The Effect Of Tropomyosin On Myosin

Objective Tropomyosin(TM), a thin filament-associated protein, is thought to play a crucial role in the regulation of muscle contraction. It has been known that TM could affect the myosin Mg²⁺-ATPase activity. However, the interaction between TM and myosin is not absolutely clear. We hypothesize that there may be an interaction between TM and myosin and this interaction may correlate to the changes of myosin Mg²⁺-ATPase activity. To test this hypothesis, we used unphosphorylated and phosphorylated myosins to investigate the interaction of tropomyosin with myosin and the influences of tropomyosin on myosin Mg²⁺-ATPase activities both in the absence and presence of actin, and in cardiac muscle and smooth muscle respectively.Methods (1) Binding assay: the binding assay was used to test the interaction of tropomyosin and myosin. (2) The measurement of the myosin Mg²⁺-ATPase activity was used to test the effects of TM on myosins in different states.Results (1) The binding assay and myosin Mg²⁺-ATPase activity were carried out in the absence of actin. 1) Tropomyosin decreased the pellets of myosins both in cardiac and smooth muscle, suggesting that it could facilitate myosin to dissolve. 2) Tropomyosin increased the myosin Mg²⁺-ATPase activities both in cardiac and smooth muscle. (2) The binding assay and myosin Mg²⁺-ATPase activity were carried out in the presence of actin. 1) Tropomyosin significantly inhibited the myosin-actin binding both in cardiac and smooth muscle. 2) Tropomyosin decreased the actin activated myosin Mg²⁺-ATPase activity in cardiac muscle, while increased that in smooth muscle.Conclusions (1)Our results suggested that TM could facilitate myosin to dissolve, and the interaction between TM and myosin is different from the interactions of actin-myosin, calponin-myosin, and caldesmon-myosin, which is precipitated after the interaction. TM could not only facilitate unphosphorylated myosin to dissolve but also facilitate phosphorylated myosin to dissolve. Moreover, the effect of TM facilitating myosin to dissolve has a concordance in cardiac and smooth muscle. (2)The effect of TM on the myosin Mg²⁺-ATPase activities in the absence of actin has also a concordance in cardiac and smooth muscle. (3) The effect of TM inhibiting the myosin-actin binding has a consistency in cardiac and smooth muscle. (4)The effect of TM on the actin activated myosin Mg²⁺-ATPase activity is contrary in cardiac and smooth muscle. This study provides information for further illustration of exact mechanism of muscle contraction.