The function of muscle is to exert force and to do mechanical work. It would therefore be highly desirable to be able to measure the force exerted upon single thin filaments by thick filaments (myosin).
In the normal in vitro motility assay the thin filaments are unloaded, therefore the interpretation of the results obtained from such systems is limited to comparisons with unloaded shortening in intact muscle.
The principle of novel in vitro motility assay is to place an internal load upon the thin filament to retard filament movement due to the myosin motor. This is achieved by using an actin-binding protein attached to the cover glass along with the immobilized myosin motor protein. The greater the force on an actin filament, the higher the concentration of actin-binding protein needed to stop movement.
Bing, W., A. Knott, and S.B. Marston, A simple method for measuring the relative force exerted by myosin on actin filaments in the in vitro motility assay: evidence that tropomyosin and troponin increase force in single thin filaments. Biochem J, 2000. 350 Pt 3: p. 693-9
