Objective To test whether in the absence of actin

actin-binding proteins such as caldesmon

calponin

and tropomyosin interact with the myosin of unphosphorylation

Ca 2+ -dependent phosphorylation （CDP）

and Ca 2+ -independent phosphorylati-on （CIP） and stimulate myosin Mg 2+ -ATPase activities. Methods Mg 2+ -ATPase activities were measured to evaluate the effects of caldesmon

calponin

and tropomyosin on the myosin in unphosphorylation

CDP by myosin light chain kinase （MLCK）

and CIP by MLCK. Results （1） At different incubation-time

i.e.

5

10

20

40

and 60 minutes

the highest Mg 2+ -ATPase activity was ob-served when myosin was in the state of CDP

the medium was CIP of myosin

and the lowest was the unphosphorylated myosin. （2） In the absence of caldesmon

calponin

and tropomyosin

the Mg 2+ -ATPase activities from high to low were in the following order: CDP

CIP

and unphosphorylated myosin. However

in the presence of caldesmon

calponin

and tropo-myosin

the order of relative value of Mg 2+ -ATPase activities from high to low was unphosphorylated

CIP

and CDP of myosin respectively compared to the corresponding controls. Conclusions The results propose that caldesmon

calponin

and tropomyosin are capable of stimulating Mg 2+ -ATPase activity of smooth muscle myosin in Ca 2+ -independent manner

since Ca 2+ is not obligating for the stimulating effects of the three proteins. The common characteristic of the three proteins is that when myosin activities are low

their activations are relatively strong and this property might be involved in smooth muscle tension keeping.