Cooperative binding to the Ca2+-specific sites of troponin C in regulated actin and actomyosin.

The Ca2+-binding component of troponin (TnC) and its proteolytic fragments containing Ca2+-binding sites I-III (TH1) or sites III and IV (TR2C) have been labeled with the fluorescent probes dansylaziridine (DANZ) at methionine 25 or 5-(iodoacetamidoethyl)amino-naphthalene-1-sulfonic acid (AEDANS) at cysteine-98. These probes report binding of Ca2+ to the low and high affinity sites, respectively. Fluorescence changes as a function of Ca2+ were measured for the free peptides, their complexes with troponin I + troponin T, and these complexes bound to actin-tropomyosin in the presence of Mg2+ and ATP with and without myosin. An apparent Hill coefficient of 1.0-1.1 has been obtained for the Ca2+-induced fluorescence changes in TnC, its fragments, and their ternary complexes regardless of the label used. When a ternary complex containing appropriately labeled TnC or its fragment is bound to the actin-tropomyosin complex, the Hill coefficient for the titration of the low affinity sites increases to 1.5-1.6 and further increases to greater than 2 in the presence of myosin. To interpret the apparent Hill coefficients, we used a model containing two binding sites and a single reporter of the conformational change. Hill coefficients between 1.0 and 1.2 can be obtained for the fluorescence change without true cooperativity in metal binding, depending on the mechanism of the fluorescence change; i.e. the contribution of the singly or doubly occupied species to the fluorescence change. A Hill coefficient between 1.2 and 2, however, always indicates cooperativity in binding independently of the mechanism. Thus, our finding that fluorescence titrations of Ca2+ binding to TnCDANZ bound to actin-tropomyosin exhibit a Hill coefficient of 1.5 in the absence of myosin and 2.4 in its presence indicates the existence of true positive cooperativity in metal binding to sites I and II. No cooperativity was observed for AEDANS-labeled complexes that reflect Ca2+-binding to the high affinity sites. Plots of the Ca2+ dependence of myosin ATPase activity activated by actin-tropomyosin in the presence of any of the troponin complexes used had apparent Hill coefficients of approximately 4. The higher value suggests cooperative interactions in the activation of ATPase beyond those involved in Ca2+-binding to the Ca2+-specific sites.