Cooperativity of the calcium switch of regulated rabbit actomyosin system.

The concentration range required for calcium activation of skeletal myofibrillar ATPase activity has previously been attributed to simultaneous binding of two calcium ions to each troponin. We present data representative of the majority of myofibrillar preparations and data with acto subfragment-1 (S-1) whose calcium activation of ATPase activity occurs over a much too narrow range of calcium concentrations to be so explained. S-1 binding significantly broadened the range of Ca2+ concentrations over which activation occurred but not to the extent that is associated with simultaneous binding of 2 calcium ions.