Differential regulation of the actomyosin interaction by skeletal and cardiac troponin isoforms.

There are significant isoform differences between the skeletal and cardiac troponin complexes. Studies of the regulatory properties of these proteins have previously shown only significant differences in the calcium dependence of their regulation. Using a sensitive myosin subfragment 1 (S1) binding assay we show that in the presence of calcium, thin filaments reconstituted with either skeletal or cardiac troponin produce virtually identical S1 binding curves. However in the absence of calcium the S1 binding curves differ considerably. Combined with kinetic measurements, curve fitting to the three-state thin filament regulatory model shows the main difference is that calcium produces a 4-fold change in K(T) (the closed-open equilibrium) for the skeletal system but little change in the cardiac system. The results show a significant difference in the range of regulatory effect between the cardiac and skeletal systems that we interpret as effects upon actin-troponin (Tn)I-TnC binding equilibria. As structural data show that the Ca(2+)-bound TnC structures differ, the additional counter-intuitive result here is that with respect to myosin binding the +Ca(2+) state of the two systems is similar whereas the -Ca(2+) state differs. This shows the regulatory tuning of the troponin complex produced by isoform variation is the net result of a complex series of interactions among all the troponin components.