Functional Effects of the H1-Helix of Rat Cardiac Troponin T on Crossbridge Detachment Rate is Differently Modulated by α- and β-Myosin Heavy Chain Isoforms

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The primary structure of the H1-helix of troponin T (TnT) varies among different types of striated muscles. Moreover, these muscles also express different myosin heavy chain (MHC) isoforms. Recently, we demonstrated that pseudo-phosphorylation of residue 204 (near the H1-helix) of cardiac TnT affected the functional state of the thin filament differently in fibers that expressed either α- or β-MHC isoforms (Michael et al., Basic Res Cardiol, 109:442, 2014). In this follow-up study, we investigated how the isoform-specific function of the H1-helix of cardiac TnT was influenced by α- and β-MHC isoforms. We generated a mutant rat cardiac TnT (RfsH1) in which the cardiac H1-helix was replaced by the fast skeletal H1-helix. Recombinant RfsH1 was reconstituted into detergent-skinned cardiac muscle fibers from either normal rats (expressing α-MHC) or propylthiouracil treated rats (expressing β-MHC). Steady-state and dynamic measurements were carried out at sarcomere length 2.3 µm. Our results demonstrated that RfsH1 decreased Ca2+-activated maximal ATPase activity differently in α-MHC (∼33%) and β-MHC (∼17%) fibers. Furthermore, RfsH1 decreased tension cost (∼31%) and crossbridge (XB) distortion dynamics (∼25%) in α-MHC but not in β-MHC fibers. Because the above mentioned parameters are indices of the rate of XB detachment, our results suggest that the interplay between the RfsH1- and α-MHC-mediated effects on the thin filament modulates XB detachment kinetics. Our findings suggest that the conformational changes in the H1-helix of TnT are sensitive to MHC isoform-mediated changes in the thin filament.