Rhodamine phalloidin F-actin : critical concentration versus tensile strength

A knowledge of actin's mechanical properties is crucial to understanding its role in motile and cytoskeletal system. It has long been assumed that thin filaments are rigid, however, this view has been challenged in recent years by accurate measurements of filament compliance. We have previously determined tensile strength, elastic modulus and work required to break the filament of phalloidin rhodamine actin be stretching filaments by a laser trap. these experiments have proved that mechanic and elastic properties and actin critical concentration are inversely related and posed the question of how thin filament bears the forces developed in muscle contraction. A solution migth be found in the interaction itself of the cross bridge with the thin filament. We have investigated this aspect by determining the yield strength of thin filament, we have found that myosin subfragment S1 delays the polimerization of F-actin. It is therefore likely that crossbridge attachment decreases the critical concentration of thin filament, by stregthening the interactions of the neighbouring actin monomers.