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| - Caldesmon−Actin−Tropomyosin Contains Two Types of Binding Sites for MyosinS1
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| - Caldesmon inhibits the activation of myosin ATPase activity byactin−tropomyosin. Caldesmonalso inhibits the binding of myosin to actin. There isdisagreement as to the degree to which competitivedisplacement of myosin subfragment binding to actin is responsible forthe inhibition of ATPase activity.We have examined the possibility that one or more molecules of S1may bind to actin−tropomyosin−caldesmon without having the normal actin activation of ATPase activity.The effect of caldesmon onthe binding and ATPase activity of S1 was measured at several initiallevels of saturation of S1 to determineif a fraction of the bound S1 was resistant to displacement bycaldesmon. In the case of both unmodifiedS1 and ρPDM-modified S1, most, but not all, of the S1 was displacedby caldesmon. The results areconsistent with a single molecule of S1 binding with low affinity foreach seven actin monomers that arefully saturated with caldesmon and tropomyosin. This single S1 isnot necessarily bound directly to actinbut may be attached to the NH2-terminal region ofcaldesmon.
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