Stepwise length changes in single invertebrate thick filaments

Previous experiments on thick filaments of the anterior byssus retractor muscle of Mytilus and the telson-levator muscle of Limulus polyphemus have shown large, reversible length changes up to 23% and 66% of initial length, respectively, within the physiological tension range. Using nanofabricated cantilevers and newly developed high-resolution detection methods, we investigated the dynamics of isolated Mytilus anterior byssus retractor muscle thick filaments. Single thick filaments were suspended between the tips of two microbeams oriented perpendicular to the filament axis: a deflectable cantilever and a stationary beam. Axial stress was applied by translating the base of the deflectable nanolever away from the stationary beam, which bent the nanolever. Tips of flexible nanolevers and stationary beam were imaged onto a photodiode array to track their positions. Filament shortening and lengthening traces, obtained immediately after the motor had imposed stress on the filament, showed steps and pauses. Step sizes were 2.7 nm and integer multiples thereof. Steps of this same size paradigm have been seen both during contraction of single sarcomeres and during active interaction between single isolated actin and myosin filaments, raising the question whether all of these phenomena might be related.