Interactions of calcium and native tropomyosin with myosin and heavy meromyosin

Abstract

It is confirmed that the presence of native tropomyosin is required in order for the removal of Ca ++ to cause inhibition of the Mg ++-activated ATPase of actomyosin and acto-heavy-meromyosin. Native tropomyosin is present in routinely purified samples of myosin at levels of 0.7–2.3%; such quantities inhibit the actomyosin ATPase by 30–60% upon removal of Ca ++. Titration with mercurial at low ionic strength causes a loss of actomyosin ATPase; it causes an even greater loss of the inhibition upon removal of Ca ++. If the mercurial is added in the presence of ATP, greater loss of the ATPase occurs than if the mercurial is added in the absence of ATP. It is proposed that binding of ATP at a second site (distinct from the substrate-binding site) either potentiates or blocks the inhibitory conformational change caused by mercurial binding, depending on the ionic strength.