Troponin I and troponin T interact with troponin C to produce different Ca2+-dependent effects on actin-tropomyosin filament motility

We have developed an in vitro motility assay to make a detailed quantitative analysis of Ca2+ control of skeletal-muscle troponin-tropomyosin control of actin-filament movement over immobilized myosin. Ca2+ regulates both filament velocity and the fraction of filaments that are motile. We have demonstrated that the two effects are due to separate interactions of troponin C with troponin I and troponin T. When 64 nM of the complex actin-tropomyosin-troponin I-troponin C was added at pCa 5, more than 80%, of filaments were moving and their velocity did not change. At pCa 9, more than 20% of the filaments were moving. When 20 nM of the complex actin-tropomyosin-troponin T+troponin I+troponin C was added at pCa 5, filament motility remained high, whereas velocity increased. The 30% increase in velocity observed when troponin T was present was also observed when heavy meromyosin fragment I labelled with N-ethylmaleimide (NEM S-l) was added after actin-tropomyosin filaments. The NEM S-I effect was not additive with the troponin T-dependent velocity increase. The pattern of motile behaviour is characteristic of myosin on silicone-treated glass and different from the behaviour on nitrocellulose-coated glass.