Effect of Muscle and Non-muscle Tropomyosins in Reconstituted Skeletal Muscle Actomyosin

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Abstract
Smooth and non-muscle tropomyosins produced a 2-3 fold Ca-insensitive stimulation of the ATPase activity of reconstituted rabbit skeletal muscle actomyosin at normal MgATP concentrations and physiological ratios of myosin to actin. Under the same conditions skeletal muscle tropomyosin had no effect. Similar effects of these 3 tropomyosins were observed for the low myosin/F-actin ratios necessary for kinetic measurements. Since it could be established that this actomyosin system, with or without tropomyosin, obeyed Michaelian kinetics, the tropomyosin effects could be interpreted in terms of their influence on maximal turnover (V) or on the affinity of myosin for actin (Kapp). Gizzard tropomyosin produced an .apprx. 2-fold increase in both V and the affinity, while skeletal muscle tropomyosin had practically no effect on the affinity and reduced only slightly the value of V, compared to pure actin. In contrast to gizzard tropomyosin, pig brain tropomyosin produced an .apprx. 2-fold increase in both Kapp and V; i.e., it increased the turnover rate but decreased the affinity. Brain tropomyosin apparently acts as an uncompetitive activator with respect to pure actin, while having the same V as the actin plus gizzard tropomyosin complex. Further studies on these tropomyosins show that only skeletal and smooth muscle tropomyosin have similar functional properties with respect to troponin inhibition and the activation of the ATPase at low ATP concentrations. The noted increases in V by tropomyosin apparently are caused by the acceleration of the dissociation of the myosin head from actin at the end point of the cross bridge movement.