Interaction of calmodulin with troponin I and the troponin-tropomyosin-actin complex. Effect of Ca²⁺ or Sr²⁺ ions

In an effort to elucidate the mechanism of calmodulin regulation of muscle contraction, we investigated the interaction between calmodulin and troponin components in the presence of Ca²⁺ or Sr²⁺ by the use of ultracentrifugation methods and polyacrylamide-gel electrophoresis. Skeletal-muscle troponin C bound to troponin I and dissociated it from the tropomyosin-actin complex in the presence of Ca²⁺ or Sr²⁺. When troponin T was absent, calmodulin bound to troponin I and dissociated it from the tropomyosin-actin complex in the presence of Ca²⁺ or Sr²⁺. When troponin T was present, calmodulin hardly bound to troponin I even in the presence of bivalent cations. Trifluoperazine, a calmodulin antagonist, inhibited the bivalent-cation-dependent interaction between calmodulin and troponin I. Calmodulin migrated more slowly in the presence of Sr²⁺ than it did in the presence of EGTA but faster than it did in the presence of Ca²⁺ on polyacrylamide-gel electrophoresis under non-denaturing conditions. It is concluded that troponin T is not required in the calmodulin regulation of muscle contraction because troponin T inhibits the bivalent-cation-dependent interaction between calmodulin and troponin I and because calmodulin binds to troponin I and dissociates it from the tropomyosin-actin complex in a bivalent-cation-dependent manner. Sr²⁺-induced exposure of the hydrophobic region enables calmodulin to bind to troponin I, as is the case with Ca²⁺.