relationship of the ATP-dependent actin–myosin sliding responsible for cytoplasmic streaming, using a centrifuge microscope with which constant centrifugal forces were applied to the moving beads to serve as external loads (Oiwa et al. 1990). It will be shown in the present paper that the force–velocity curve in the positive load region is nearly straight in shape, with a large value of a/P0, where a is a constant from the Hill equation and P0 is maximum isometric force, suggesting a high cycling rate and a low efficiency of the actin–myosin interaction in cytoplasmic streaming. Since cytoplasmic streaming is known to be inhibited by Ca2+ as well as by removal of ATP (Williamson, 1975; Tominaga et al. 1983), we also studied the behaviour of the beads in the absence of ATP and in the presence of Ca2+