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Contractility of the heart muscle is a result of sliding movements between thick and thin filaments, produced by interactions between actin and myosin during the cross-bridge cycle. Activation of the myofilament is triggered by Ca 2+  binding to cardiac troponin C and is regulated through an “on/off” switching process occurring in the thin filament. Beside Ca 2+  regulation, strongly bound cross-bridges exert a positive feedback on myofilament regulation. Despite the importance of this positive feedback mechanism, its full molecular basis has so far remained elusive. Ca 2+ -regulated interactions between thick and thin filaments are widely regarded as an allosteric system, which means that multiple protein-protein interactions at their interface may exert alternative feedback effects on myofilament activation. To advance knowledge about these regulatory feedback mechanisms, we investigated a previously unstudied, hypothetical interaction between cardiac troponin and myosin, and how this interaction affects the function of myosin. Our results strongly suggest that myosin does indeed interact with the N-terminus of cardiac troponin I and the C-terminus of cardiac troponin T, suggesting a possible direct interaction between myosin and the IT-arm of troponin. We also conducted an in vitro heavy meromyosin (HMM) ATPase assay, and found that troponin significantly enhanced the actin-activated ATPase activity of HMM, both in the absence of tropomyosin and at the activated state of thin filament.

Direct interaction between troponin and myosin enhances the ATPase activity of heavy meromyosin