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

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 Ca2+ binding to cardiac troponin C and is regulated through an “on/off” switching process occurring in the thin filament. Beside Ca2+ 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. Ca2+-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.