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      Blebbistatin Effects Expose Hidden Secrets in the Force-Generating Cycle of Actin and Myosin


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          Cyclic interactions between myosin II motors and actin filaments driven by ATP turnover underlie muscle contraction and have key roles in the motility of nonmuscle cells. A remaining enigma in the understanding of this interaction is the relationship between the force-generating structural change and the release of the ATP-hydrolysis product, inorganic phosphate (Pi), from the active site of myosin. Here, we use the small molecular compound blebbistatin to probe otherwise hidden states and transitions in this process. Different hypotheses for the Pi release mechanism are tested by interpreting experimental results from in vitro motility assays and isolated muscle fibers in terms of mechanokinetic actomyosin models. The data fit with ideas that actomyosin force generation is preceded by Pi release, which in turn is preceded by two serial transitions after/coincident with cross-bridge attachment. Blebbistatin changes the rate limitation of the cycle from the first to the second of these transitions, uncovering functional roles of an otherwise short-lived pre-power stroke state that has been implicated by structural data.

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          Mechanism of blebbistatin inhibition of myosin II.

          Blebbistatin is a recently discovered small molecule inhibitor showing high affinity and selectivity toward myosin II. Here we report a detailed investigation of its mechanism of inhibition. Blebbistatin does not compete with nucleotide binding to the skeletal muscle myosin subfragment-1. The inhibitor preferentially binds to the ATPase intermediate with ADP and phosphate bound at the active site, and it slows down phosphate release. Blebbistatin interferes neither with binding of myosin to actin nor with ATP-induced actomyosin dissociation. Instead, it blocks the myosin heads in a products complex with low actin affinity. Blind docking molecular simulations indicate that the productive blebbistatin-binding site of the myosin head is within the aqueous cavity between the nucleotide pocket and the cleft of the actin-binding interface. The property that blebbistatin blocks myosin II in an actin-detached state makes the compound useful both in muscle physiology and in exploring the cellular function of cytoplasmic myosin II isoforms, whereas the stabilization of a specific myosin intermediate confers a great potential in structural studies.
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            Cardiac myosin activation: a potential therapeutic approach for systolic heart failure.

            Decreased cardiac contractility is a central feature of systolic heart failure. Existing drugs increase cardiac contractility indirectly through signaling cascades but are limited by their mechanism-related adverse effects. To avoid these limitations, we previously developed omecamtiv mecarbil, a small-molecule, direct activator of cardiac myosin. Here, we show that it binds to the myosin catalytic domain and operates by an allosteric mechanism to increase the transition rate of myosin into the strongly actin-bound force-generating state. Paradoxically, it inhibits adenosine 5'-triphosphate turnover in the absence of actin, which suggests that it stabilizes an actin-bound conformation of myosin. In animal models, omecamtiv mecarbil increases cardiac function by increasing the duration of ejection without changing the rates of contraction. Cardiac myosin activation may provide a new therapeutic approach for systolic heart failure.
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              Muscle structure and theories of contraction.


                Author and article information

                Biophys J
                Biophys. J
                Biophysical Journal
                The Biophysical Society
                17 July 2018
                17 July 2018
                : 115
                : 2
                : 386-397
                [1 ]Department of Chemistry and Biomedical Sciences, Linnaeus University, Kalmar, Sweden
                [2 ]Department of Kinesiology and Physical Education, McGill University, Montreal, Canada
                Author notes
                []Corresponding author alf.mansson@ 123456lnu.se
                © 2018 Biophysical Society.

                This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

                : 12 February 2018
                : 30 May 2018
                Molecular Machines, Motors, and Nanoscale Biophysics



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