A soluble motor from the alga Nitella supports fast movement of actin filaments in vitro

In the streaming cytoplasm of the Characean algae cell, the movement of organelles along actin bundles occurs at a striking rate of up to 60 μm s^-1. To further characterize the molecular mechanisms responsible for this phenomenon, we have reconstituted the movement of actin filaments in vitro using defined biochemical components. We report that only a soluble cytoplasmic fraction devoid of organelles and filamentous material supports the movement of fluorescent-labeled actin filaments on glass at a rate of up to 60 μm s^-1. This fraction also contains the K⁺-EDTA ATPase and the actin-activated Mg²⁺ ATPase activities characteristic of myosin proteins. Therefore, on the basis of these observations, we conclude that Nitella cells have a soluble pool of non-filamentous myosin molecules with the mechanochemical properties expected for a motor responsible for cytoplasmic streaming in vivo. The preparation and conditions described here should be useful for the purification of this translocator.