Interference x-ray diffraction from single muscle cells reveals the molecular basis of muscle braking

Muscle is a machine that converts metabolic energy into mechanical work by cyclic ATP-driven interactions of the molecular motor, myosin II, with the actin filament. Muscle can also act as a brake, generating a high resistive force with reduced ATP consumption, when the load is increased above the isometric force. To investigate the molecular basis of the braking action of muscle, we used time-resolved X-ray diffraction from intact cells isolated from skeletal muscle of the frog. The results indicate that a stretch of 2-6nm per half-sarcomere imposed on the actively contracting cell induces a rapid attachment to actin of the second motor domain of the myosin molecules that have the first motor domain already attached before the stretch. This mechanism allows skeletal muscle to almost instantaneously resist an external stretch, while minimising the stress on an individual motor. © Springer-Verlag Berlin Heidelberg 2011. All rights are reserved.