Role of myosin in the stimulatory effect of caldesmon on the interaction between actin, myosin, and ATP.

We have previously shown that caldesmon at low concentrations stimulates the interaction between actin, myosin, and ATP, while at high concentrations it inhibits the interaction [Ishikawa, R., Okagaki, T., Higashi-Fujime, S., & Kohama, K. (1991) J. Biol. Chem. 266, 21784-21790]. When the effect of caldesmon at low concentrations was monitored by measuring myosin ATPase activity in the absence of actin, the effect was slightly but significantly stimulatory; and at higher concentrations no inhibitory effect was observed. Therefore, we related the stimulatory effect with the myosin-binding property of caldesmon. In the presence of actin, a low concentration of caldesmon was not enough to evince the stimulatory effect: myosin concentration must also be low. This is because the stimulatory effect was obscured when myosin concentration was elevated. Ca(2+)-calmodulin abolished the stimulatory effect of caldesmon. However, the concentration of calmodulin required to abolish the stimulation was higher than that required to abolish the inhibition.