Direct measurement of single synthetic vertebrate thick filament elasticity using nanofabricated cantilevers

Thick filaments are generally thought to be effectively inextensible. Here we use novel nanofabricated cantilevers to carry out the first direct force-elongation measurements on single vertebrate thick filaments. Cantilevers are ideal for these experiments: force ranges are from pico- to micronewtons, specimens can be visualized during the experiment, and attachment surfaces are in the same plane as the filament. Synthetic thick filaments from rabbit myosin were suspended between two cantilevers and stretched. With stretch, stiffness increased gradually and then became nearly constant after approximately 100 pN. Stretch rate had little or no effect on force-elongation behavior. Under physiological loads (approximately 240 pN axially averaged with full activation) filaments elongated by 1.1 +/- 0.3%. Previous x-ray diffraction results showed a 1.0 to 1.5% increase in myosin head spacing with activation; however, this increase in spacing has been interpreted as change in the state of the cross-bridges, not as elasticity in the thick filament backbone. Comparison with our data suggests that changes in the myosin x-ray reflections seen during activation may be due to elongation of the thick filament backbone. Recognition of thick filament elasticity is important because it affects the interpretation of mechanical experiments and inferences drawn on the molecular mechanism of contraction.