Enhancement of actomyosin ATPase activity by tropomyosin. Recombination of myosin and tropomyosin between muscles and platelet.
In skeletal muscle, the physiological role of tropomyosin has been assumed to be the 'blocking' of the actin-myosin interaction. In smooth muscle and platelet, however, tropomyosin was shown to 'enhance' the interaction. To investigate the reason for this apparent contradiction, we carried out recombination experiments using reconstituted actomyosins and different tropomyosins. Tropomyosins from skeletal muscle, arterial smooth muscle and platelet were recombined with skeletal, arterial and platelet myosins. The effects of tropomyosins on the actin-activated ATPase activities of myosins were then examined. The results are as follows. (i) Although tropomyosins from artery and platelet are distinctively different in molecular weight, they are interchangeable in enhancing the ATPase activities of both arterial and platelet actomyosins. The enhancement, however, is reduced by increasing the concentration of Mg X ATP and decreasing the concentration of myosin. (ii) Arterial and platelet tropomyosins are not capable of inhibiting the ATPase activity of skeletal actomyosin. (iii) Skeletal tropomyosin enhances arterial and platelet actomyosin ATPase activities in the same way as arterial and platelet tropomyosins. The results indicate that the major determinant of the effect of tropomyosin on the actomyosin-ATPase activity is the state of actomyosin. We suggest that any tropomyosin enhances the actin-activated ATPase activity of myosin recombined with skeletal actin, under the condition where actin and myosin form a 'rigor' (tight) complex.