Inhibition of actin-myosin subfragment 1 ATPase activity by troponin I and IC: Relationship to the thin filament states of muscle

Troponin I (TnI) is the component of the troponin complex that inhibits actomyosin ATPase activity, and Ca2+ binding to the troponin C (TnC) component reverses the inhibition. Effects of the binding of TnI and the TnI-TnC (TnIC) complex to actin-tropomyosin (actinTm) on ATPase and on the binding kinetics of myosin subfragment 1 (S1) were studied to clarify the mechanism of the inhibition. TnI and TnIC in the absence of Ca2+ bind to actinTm and inhibit ATPase to similar levels with a stoichiometry of one TnI or one TnIC per one Tm and seven actin subunits. Tnf also binds to actinTmTn in the presence of Ca2+ with a stoichiometry and inhibition constant similar to those for the binding to actinTm of TnI and Tn in the absence of Ca2+. Thus, in the presence of Ca2+, the intrinsic TnI which is released from its binding site on actinTm does not interfere with the binding of an extra molecule of TnI to actinTmTn. The rate of Si binding to actinTmTnI and to actinTmTnTnI in the presence of Ca2+ was inhibited to the same extent as upon removal of Ca2+ from actinTmTn. These studies show that TnI inhibits ATPase by the same mechanism as Tn in the absence of Ca2+, by shifting the thin filament equilibria from the open state to the closed and blocked states.