Abstract
Muscle contraction is driven by the motor protein myosin II, which binds transiently to an actin filament, generates a unitary filament displacement or ‘working stroke’, then detaches and repeats the cycle. The stroke size has been measured previously using isolated myosin II molecules at low load, with rather variable results1,2,3,4, but not at the higher loads that the motor works against during muscle contraction. Here we used a novel X-ray-interference technique5,6 to measure the working stroke of myosin II at constant load7 in an intact muscle cell, preserving the native structure and function of the motor. We show that the stroke is smaller and slower at higher load. The stroke size at low load is likely to be set by a structural limit8,9; at higher loads, the motor detaches from actin before reaching this limit. The load dependence of the myosin II stroke is the primary molecular determinant of the mechanical performance and efficiency of skeletal muscle.
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Acknowledgements
This work was supported by Ministero dell'Istruzione, dell'Università e della Ricerca, Telethon-945 (Italy), the National Institutes of Health (NIH, USA), the Medical Research Council (UK), the European Molecular Biology Laboratory, the European Union and the European Synchrotron Radiation Facility. Use of the Advanced Photon Source was supported by the US Department of Energy, Basic Energy Sciences, Office of Science. BioCAT is an NIH-supported research centre. We thank A. Aiazzi, M. Dolfi and J. Gorini for mechanical and electronics support.
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Supplementary information
Supplementary Figure 1
Comparison of experimental RM3 (a) and IM3 (b) with predictions from a structural model. (JPG 142 kb)
Supplementary Figure 2
Interference fine structure and intensity of the M6 X-ray reflection during velocity transients under constant load. (JPG 68 kb)
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Reconditi, M., Linari, M., Lucii, L. et al. The myosin motor in muscle generates a smaller and slower working stroke at higher load. Nature 428, 578–581 (2004). https://doi.org/10.1038/nature02380
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DOI: https://doi.org/10.1038/nature02380
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