1. Laboratorio di Fisiologia, DBAG, Università di Firenze, I-50134 Firenze, and OGG, Istituto Nazionale di Fisica della Materia, Italy
2. Rosenstiel Center, Brandeis University, Waltham, Massachusetts 02545, USA
3. Randall Division of Cell and Molecular Biophysics, King's College London, London SE1 1UL, UK
4. European Synchrotron Radiation Facility, F-38043 Grenoble Cedex, France
5. BioCAT Advanced Photon Source, Argonne, Illinois 60439, USA

Correspondence to: Vincenzo Lombardi¹ Email: vincenzo.lombardi@unifi.it

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 results^{1, 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 technique^{5, 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 limit^{8, 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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