Characterizations of myosin essential light chain’s N-terminal truncation mutant Δ43 in transgenic mouse papillary muscles by using tension transients in response to sinusoidal length alterations
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Cross-bridge kinetics were studied at 20 °C in cardiac muscle strips from transgenic (Tg) mice expressing N-terminal 43 amino acid truncation mutation (Δ43) of myosin essential light chain (ELC), and the results were compared to those from Tg-wild type (WT) mice. Sinusoidal length changes were applied to activated skinned papillary muscle strips to induce tension transients, from which two exponential processes were deduced to characterize the cross-bridge kinetics. Their two rate constants were studied as functions of ATP, phosphate (Pi), ADP, and Ca2+ concentrations to characterize elementary steps of the cross-bridge cycle consisting of six states. Our results demonstrate for the first time that the cross-bridge kinetics of Δ43 are accelerated owing to an acceleration of the rate constant k2 of the cross-bridge detachment step, and that the number of strongly attached cross-bridges are decreased because of a reduction of the equilibrium constant K4 of the force generation step. The isometric tension and stiffness of Δ43 are diminished compared to WT, but the force per cross-bridge is not changed. Stiffness measurement during rigor induction demonstrates a reduction in the stiffness in Δ43, indicating that the N-terminal extension of ELC forms an extra linkage between the myosin cross-bridge and actin. The tension-pCa study demonstrates that there is no Ca2+ sensitivity change with Δ43, but the cooperativity is diminished. These results demonstrate the importance of the N-terminal extension of ELC in maintaining the myosin motor function during force generation and optimal cardiac performance.
KeywordsCardiac muscle Muscle mechanics Elementary steps Sinusoidal analysis Cross-bridge cycle Cardiomyopathy Cross-bridge kinetics Tension transients Rate constants
We thank Ana I. Rojas and Jingsheng Liang (University of Miami) for their excellent technical assistance with transgenic mice and cardiac muscle. This work was supported in part by grants from the National Institutes of Health HL070041 (M.K.); HL108343, HL071778 and HL090786 (D.S–C.); and the American Heart Association 10POST3420009 (P.M.). The content is solely the responsibility of the authors and does not necessarily reflect the official views of the National Center for Research Resources or the NIH.
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