Effects of Ca2+ and calmodulin on the motile activity of characean myosin in vitro

It is well known that the cytoplasmic streaming of characean cells is readi ly inhibited by Ca2+. However, neither the actin-activated MgATPase nor the in vitro motile activity of purified characean myosin were inhibited by Ca 2+. Recently, amino acid sequence of characean myosin was determined in our laboratory and the sequence revealed that characean myosin contains six ca lmodulin binding sites in the neck region. We also detected calmodulin in q uickly prepared characean myosin fraction. It is, therefore, possible that the insensitivity of characean myosin to Ca2+ is due to the dissociation of some calmodulin molecules from the neck region during the course of protei n purification. To determine strictly the Ca2+ sensitivity of characean myo sin, we intentionally used crude preparation of characean myosin to reduce the possibility of calmodulin dissociation and examined the motile activity of characean myosin in vitro in the presence of excess characean calmoduli n. We could not observe any drastic inhibition of characean myosin activity by Ca2+. The results suggest that the brief cessation of cytoplamic stream ing is not caused by the direct inhibition of myosin activity by Ca2+.